Publications

2024

1. A Post-Launch Summary of the Science of NASA’s Psyche Mission

   \relax Steven. D. Dibb, Erik Asphaug, James F. Bell, and 13 more authors

   AGU Advances, 2024.

   abstract url

   Astronomical observations indicate that asteroid (16) Psyche is a large, high-density (likely >3,400 kg⋅m-3), metal-rich (30–55 vol. %) asteroid. Psyche may be remnant core material or it could be a primordial, undifferentiated metal-rich object. We discuss the science objectives of the upcoming Psyche mission, which will employ three instruments (the Magnetometer, Multispectral Imager, and Gamma-Ray and Neutron Spectrometer) and will use Doppler tracking of the spacecraft to explore the asteroid. This mission will shed light on the nature and origins of metal-rich objects in the solar system and beyond, including the cores of the terrestrial planets.

2. Constraints on Lateral Variations of Martian Crustal Thickness From Seismological and Gravity Field Measurements

   Mélanie Drilleau, Henri Samuel, Raphaël F. Garcia, and 5 more authors

   Geophysical Research Letters, 2024.

   abstract url

   Using body wave arrival times from 31 seismic events recorded on Mars by the InSight mission, combined with topography and gravity field modeling, we constrained lateral variations of crustal thickness through a Bayesian inversion approach. The parameterization of the seismic structure relies on quantities that influence the thermochemical evolution of Mars, enabling the seismic velocities and densities in the different planetary envelopes to be consistently linked through common physical assumptions. Compared to a 1D structure, models with lateral variations of crustal thickness show two possible interpretations of the thermal evolution of Mars, with either a hot or cold scenario at the present-day. We found the hot scenario to be more compatible with InSight’s radiotracking data and the tidal Love number. We relocated the marsquakes and derived maps of seismicity recorded by InSight, which is mostly located along or North of the boundary between the Northern lowlands and the Southern highlands.

3. Constraints on the Spatial Distribution of Lunar Crustal Magnetic Sources From Orbital Magnetic Field Data

   Joana S. Oliveira, Foteini Vervelidou, Mark A. Wieczorek, and 1 more author

   Journal of Geophysical Research: Planets, 2024.

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   Spacecraft measurements show that the crust of the Moon is heterogeneously magnetized. The sources of these magnetic anomalies are yet not fully understood, with most not being related to known geological structures or processes. Here, we use an inversion methodology that relies on the assumption of unidirectional magnetization, commonly referred to as Parker’s method, to elucidate the origin of the magnetic sources by constraining the location and geometry of the underlying magnetization. This method has been used previously to infer the direction of the underlying magnetization but it has not been tested as to whether it can infer the geometry of the source. The performance of the method is here assessed by conducting a variety of tests, using synthetic magnetized bodies of different geometries mimicking the main geological structures potentially magnetized within the lunar crust. Results from our tests show that this method successfully localizes and delineates the two-dimensional surface projection of subsurface three-dimensional magnetized bodies, provided their magnetization is close to unidirectional and the magnetic field data are of sufficient spatial resolution and reasonable signal-to-noise ratio. We applied this inversion method to two different lunar magnetic anomalies, the Mendel-Rydberg impact basin and the Reiner Gamma swirl. For Mendel-Rydberg, our analysis shows that the strongest magnetic sources are located within the basin’s inner ring, whereas for Reiner Gamma, the strongest magnetic sources form a narrow dike-like body that emanates from the center of the Marius Hills volcanic complex.

4. Geophysical Characterization of the Interiors of Ganymede, Callisto and Europa by ESA’s JUpiter ICy Moons Explorer

   Tim Van Hoolst, Gabriel Tobie, Claire Vallat, and 54 more authors

   Space Science Reviews, 2024.

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   The JUpiter ICy moons Explorer (JUICE) of ESA was launched on 14 April 2023 and will arrive at Jupiter and its moons in July 2031. In this review article, we describe how JUICE will investigate the interior of the three icy Galilean moons, Ganymede, Callisto and Europa, during its Jupiter orbital tour and the final orbital phase around Ganymede. Detailed geophysical observations about the interior of the moons can only be performed from close distances to the moons, and best estimates of signatures of the interior, such as an induced magnetic field, tides and rotation variations, and radar reflections, will be obtained during flybys of the moons with altitudes of about 1000 km or less and during the Ganymede orbital phase at an average altitude of 490 km. The 9-month long orbital phase around Ganymede, the first of its kind around another moon than our Moon, will allow an unprecedented and detailed insight into the moon’s interior, from the central regions where a magnetic field is generated to the internal ocean and outer ice shell. Multiple flybys of Callisto will clarify the differences in evolution compared to Ganymede and will provide key constraints on the origin and evolution of the Jupiter system. JUICE will visit Europa only during two close flybys and the geophysical investigations will focus on selected areas of the ice shell. A prime goal of JUICE is the characterisation of the ice shell and ocean of the Galilean moons, and we here specifically emphasise the synergistic aspects of the different geophysical investigations, showing how different instruments will work together to probe the hydrosphere. We also describe how synergies between JUICE instruments will contribute to the assessment of the deep interior of the moons, their internal differentiation, dynamics and evolution. In situ measurements and remote sensing observations will support the geophysical instruments to achieve these goals, but will also, together with subsurface radar sounding, provide information about tectonics, potential plumes, and the composition of the surface, which will help understanding the composition of the interior, the structure of the ice shell, and exchange processes between ocean, ice and surface. Accurate tracking of the JUICE spacecraft all along the mission will strongly improve our knowledge of the changing orbital motions of the moons and will provide additional insight into the dissipative processes in the Jupiter system. Finally, we present an overview of how the geophysical investigations will be performed and describe the operational synergies and challenges.

5. 

   Magnetic Signatures of Lunar Impact Craters

   Xi Yang, and Mark Wieczorek

   Icarus, 2024.

   abstract url

   The Moon almost certainly had an internally generated magnetic field in its past, but the duration of the dynamo, its temporal stability, and the surface field intensity are poorly known. Impact cratering events heat portions of the crust above the Curie temperature, allowing the ambient magnetic field strength to be recorded as the crater cools. We systematically analyzed the magnetic signatures of lunar impact craters with diameters greater than 90 km using recent magnetic field models and crater databases. Craters were classified as having evidence for impact-related central magnetization or demagnetization, and synthetic magnetic field models were used to estimate the number of incorrect identifications. In total, about 15% of craters were found to have impact-related magnetized or demagnetized signatures. The proportion of pre-Nectarian and Nectarian aged craters in the magnetized class is about 2%–3%, and there is little evidence for magnetized craters in the Imbrian and younger periods. The percentage of craters in the demagnetized class is about 0.3%–3% in the pre-Nectarian and Nectarian periods and abruptly increases to 16% in the Imbrian period. Our observations are consistent with the presence of strong dynamo fields during portions of the older pre-Nectarian and Nectarian periods, with a weakening (or cessation) of the dynamo at the beginning of the younger Imbrian period. These results differ from paleomagnetic analyses of lunar samples that imply the existence of strong dynamo field strengths from the pre-Nectarian up until at least the first half of the Imbrian period.

2023

1. Regional Variations of Mercury’s Crustal Density and Porosity from MESSENGER Gravity Data

   Antonio Genova, Sander Goossens, Edoardo Del Vecchio, and 10 more authors

   Icarus, 2023.

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   A new solution of Mercury’s gravity field to degree and order 160, named HgM009, is retrieved through a reprocessing of MESSENGER radio science measurements. By combining our latest gravity field with topography data, localized spectral admittance analyses are carried out to investigate Mercury’s crustal and lithospheric properties across the northern hemisphere. The measured spectra are compared with admittances predicted by lithospheric flexure models. The localized gravity/topography admittance analyses yield key information on the lateral variations of the bulk density of the upper crust. Elastic and crustal thicknesses are also adjusted in our study, but the local admittance spectra allow us to constrain these parameters only over a few regions. The average bulk density across the observed areas in the northern hemisphere is 2540 \textpm60 kg m-3. The crustal porosity is then constrained by using an estimate of the pore-free grain density of surface materials with our measured bulk density. Our estimate of the mean porosity is 14.7 \textpm1.6 %, which is comparable to, but slightly higher than, the average value measured on the Moon. Larger crustal porosities are observed over heavily cratered regions, suggesting that impact bombardment is the main cause of the crustal porosity.

2. Joint Inversion of Receiver Functions and Apparent Incidence Angles to Determine the Crustal Structure of Mars

   Rakshit Joshi, Brigitte Knapmeyer-Endrun, Klaus Mosegaard, and 4 more authors

   Geophysical Research Letters, 2023.

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   Recent estimates of the crustal thickness of Mars show a bimodal result of either ∼20 or ∼40 km beneath the InSight lander. We propose an approach based on random matrix theory applied to receiver functions (RFs) to further constrain the subsurface structure. Assuming a spiked covariance model for our data, we first use the phase transition properties of the singular value spectrum of random matrices to detect coherent arrivals in the waveforms. Examples from terrestrial data show how the method works in different scenarios. We identify three previously undetected converted arrivals in the InSight data, including the first multiple from a deeper third interface. We then use this information to jointly invert RFs with the absolute S-wave velocity information in the polarization of body waves. Results show a crustal thickness of 43 \textpm 5 km beneath the lander with two mid-crustal interfaces at depths of 8 \textpm 1 and 21 \textpm 3 km.

3. Structure Along the Martian Dichotomy Constrained by Rayleigh and Love Waves and Their Overtones

   D. Kim, S. C. Stähler, S. Ceylan, and 12 more authors

   Geophysical Research Letters, 2023.

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   Using seismic recordings of event S1222a, we measure dispersion curves of Rayleigh and Love waves, including their first overtones, and invert these for shear velocity (VS) and radial anisotropic structure of the Martian crust. The crustal structure along the topographic dichotomy is characterized by a fairly uniform vertically polarized shear velocity (VSV) of 3.17 km/s between ∼5 and 30 km depth, compatible with the previous study by Kim et al. (2022), https://doi.org/10.1126/science.abq7157. Radial anisotropy as large as 12% (VSH > VSV) is required in the crust between 5 and 40 km depth. At greater depths, we observe a large discontinuity near 63 \textpm 10 km, below which VSV reaches 4.1 km/s. We interpret this velocity increase as the crust-mantle boundary along the path. Combined gravimetric modeling suggests that the observed average crustal thickness favors the absence of large-scale density differences across the topographic dichotomy.

4. Global Crustal Thickness Revealed by Surface Waves Orbiting Mars

   D. Kim, C. Duran, D. Giardini, and 14 more authors

   Geophysical Research Letters, 2023.

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   We report observations of Rayleigh waves that orbit around Mars up to three times following the S1222a marsquake. Averaging these signals, we find the largest amplitude signals at 30 and 85 s central period, propagating with distinctly different group velocities of 2.9 and 3.8 km/s, respectively. The group velocities constraining the average crustal thickness beneath the great circle path rule out the majority of previous crustal models of Mars that have a >200 kg/m3 density contrast across the equatorial dichotomy between northern lowlands and southern highlands. We find that the thickness of the Martian crust is 42–56 km on average, and thus thicker than the crusts of the Earth and Moon. Considered with the context of thermal evolution models, a thick Martian crust suggests that the crust must contain 50%–70% of the total heat production to explain present-day local melt zones in the interior of Mars.

5. Spin State and Deep Interior Structure of Mars from InSight Radio Tracking

   Sébastien Le Maistre, Attilio Rivoldini, Alfonso Caldiero, and 22 more authors

   Nature, 2023.

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   Knowledge of the interior structure and atmosphere of Mars is essential to understanding how the planet has formed and evolved. A major obstacle to investigations of planetary interiors, however, is that they are not directly accessible. Most of the geophysical data provide global information that cannot be separated into contributions from the core, the mantle and the crust. The NASA InSight mission changed this situation by providing high-quality seismic and lander radio science data1,2. Here we use the InSight’s radio science data to determine fundamental properties of the core, mantle and atmosphere of Mars. By precisely measuring the rotation of the planet, we detected a resonance with a normal mode that allowed us to characterize the core and mantle separately. For an entirely solid mantle, we found that the liquid core has a radius of 1,835\,\textpm 55 km and a mean density of 5,955–6,290 kg m-3, and that the increase in density at the core–mantle boundary is 1,690–2,110 kg m-3. Our analysis of InSight’s radio tracking data argues against the existence of a solid inner core and reveals the shape of the core, indicating that there are internal mass anomalies deep within the mantle. We also find evidence of a slow acceleration in the Martian rotation rate, which could be the result of a long-term trend either in the internal dynamics of Mars or in its atmosphere and ice caps.

6. Different Martian Crustal Seismic Velocities Across the Dichotomy Boundary From Multi-Orbiting Surface Waves

   Jiaqi Li, Caroline Beghein, Philippe Lognonné, and 6 more authors

   Geophysical Research Letters, 2023.

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   We have observed both minor-arc (R1) and major-arc (R2) Rayleigh waves for the largest marsquake (magnitude of 4.7 \textpm 0.2) ever recorded. Along the R1 path (in the lowlands), inversion results show that a simple, two-layer model with an interface located at 21–29 km and an upper crustal shear-wave velocity of 3.05–3.17 km/s can fit the group velocity measurements. Along the R2 path, observations can be explained by upper crustal thickness models constrained from gravity data and upper crustal shear-wave velocities of 2.61–3.27 and 3.28–3.52 km/s in the lowlands and highlands, respectively. The shear-wave velocity being faster in the highlands than in the lowlands indicates the possible existence of sedimentary rocks, and relatively higher porosity in the lowlands.

7. 

   The Mantle Viscosity Structure of Venus

   J. S. Maia, M. A. Wieczorek, and A.-C. Plesa

   Geophysical Research Letters, 2023.

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   The long-wavelength gravity and topography of Venus are dominated by mantle convective flows, and are hence sensitive to the planet’s viscosity structure and mantle density anomalies. By modeling the dynamic gravity and topography signatures and by making use of a Bayesian inference approach, we investigate the viscosity structure of the Venusian mantle by constraining radial viscosity variations. We performed inversions under a wide range of model assumptions that consistently predicted the existence of a thin low-viscosity zone in the uppermost mantle. The zone is about 235 km thick and has a viscosity reduction of 5–15 times with respect to the underlying mantle. Drawing a parallel with the Earth, the reduced viscosity could be a result of partial melting as suggested for the origin of the asthenosphere. These results support the interpretation that Venus is a geologically active world predominantly governed by ongoing magmatic processes.

8. 

   Lunar Magnetism

   Mark A. Wieczorek, Benjamin P. Weiss, Doris Breuer, and 14 more authors

   Reviews in Mineralogy and Geochemistry, 2023.

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2022

1. A Positive Feedback Between Crustal Thickness and Melt Extraction for the Origin of the Martian Dichotomy

   Valentin Bonnet Gibet, Chloé Michaut, Mark Wieczorek, and 1 more author

   Journal of Geophysical Research: Planets, 2022.

   abstract url

   A North/South difference in crustal thickness is likely at the origin of the Martian dichotomy in topography. Recent crustal thickness maps were obtained by inversion of topography and gravity data seismically anchored at the InSight station. On average, the Martian crust is 51–71 km thick with a southern crust thicker by 18–28 km than the northern one. The origin of this crustal dichotomy is still debated although the hypothesis of a large impact is at present very popular. Here, we propose a new mechanism for the formation of this dichotomy that involves a positive feedback between crustal growth and mantle melting. As the crust is enriched in heat-producing elements, the lid of a one-plate planet is hotter and thinner where the crust is thicker, inducing a larger amount of partial melt below the lid and hence a larger rate of melt extraction and crustal growth. We first demonstrate analytically that larger wavelength perturbations, that is, hemispherical perturbations, grow faster because smaller wavelengths are more attenuated by thermal diffusion. We then use a parameterized thermal evolution model with a well-mixed mantle topped by two different lids characterized by their thermal structures and thicknesses to study the growth of the crust in the two hemispheres. Our results demonstrate that this positive feedback can generate a significant crustal dichotomy.

2. Marsquake Locations and 1-D Seismic Models for Mars From InSight Data

   Mélanie Drilleau, Henri Samuel, Raphaël F. Garcia, and 9 more authors

   Journal of Geophysical Research: Planets, 2022.

   abstract url

   We present inversions for the structure of Mars using the first Martian seismic record collected by the InSight lander. We identified and used arrival times of direct, multiples, and depth phases of body waves, for 17 marsquakes to constrain the quake locations and the one-dimensional average interior structure of Mars. We found the marsquake hypocenters to be shallower than 40 km depth, most of them being located in the Cerberus Fossae graben system, which could be a source of marsquakes. Our results show a significant velocity jump between the upper and the lower part of the crust, interpreted as the transition between intrusive and extrusive rocks. The lower crust makes up a significant fraction of the crust, with seismic velocities compatible with those of mafic to ultramafic rocks. Additional constraints on the crustal thickness from previous seismic analyses, combined with modeling relying on gravity and topography measurements, yield constraints on the present-day thermochemical state of Mars and on its long-term history. Our most constrained inversion results indicate a present-day surface heat flux of 22 \textpm 1 mW/m2, a relatively hot mantle (potential temperature: 1740 \textpm 90 K) and a thick lithosphere (540 \textpm 120 km), associated with a lithospheric thermal gradient of 1.9 \textpm 0.3 K/km. These results are compatible with recent seismic studies using a reduced data set and different inversion approaches, confirming that Mars’ potential mantle temperature was initially relatively cold (1780 \textpm 50 K) compared to that of its present-day state, and that its crust contains 10–12 times more heat-producing elements than the primitive mantle.

3. Distinguishing the Origin of Asteroid (16) Psyche

   Linda T. Elkins-Tanton, Erik Asphaug, James F. Bell Iii, and 19 more authors

   Space Science Reviews, 2022.

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   The asteroid (16) Psyche may be the metal-rich remnant of a differentiated planetesimal, or it may be a highly reduced, metal-rich asteroidal material that never differentiated. The NASA Psyche mission aims to determine Psyche’s provenance. Here we describe the possible solar system regions of origin for Psyche, prior to its likely implantation into the asteroid belt, the physical and chemical processes that can enrich metal in an asteroid, and possible meteoritic analogs. The spacecraft payload is designed to be able to discriminate among possible formation theories. The project will determine Psyche’s origin and formation by measuring any strong remanent magnetic fields, which would imply it was the core of a differentiated body; the scale of metal to silicate mixing will be determined by both the neutron spectrometers and the filtered images; the degree of disruption between metal and rock may be determined by the correlation of gravity with composition; some mineralogy (e.g., modeled silicate/metal ratio, and inferred existence of low-calcium pyroxene or olivine, for example) will be detected using filtered images; and the nickel content of Psyche’s metal phase will be measured using the GRNS.

4. Geodetic Investigations of the Mission Concept MAGIC to Reveal Callisto’s Internal Structure

   Antonio Genova, David E. Smith, Robin Canup, and 8 more authors

   Acta Astronautica, 2022.

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   Geodetic and geophysical investigations of the Galilean moon Callisto address fundamental questions regarding the formation and evolution of the Jovian system. Callisto’s evolution and internal structure appear to significantly differ from the other Jovian satellites. Similarly-sized Ganymede is a highly evolved ice-rock moon with a differentiated interior, intrinsic magnetic field, and abundant surface evidence of internal activity. In contrast, Callisto’s surface is ancient, and Galileo spacecraft data suggest its interior is only incompletely differentiated, despite the presumed presence of a sub-surface ocean. These properties make Callisto uniquely able to constrain the timing and nature of the Jovian system formation. The Magnetics, Altimetry, Gravity, and Imaging of Callisto (MAGIC) mission concept is conceived to fully characterize the properties of this enigmatic moon from its deep interior to the icy shell. Three main instruments are included as a scientific payload. Highly accurate measurements of Callisto’s topography, magnetic field, and morphology are obtained by the onboard laser altimeter, magnetometer, and camera, respectively. The telecommunication system supports an additional gravity and radio science investigation. Long- and short-wavelength gravity anomalies afford powerful constraints on internal differentiation and the properties of the hydrosphere (water and ice). Comprehensive numerical simulations and covariance analyses of MAGIC mission scenarios presented in this paper show that the gravitational degree-2 normalized coefficients and the pole obliquity enable the determination of the moment of inertia with an accuracy better than 0.015%. The combination of gravity and altimetry measurements acquired by MAGIC are essential to the characterization of Callisto’s interior if – as is likely – the degree-2 gravity includes non-hydrostatic terms. MAGIC’s radio science data yield the estimation of Callisto’s gravity field with spatial resolutions of <100 km. The combination of gravitational and deformation tides that are retrieved by the radio science and altimetry investigations, respectively, leads to the recovery of the rigid ice shell thickness to within ∼3 km. Together these datasets would resolve ambiguities inherent in Galileo flyby data, revealing Callisto’s interior structure as well as the existence and properties of its postulated internal ocean.

5. The Psyche Topography and Geomorphology Investigation

   Ralf Jaumann, James F. Bell, Carol A. Polanskey, and 16 more authors

   Space Science Reviews, 2022.

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   Detailed mapping of topography is crucial for the understanding of processes shaping the surfaces of planetary bodies. In particular, stereoscopic imagery makes a major contribution to topographic mapping and especially supports the geologic characterization of planetary surfaces. Image data provide the basis for extensive studies of the surface structure and morphology on local, regional and global scales using photogeologic information from images, the topographic information from stereo-derived digital terrain models and co-registered spectral terrain information from color images. The objective of the Psyche topography and geomorphology investigation is to derive the detailed shape of (16) Psyche to generate orthorectified image mosaics, which are needed to study the asteroids’ landforms, interior structure, and the processes that have modified the surface over geologic time. In this paper we describe our approaches for producing shape models, and our plans for acquiring requested image data to quantify the expected accuracy of the results. Multi-angle images obtained by Psyche’s camera will be used to create topographic models with about 15 m/pixel horizontal resolution and better than 10 m height accuracy on a global scale. This is slightly better as global imaging obtained during the Dawn mission, however, both missions yield resolutions of a few m/pixel locally. Two different techniques, stereophotogrammetry and stereophotoclinometry, are used to model the shape; these models will be merged with the gravity fields obtained by the Psyche spacecraft to produce geodetically controlled topographic models. The resulting digital topography models, together with the gravity data, will reveal the tectonic, volcanic, impact, and gradational history of Psyche, and enable co-registration of data sets to determine Psyche’s geologic history.

6. Correction to: The Psyche Topography and Geomorphology Investigation

   Ralf Jaumann, James F. Bell, Carol A. Polanskey, and 16 more authors

   Space Science Reviews, 2022.

   url
7. An Autonomous Lunar Geophysical Experiment Package (ALGEP) for Future Space Missions

   Taichi Kawamura, Matthias Grott, Raphael Garcia, and 36 more authors

   Experimental Astronomy, 2022.

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   Geophysical observations will provide key information about the inner structure of the planets and satellites and understanding the internal structure is a strong constraint on the bulk composition and thermal evolution of these bodies. Thus, geophysical observations are a key to uncovering the origin and evolution of the Moon. In this article, we propose the development of an autonomous lunar geophysical experiment package, composed of a suite of instruments and a central station with standardized interface, which can be installed on various future lunar missions. By fixing the interface between instruments and the central station, it would be possible to easily configure an appropriate experiment package for different missions. We describe here a series of geophysical instruments that may be included as part of the geophysical package: a seismometer, a magnetometer, a heat flow probe, and a laser reflector. These instruments will provide mechanical, thermal, and geodetic parameters of the Moon that are strongly related to the internal structure. We discuss the functionality required for future geophysical observations of the Moon, including the development of the central station that will be used commonly by different payloads.

8. 

   Surface Waves and Crustal Structure on Mars

   D. Kim, W B Banerdt, S Ceylan, and 42 more authors

   Science, 2022.

   abstract url

   We detected surface waves from two meteorite impacts on Mars. By measuring group velocity dispersion along the impact-lander path, we obtained a direct constraint on crustal structure away from the InSight lander. The crust north of the equatorial dichotomy had a shear wave velocity of approximately 3.2 kilometers per second in the 5- to 30-kilometer depth range, with little depth variation. This implies a higher crustal density than inferred beneath the lander, suggesting either compositional differences or reduced porosity in the volcanic areas traversed by the surface waves. The lower velocities and the crustal layering observed beneath the landing site down to a 10-kilometer depth are not a global feature. Structural variations revealed by surface waves hold implications for models of the formation and thickness of the martian crust.

9. 

   Lithospheric Structure of Venusian Crustal Plateaus

   J. S. Maia, and M. A. Wieczorek

   Journal of Geophysical Research: Planets, 2022.

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   Crustal plateaus are Venusian highlands characterized by tectonized terrains. It is commonly interpreted that their topography is isostatically supported and that they represent fossils of an extinct tectonic regime. Using gravity and topography we perform a comprehensive investigation of the lithospheric structure of six crustal plateaus. We computed the admittance (gravity to topography wavelength-dependent ratio) for each region and compared them to modeled admittances. Three compensation scenarios were tested: Airy isostasy, a surface-loading flexural model, and a flexural model with surface and subsurface loads. Our results show that the topography of most plateaus is supported by crustal thickening and that the addition of a mantle support component is not necessary at the investigated wavelengths. The elastic thickness was constrained to be less than 35 km with a best-fitting average of 15 km, confirming that these regions are consistent with an isostatic regime. The average crustal thickness of the plateaus ranges from 15 to 34 km, and if they are in Airy isostasy, this implies that the global average crustal thickness of Venus is about 20 km. Phoebe Regio is the sole exception of our analysis in that crustal thicknesses that are compatible with the other plateaus are obtained only when a buoyant layer is included. Heat flow estimations computed from the elastic thickness indicate that the plateaus formed under higher heat flow conditions compared to the current global average and could have caused localized melting. Present-day heat flow predictions suggest that eclogitization could occur where the crust is thickest.

10. Interior Dynamics and Thermal Evolution of Mars – a Geodynamic Perspective

    Ana-Catalina Plesa, Mark Wieczorek, Martin Knapmeyer, and 3 more authors

    Advances in Geophysics, 2022.

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    Over the past decades, global geodynamic models have been used to investigate the thermal evolution of terrestrial planets. With the increase of computational power and improvement of numerical techniques, these models have become more complex, and simulations are now able to use a high resolution 3D spherical shell geometry and to account for strongly varying viscosity, as appropriate for mantle materials. In this study we review global 3D geodynamic models that have been used to study the thermal evolution and interior dynamics of Mars. We discuss how these models can be combined with local and global observations to constrain the planet’s thermal history. In particular, we use the recent InSight estimates of the crustal thickness, upper mantle structure, and core size to show how these constraints can be combined with 3D geodynamic models to improve our understanding of the interior dynamics, present-day thermal state and temperature variations in the interior of Mars. Our results show that the crustal thickness variations control the surface heat flow and the elastic thickness pattern, as well as the location of melting zones in the present-day martian mantle. The lithospheric temperature and the seismic velocities pattern in the shallow mantle reflect the crustal thickness pattern. The large size of the martian core leads to a smaller scale convection pattern in the mantle than previously suggested. Strong mantle plumes that produce melt up to recent times become focused in Tharsis and Elysium, while weaker plumes are distributed throughout the mantle. The thickness of the seismogenic layer, where seismic events can occur, can be used to discriminate between geodynamic models, if the source depth and location of seismic events is known. Furthermore model predictions of present-day martian seismicity can be compared to the values measured by InSight. Future models need to consider recent estimates from the present-day elastic lithosphere thickness at the north pole of Mars, the effects of lateral variations of seismic velocities on waves propagation through the mantle and lithosphere, and to test the spatial distribution of seismicity by comparing model predictions to observations.

11. 

    InSight Constraints on the Global Character of the Martian Crust

    Mark A. Wieczorek, Adrien Broquet, Scott M. McLennan, and 23 more authors

    Journal of Geophysical Research: Planets, 2022.

    abstract url

    Analyses of seismic data from the InSight mission have provided the first in situ constraints on the thickness of the crust of Mars. These crustal thickness constraints are currently limited to beneath the lander that is located in the northern lowlands, and we use gravity and topography data to construct global crustal thickness models that satisfy the seismic data. These models consider a range of possible mantle and core density profiles, a range of crustal densities, a low-density surface layer, and the possibility that the crustal density of the northern lowlands is greater than that of the southern highlands. Using the preferred InSight three-layer seismic model of the crust, the average crustal thickness of the planet is found to lie between 30 and 72 km. Depending on the choice of the upper mantle density, the maximum permissible density of the northern lowlands and southern highlands crust is constrained to be between 2,850 and 3,100 kg m\mkern1mu-3. These crustal densities are lower than typical Martian basaltic materials and are consistent with a crust that is on average more felsic than the materials found at the surface. We argue that a substantial portion of the crust of Mars is a primary crust that formed during the initial differentiation of the planet. Various hypotheses for the origin of the observed intracrustal seisimic layers are assessed, with our preferred interpretation including thick volcanic deposits, ejecta from the Utopia basin, porosity closure, and differentiation products of a Borealis impact melt sheet.

12. The Psyche Gravity Investigation

    Maria T. Zuber, Ryan S. Park, Linda T. Elkins-Tanton, and 13 more authors

    Space Science Reviews, 2022.

    abstract url

    The objective of the NASA Psyche mission gravity science investigation is to map the mass distribution within asteroid (16) Psyche to elucidate interior structure and to resolve the question of whether this metal-rich asteroid represents a remnant metal core or whether it is a primordial body that never melted. Measurements of gravity will be obtained via the X-band telecommunication system on the Psyche spacecraft, collected from progressively lower mapping altitudes. Orbital gravity will allow an estimate of GM to better than 0.001 km(3) s(-2). A spherical harmonic model of gravity to degree and order 10 will be achievable and, in concert with spherical harmonic data sets from topography and magnetometry, as well as surface composition data, will provide information regarding the spatial and radial distribution of mass that will be used to constrain the origin and evolution of (16) Psyche.

2021

1. Impacts on the Moon: Analysis Methods and Size Distribution of Impactors

   Chrysa Avdellidou, Edhah Munaibari, Raven Larson, and 6 more authors

   Planetary and Space Science, 2021.

   abstract url

   We are preparing a telescope system to carry out a survey of detection and analysis of lunar impact flashes. In the framework of this project, here we present all necessary methods to automatically identify these luminous events, their lunar impact coordinates, the origin of the impacting meteoroids, as well as the estimation of their physical properties such as mass and size. We tested our methods against confirmed impact events and constructed the meteoroid size frequency distribution of impactors using literature data of the last 20 years. In addition, we present the first lunar impact event observed from the Observatoire de la Côte d’Azur that was detected during the testing phase of our project.

2. The Composition of the South Polar Cap of Mars Derived From Orbital Data

   A. Broquet, M. A. Wieczorek, and W. Fa

   Journal of Geophysical Research: Planets, 2021.

   abstract url

   The flexure of the lithosphere under stresses imposed by the geologically young south polar cap is one of the few clues we have regarding the south polar cap composition and the present-day thermal state of Mars. Here, we combine radar, gravity, and topography data with a flexural loading model to estimate the bulk density () and average real dielectric constant ( ) of the south polar cap, and the elastic thickness of the lithosphere (Te). Given the uncertainties of the data, our results constrain  to be 1,100–1,300 kg m3 (best fit of 1,220 kg m3),   to be 2.5–3.4 (best fit of 3.3), and Te to be greater than 150 km (best fit of 360 km). Based on these results, the maximum lithospheric flexure is 770 m, and the polar cap volume could be up to 26% larger than previous estimates that did not account for lithospheric flexure. Our inferred compositions imply that the dust concentration would be at least 9 vol% if the CO2 ice content were negligible, and that the CO2 ice concentration would be more than the known 1 vol% CO2 if the dust concentration were less than 9 vol%. The 1- lower limit on Te implies a surface heat flow that is less than 23.5 mW m2. This lower limit is significantly less than the range of acceptable values at the north pole (330–450 km, heat flow of 11–16 mW m2), and helps satisfy global thermal evolution simulations that predict hemispheric differences in surface heat flow.

3. Evidence for Differentiation of the Most Primitive Small Bodies

   B. Carry, P. Vernazza, F. Vachier, and 41 more authors

   Astronomy & Astrophysics, 2021.

   abstract url

   \emphContext. Dynamical models of Solar System evolution have suggested that the so-called P- and D-type volatile-rich asteroids formed in the outer Solar System beyond Neptune’s orbit and may be genetically related to the Jupiter Trojans, comets, and small Kuiper belt objects (KBOs). Indeed, the spectral properties of P- and D-type asteroids resemble that of anhydrous cometary dust.\emphAims. We aim to gain insights into the above classes of bodies by characterizing the internal structure of a large P- and D-type asteroid.\emphMethods. We report high-angular-resolution imaging observations of the P-type asteroid (87) Sylvia with the Very Large Telescope Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE) instrument. These images were used to reconstruct the 3D shape of Sylvia. Our images together with those obtained in the past with large ground-based telescopes were used to study the dynamics of its two satellites. We also modeled Sylvia’s thermal evolution.\emphResults. The shape of Sylvia appears flattened and elongated (a/b ~1.45; a/c ~1.84). We derive a volume-equivalent diameter of 271 \textpm 5 km and a low density of 1378 \textpm 45 kg m\textsuperscript-3. The two satellites orbit Sylvia on circular, equatorial orbits. The oblateness of Sylvia should imply a detectable nodal precession which contrasts with the fully-Keplerian dynamics of its two satellites. This reveals an inhomogeneous internal structure, suggesting that Sylvia is differentiated.\emphConclusions. Sylvia’s low density and differentiated interior can be explained by partial melting and mass redistribution through water percolation. The outer shell should be composed of material similar to interplanetary dust particles (IDPs) and the core should be similar to aqueously altered IDPs or carbonaceous chondrite meteorites such as the Tagish Lake meteorite. Numerical simulations of the thermal evolution of Sylvia show that for a body of such a size, partial melting was unavoidable due to the decay of long-lived radionuclides. In addition, we show that bodies as small as 130–150 km in diameter should have followed a similar thermal evolution, while smaller objects, such as comets and the KBO Arrokoth, must have remained pristine, which is in agreement with in situ observations of these bodies. NASA Lucy mission target (617) Patroclus (diameter ≈140 km) may, however, be differentiated.

4. Depth of Martian Magnetization From Localized Power Spectrum Analysis

   Shengxia Gong, and Mark Wieczorek

   Journal of Geophysical Research: Planets, 2021.

   abstract url

   Magnetic field measurements show that Mars possesses strong crustal magnetic anomalies that formed when the planet had an active dynamo. To investigate the origin of this magnetization, we used localized power spectrum analyses to constrain the equivalent depths of thin magnetic layers in the crust. Using a new martian magnetic field model that incorporates data from both the Mars Atmosphere and Volatile EvolutioN and Mars Global Surveyor missions, we found that the equivalent magnetization depths on Mars vary from the surface to 72 km. In the northern hemisphere the magnetization depths are found to be on average 9 km, whereas in the southern hemisphere the depths are on average 32 km. If these depths are interpreted in terms of a thick magnetic layer, magnetization in the northern lowlands could extend from the surface to about 18 km depth, whereas for the southern highlands the magnetic layer could extend from about 20 km depth to the base of the crust. The strongest magnetic anomalies are, in general, associated with deep source depths. Our results are consistent with the hypothesis where strong primordial magnetic materials of the northern hemisphere were excavated from the Borealis impact basin, leaving only strong deep remanent magnetization in the southern hemisphere.

5. A New Large-Scale Map of the Lunar Crustal Magnetic Field and Its Interpretation

   Lon L. Hood, Cecilyn B. Torres, Joana S. Oliveira, and 2 more authors

   Journal of Geophysical Research: Planets, 2021.

   abstract url

   A new large-scale map of the lunar crustal magnetic field at 30 km altitude covering latitudes from 65^∘S to 65^∘N has been produced using high-quality vector magnetometer data from two complementary polar orbital missions, Lunar Prospector and SELENE (Kaguya). The map has characteristics similar to those of previous maps but better resolves the shapes and distribution of weaker anomalies. The strongest group of anomalies is located on the northwest side of the South Pole-Aitken basin approximately antipodal to the Imbrium basin. On the near side, both strong isolated anomalies and weaker elongated anomalies tend to lie along lines oriented radial to Imbrium. These include named anomalies such as Reiner Gamma, Hartwig, Descartes, Abel, and Airy. The statistical significance of this tendency for elongated anomalies is verified by Monte Carlo simulations. Great circle paths determined by end points of elongated anomaly groups and the locations of five individual strong anomalies converge within the inner rim of Imbrium and intersect within the Imbrium antipode zone. Statistically significant evidence for similar alignments northwest of the Orientale basin is also found. The observed distribution of anomalies on the near side and the location of the strongest anomaly group antipodal to Imbrium are consistent with the hypothesis that iron from the Imbrium impactor was mixed into ejecta that was inhomogeneously deposited downrange in groups aligned radial to the basin and concentrated antipodal to the basin.

6. Magnetic Anomalies in Five Lunar Impact Basins: Implications for Impactor Trajectories and Inverse Modeling

   L. L. Hood, J. S. Oliveira, J. Andrews-Hanna, and 2 more authors

   Journal of Geophysical Research: Planets, 2021.

   abstract url

   A recent large-scale map of the lunar crustal magnetic field is examined for the existence of magnetic anomalies internal to ringed impact basins. It is found that, of 25 basins with upper preNectarian and younger ages, 18 contain mapped internal anomalies with amplitudes of at least 1 nT at 30 km altitude. Of these, five are most confidently judged to contain intrinsic anomalies (i.e., anomalies located within the inner basin rims and originating at the times of basin formation): Crisium, Humboldtianum, Mendel-Rydberg, Moscoviense, and Nectaris. Comparing the anomaly distributions with previous numerical simulations of the impact of iron-rich planetesimals to form a large (SPA-sized) basin, inferences are drawn about the likely trajectories of the impactors. Specifically, results suggest that impactor trajectories for these basins were within ∼45^∘ of being vertical and tended to lie on average parallel to the lunar equatorial plane and the ecliptic plane. Inverse modeling of anomalies within these basins yields inferred directions of magnetization that are difficult to reconcile with the axial centered dipole hypothesis for the geometry of the internal lunar dynamo field: Paleomagnetic pole positions are widely scattered and, in agreement with a recent independent study, the two main anomalies within Crisium yield significantly different directions of magnetization.

7. Improving Constraints on Planetary Interiors With PPs Receiver Functions

   D. Kim, V. Lekic, J. C. E. Irving, and 14 more authors

   Journal of Geophysical Research Planets, 2021.

   abstract url

   Seismological constraints obtained from receiver function (RF) analysis provide important information about the crust and mantle structure. Here, we explore the utility of the free-surface multiple of the P-wave (PP) and the corresponding conversions in RF analysis. Using earthquake records, we demonstrate the efficacy of PPs-RFs before illustrating how they become especially useful when limited data is available in typical planetary missions. Using a transdimensional hierarchical Bayesian deconvolution approach, we compute robust P-to-S (Ps)- and PPs-RFs with InSight recordings of five marsquakes. Our Ps-RF results verify the direct Ps converted phases reported by previous RF analyses with increased coherence and reveal other phases including the primary multiple reverberating within the uppermost layer of the Martian crust. Unlike the Ps-RFs, our PPs-RFs lack an arrival at 7.2 s lag time. Whereas Ps-RFs on Mars could be equally well fit by a two- or three-layer crust, synthetic modeling shows that the disappearance of the 7.2 s phase requires a three-layer crust, and is highly sensitive to velocity and thickness of intra-crustal layers. We show that a three-layer crust is also preferred by S-to-P (Sp)-RFs. While the deepest interface of the three-layer crust represents the crust-mantle interface beneath the InSight landing site, the other two interfaces at shallower depths could represent a sharp transition between either fractured and unfractured materials or thick basaltic flows and pre-existing crustal materials. PPs-RFs can provide complementary constraints and maximize the extraction of information about crustal structure in data-constrained circumstances such as planetary missions.

8. Thickness and Structure of the Martian Crust from InSight Seismic Data

   Brigitte Knapmeyer-Endrun, Mark P. Panning, Felix Bissig, and 36 more authors

   Science, 2021.

   abstract url

   A planet’s crust bears witness to the history of planetary formation and evolution, but for Mars, no absolute measurement of crustal thickness has been available. Here, we determine the structure of the crust beneath the InSight landing site on Mars using both marsquake recordings and the ambient wavefield. By analyzing seismic phases that are reflected and converted at subsurface interfaces, we find that the observations are consistent with models with at least two and possibly three interfaces. If the second interface is the boundary of the crust, the thickness is 20 \textpm 5 kilometers, whereas if the third interface is the boundary, the thickness is 39 \textpm 8 kilometers. Global maps of gravity and topography allow extrapolation of this point measurement to the whole planet, showing that the average thickness of the martian crust lies between 24 and 72 kilometers. Independent bulk composition and geodynamic constraints show that the thicker model is consistent with the abundances of crustal heat-producing elements observed for the shallow surface, whereas the thinner model requires greater concentration at depth.

9. Large Impact Cratering during Lunar Magma Ocean Solidification

   K. Miljković, M. A. Wieczorek, M. Laneuville, and 3 more authors

   Nature Communications, 2021.

   abstract url

   Abstract The lunar cratering record is used to constrain the bombardment history of both the Earth and the Moon. However, it is suggested from different perspectives, including impact crater dating, asteroid dynamics, lunar samples, impact basin-forming simulations, and lunar evolution modelling, that the Moon could be missing evidence of its earliest cratering record. Here we report that impact basins formed during the lunar magma ocean solidification should have produced different crater morphologies in comparison to later epochs. A low viscosity layer, mimicking a melt layer, between the crust and mantle could cause the entire impact basin size range to be susceptible to immediate and extreme crustal relaxation forming almost unidentifiable topographic and crustal thickness signatures. Lunar basins formed while the lunar magma ocean was still solidifying may escape detection, which is agreeing with studies that suggest a higher impact flux than previously thought in the earliest epoch of Earth-Moon evolution.

10. Seismic Velocity Variations in a 3D Martian Mantle: Implications for the InSight Measurements

    A.-C. Plesa, E. Bozdağ, A. Rivoldini, and 11 more authors

    Journal of Geophysical Research: Planets, 2021.

    abstract url

    We use a large data set of 3D thermal evolution models to predict the distribution of present-day seismic velocities in the Martian interior. Our models show a difference between maximum and minimum S wave velocity of up to 10% either below the crust, where thermal variations are largest, or at the depth of the olivine to wadsleyite phase transition, located at around 1,000–1,200 km depth. Models with thick lithospheres on average have weak low-velocity zones that extend deeper than 400 km and seismic velocity variations in the uppermost 400–600 km that closely follow the crustal thickness pattern. For these cases, the crust contains more than half of the total amount of heat-producing elements. Models with limited crustal heat production have thinner lithospheres and shallower but prominent low-velocity zones that are incompatible with Interior exploration using Seismic Investigations, Geodesy and Heat Transport (InSight) observations. Seismic events suggested to originate in Cerberus Fossae indicate the absence of S wave shadow zones in 25^∘–30^∘ epicentral distance. This result is compatible with previous best fit models that require a large average lithospheric thickness and a crust containing more than half of the bulk amount of heat-producing elements to be compatible with geological and geophysical constraints. Ongoing and future InSight measurements that will determine the existence of a weak low-velocity zone will directly bear on the crustal heat production.

11. Seismic Efficiency for Simple Crater Formation in the Martian Top Crust Analog

    A. Rajšić, K. Miljković, G. S. Collins, and 4 more authors

    Journal of Geophysical Research: Planets, 2021.

    abstract url

    The first seismometer operating on the surface of another planet was deployed by the NASA InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) mission to Mars. It gives us an opportunity to investigate the seismicity of Mars, including any seismic activity caused by small meteorite bombardment. Detectability of impact generated seismic signals is closely related to the seismic efficiency, defined as the fraction of the impactor’s kinetic energy transferred into the seismic energy in a target medium. This work investigated the seismic efficiency of the Martian near surface associated with small meteorite impacts on Mars. We used the iSALE-2D (Impact-Simplified Arbitrary Lagrangian Eulerian) shock physics code to simulate the formation of the meter-size impact craters, and we used a recently formed 1.5 m diameter crater as a case study. The Martian crust was simulated as unfractured nonporous bedrock, fractured bedrock with 25% porosity, and highly porous regolith with 44% and 65% porosity. We used appropriate strength and porosity models defined in previous works, and we identified that the seismic efficiency is very sensitive to the speed of sound and elastic threshold in the target medium. We constrained the value of the impact-related seismic efficiency to be between the order of ∼10-7 to 10-6 for the regolith and ∼10-4 to 10-3 for the bedrock. For new impacts occurring on Mars, this work can help understand the near-surface properties of the Martian crust, and it contributes to the understanding of impact detectability via seismic signals as a function of the target media.

12. Numerical Simulations of the Apollo S-IVB Artificial Impacts on the Moon

    A. Rajsic, K. Miljkovic, N. Wojcicka, and 6 more authors

    Earth and Space Science, 2021.

    abstract url

    The third stage of the Saturn IV rocket used in the five Apollo missions made craters on the Moon similar to 30 m in diameter. Their initial impact conditions were known, so they can be considered controlled impacts. Here, we used the iSALE-2D shock physics code to numerically simulate the formation of these craters, and to calculate the vertical component of seismic moment (similar to 4 x 10(10) Nm) and seismic efficiency (similar to 10(-6)) associated with these impacts. The irregular booster shape likely caused the irregular crater morphology observed. To investigate this, we modeled six projectile geometries, with footprint area between 3 and 105 m(2), keeping the mass and velocity of the impactor constant. We showed that the crater depth and diameter decreased as the footprint area increased. The central mound observed in lunar impact sites could be a result of layering of the target and/or low density of the projectile. Understanding seismic signatures from impact events is important for planetary seismology. Calculating seismic parameters and validating them against controlled experiments in a planetary setting will help us understand the seismic data received, not only from the Moon, but also from the InSight Mission on Mars and future seismic missions. Plain Language Summary Observations of meteoroid strikes on the Moon, including artificial impacts made by the Saturn boosters from the Apollo missions, present valuable information for connecting impact conditions with seismic properties of the lunar and planetary crusts. These artificial impacts on the Moon were made by irregularly shaped spacecraft with low density. We numerically simulated these artificial impacts, using several different projectile geometries in order to represent the spacecraft’s low density and shape. We found that the projectile representation affected the crater size. We calculated the amount of energy transferred into seismic waves and seismic moment created in these impacts to be consistent with previous studies. These parameters did not change with different projectile representations, but were affected by the material properties of the impact site. The aim is to use the results from the controlled lunar impacts to help understand the seismic signatures of impacts on Mars.

13. Ctplanet: Version 0.2.1

    M. A. Wieczorek

    Zenodo, 2021.

    url

2020

1. The Science Mission of SpaceIL’s Beresheet Lander

   Oded Aharonson, Christopher T. Russell, James W. Head, and 12 more authors

   Planetary and Space Science, 2020.

   abstract url

   SpaceIL’s lunar lander mission Beresheet was launched on February 22, 2019 and impacted its targeted landing site in Mare Serenitatis on the Moon on April 11, 2019. The spacecraft carried a package of scientific instruments including a fluxgate magnetometer and a retroreflector array for laser ranging, as well as a suite of cameras. Orbital measurements of the magnetic field from Kaguya and Lunar Prospector guided the selection of the landing site to a location west of Posidonius crater in the Serenitatis plains, where the magnitude of the modeled magnetic field reaches 8–10 nT at the surface. Data was collected by the SILMAG magnetometer from Earth orbit, lunar orbit, and during the descent maneuver, although its interpretation is hindered by the presence of a spacecraft field.

2. Initial Results from the InSight Mission on Mars

   W. Bruce Banerdt, S. E. Smrekar, D. Banfield, and 64 more authors

   Nature Geoscience, 2020.

   abstract url

   NASA’s InSight (Interior exploration using Seismic Investigations, Geodesy and Heat Transport) mission landed in Elysium Planitia on Mars on 26 November 2018. It aims to determine the interior structure, composition and thermal state of Mars, as well as constrain present-day seismicity and impact cratering rates. Such information is key to understanding the differentiation and subsequent thermal evolution of Mars, and thus the forces that shape the planet’s surface geology and volatile processes. Here we report an overview of the first ten months of geophysical observations by InSight. As of 30 September 2019, 174 seismic events have been recorded by the lander’s seismometer, including over 20 events of moment magnitude Mw = 3–4. The detections thus far are consistent with tectonic origins, with no impact-induced seismicity yet observed, and indicate a seismically active planet. An assessment of these detections suggests that the frequency of global seismic events below approximately Mw = 3 is similar to that of terrestrial intraplate seismic activity, but there are fewer larger quakes; no quakes exceeding Mw = 4 have been observed. The lander’s other instruments—two cameras, atmospheric pressure, temperature and wind sensors, a magnetometer and a radiometer—have yielded much more than the intended supporting data for seismometer noise characterization: magnetic field measurements indicate a local magnetic field that is ten-times stronger than orbital estimates and meteorological measurements reveal a more dynamic atmosphere than expected, hosting baroclinic and gravity waves and convective vortices. With the mission due to last for an entire Martian year or longer, these results will be built on by further measurements by the InSight lander.

3. Flexure of the Lithosphere Beneath the North Polar Cap of Mars: Implications for Ice Composition and Heat Flow

   A. Broquet, M. A. Wieczorek, and W. Fa

   Geophysical Research Letters, 2020.

   abstract url

   The geodynamical response of the lithosphere under stresses imposed by the geologically young north polar cap is one of the few clues we have to constrain both the polar cap composition and the present-day thermal state of Mars. Here we combine radar data with a flexural loading model to self-consistently estimate the density (ρ) and real dielectric constant (E\prime) of the polar cap, and the elastic thickness of the lithosphere underneath (Te). Our results show that ρ ranges from 920 to 1,520 kg m-3, E\prime is 2.75 (+0.40, -0.35), and Te is between 330 and 450 km. We determine a polar cap volume that is up to 30% larger than current estimates that all neglect lithospheric flexure. Our inferred compositions suggest that, for dust content larger than about 6 vol%, 10 vol% CO2 ice is mixed within the polar deposits, which has important implications for the climate evolution of Mars.

4. Observations, Meteorites, and Models: A Preflight Assessment of the Composition and Formation of (16) Psyche

   L. T. Elkins-Tanton, E. Asphaug, J. F. Bell, and 19 more authors

   Journal of Geophysical Research: Planets, 2020.

   abstract url

   Some years ago, the consensus was that asteroid (16) Psyche was almost entirely metal. New data on density, radar properties, and spectral signatures indicate that the asteroid is something perhaps even more enigmatic: a mixed metal and silicate world. Here we combine observations of Psyche with data from meteorites and models for planetesimal formation to produce the best current hypotheses for Psyche’s properties and provenance. Psyche’s bulk density appears to be between 3,400 and 4,100 kg m-3. Psyche is thus predicted to have between ~30 and ~60 vol% metal, with the remainder likely low-iron silicate rock and not more than ~20% porosity. Though their density is similar, mesosiderites are an unlikely analog to bulk Psyche because mesosiderites have far more iron-rich silicates than Psyche appears to have. CB chondrites match both Psyche’s density and spectral properties, as can some pallasites, although typical pallasitic olivine contains too much iron to be consistent with the reflectance spectra. Final answers, as well as resolution of contradictions in the data set of Psyche physical properties, for example, the thermal inertia measurements, may not be resolved until the NASA Psyche mission arrives in orbit at the asteroid. Despite the range of compositions and formation processes for Psyche allowed by the current data, the science payload of the Psyche mission (magnetometers, multispectral imagers, neutron spectrometer, and a gamma-ray spectrometer) will produce data sets that distinguish among the models. Plain Language Summary Since the NASA mission to asteroid (16) Psyche was selected, interest from the public and from the scientific community in the asteroid has risen considerably. New observations of the asteroid’s physical properties indicate a different composition than earlier data had shown. A decade ago, much of the community thought the asteroid was 90% metal on its surface. There are still contradictions in the compilation of all current data, but the best analysis indicates that Psyche’s density is between 3,400 and 4,100 kg m-3, indicating it is a mixture of rock with between 30 and 60 vol% metal.

5. The Seismicity of Mars

   D. Giardini, P. Lognonné, W. B. Banerdt, and 60 more authors

   Nature Geoscience, 2020.

   abstract url

   The InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) mission landed in Elysium Planitia on Mars on 26 November 2018 and fully deployed its seismometer by the end of February 2019. The mission aims to detect, characterize and locate seismic activity on Mars, and to further constrain the internal structure, composition and dynamics of the planet. Here, we present seismometer data recorded until 30 September 2019, which reveal that Mars is seismically active. We identify 174 marsquakes, comprising two distinct populations: 150 small-magnitude, high-frequency events with waves propagating at crustal depths and 24 low-frequency, subcrustal events of magnitude Mw 3–4 with waves propagating at various depths in the mantle. These marsquakes have spectral characteristics similar to the seismicity observed on the Earth and Moon. We determine that two of the largest detected marsquakes were located near the Cerberus Fossae fracture system. From the recorded seismicity, we constrain attenuation in the crust and mantle, and find indications of a potential low-S-wave-velocity layer in the upper mantle.

6. Is the Lunar Magnetic Field Correlated With Gravity or Topography?

   Shengxia Gong, and Mark A. Wieczorek

   Journal of Geophysical Research: Planets, 2020.

   abstract url

   Magnetic field measurements made from orbit show that there are strong magnetic anomalies on the Moon, but many of these show no clear correlation with known geological processes. Given that the primary magnetic carrier on the Moon is metallic iron, which is considerably denser than the silicate minerals that make up the crust, we might expect that there would be a correlation between the magnetic field and gravity. If the magnetic anomaly were related to iron-rich impact ejecta, there might also be a correlation between the magnetic field and topography. We use magnetic field, topography, and gravity data to test whether such correlations exist. Our results demonstrate that some magnetic anomalies show statistically significant positive correlations with free-air gravity and topography, and the magnetic sources within these regions could potentially be iron-rich impact ejecta. In a few cases, the magnetic anomalies show statistically significant positive correlations with Bouguer gravity, implying that the magnetic anomalies are associated with density anomalies within either the crust or upper mantle. The origin of the vast remainder of lunar magnetic anomalies remains enigmatic.

7. High-Resolution Gravity Field Models from GRAIL Data and Implications for Models of the Density Structure of the Moon’s Crust

   S. Goossens, T. J. Sabaka, M. A. Wieczorek, and 6 more authors

   Journal of Geophysical Research: Planets, 2020.

   abstract url

   We present our latest high-resolution lunar gravity field model of degree and order 1200 in spherical harmonics using Gravity Recovery and Interior Laboratory (GRAIL) data. In addition to a model with the standard spectral Kaula regularization constraint, we determine models by applying a constraint based on topography called rank-minus-one (RM1). The new models using this RM1 constraint have high correlations with topography over the entire degree range by design. The RM1 models allow the determination of apparent crustal densities at all spatial scales (called effective density) covered by the model, whereas the Kaula-constrained model can only be used globally up to spherical harmonic degree 700. We find that the effective density spectrum has a smaller slope for the high degrees when compared to the medium degrees. We interpret this as indicative of a global average surface density, as opposed to an ever-decreasing effective density as one approaches the surface. We use the RM1 models to derive maps of lateral and vertical density variations in the lunar crust. These models allow us to increase the resolution of this analysis compared to previous studies, by increasing the degree range over which to fit theoretical models of vertical density variations, and by decreasing the size of the spherical caps used in a localized analysis. Several regions on the Moon, such as South Pole-Aitken and Mare Orientale, are distinct from their surroundings in terms of surface densities. The RM1 models are especially valuable in (localized) spectral studies of the structure of the lunar crust.

8. Constraints on Thermal History of Mars From Depth of Pore Closure Below InSight

   Szilárd Gyalay, Francis Nimmo, Ana-Catalina Plesa, and 1 more author

   Geophysical Research Letters, 2020.

   abstract url

   Planetary crusts undergo viscous closure of pores at depth; if the thickness of this porous layer can be measured, constraints on crustal thermal evolution can be derived. We apply a pore closure model developed for the Moon to Mars and take into account the geological processes that may alter the depth of this transition region. If the 8–11 km deep discontinuity in seismic wave speed detected by the InSight lander marks the base of the porous layer, the heat flux at the time the porosity was created must have exceed 60 mW m-2, probably indicating a time prior to 4 Ga.

9. Crustal and Time-Varying Magnetic Fields at the InSight Landing Site on Mars

   Catherine L. Johnson, Anna Mittelholz, Benoit Langlais, and 21 more authors

   Nature Geoscience, 2020.

   abstract url

   Magnetic fields provide a window into a planet’s interior structure and evolution, including its atmospheric and space environments. Satellites at Mars have measured crustal magnetic fields indicating an ancient dynamo. These crustal fields interact with the solar wind to generate transient fields and electric currents in Mars’s upper atmosphere. Surface magnetic field data play a key role in understanding these effects and the dynamo. Here we report measurements of magnetic field strength and direction at the InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) landing site on Mars. We find that the field is ten times stronger than predicted by satellite-based models. We infer magnetized rocks beneath the surface, within ~150 km of the landing site, consistent with a past dynamo with Earth-like strength. Geological mapping and InSight seismic data suggest that much or all of the magnetization sources are carried in basement rocks, which are at least 3.9 billion years old and are overlain by between 200 m and ~10 km of lava flows and modified ancient terrain. Daily variations in the magnetic field indicate contributions from ionospheric currents at 120 km to 180 km altitude. Higher-frequency variations are also observed; their origin is unknown, but they probably propagate from even higher altitudes to the surface. We propose that the time-varying fields can be used to investigate the electrical conductivity structure of the martian interior.

10. Impact Cratering Rate Consistency Test from Ages of Layered Ejecta on Mars

    Anthony Lagain, Sylvain Bouley, David Baratoux, and 2 more authors

    Planetary and Space Science, 2020.

    abstract url

    Ages of geological units of planetary bodies are determined from impact crater counts on their surface. These ages are model-dependent, and several models largely used in the community assume a constant production function and a constant cratering rate over the last 3 Ga. We have mapped the population of small impact craters (>200 m in diameter) formed over a population of large impact craters (>5 km in diameter) with layered ejecta on Acidalia Planitia, Mars. We have deduced the age of each large impact crater under the assumption of a constant impact rate and constant production function. The impact rate inferred from this set of ages is, however, not constant and show a significant increasing during the last ~1 Ga compared to chronology models commonly used. We interpret this inconsistency as an evidence for temporal variations in the size-frequency distribution (SFD) of impactors in the main belt, consistent with recent studies argued for a late increasing of the large impactor flux on Earth and the Moon.

11. Constraints on the Shallow Elastic and Anelastic Structure of Mars from InSight Seismic Data

    P. Lognonné, W. B. Banerdt, W. T. Pike, and 106 more authors

    Nature Geoscience, 2020.

    abstract url

    Mars’s seismic activity and noise have been monitored since January 2019 by the seismometer of the InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) lander. At night, Mars is extremely quiet; seismic noise is about 500 times lower than Earth’s microseismic noise at periods between 4 s and 30 s. The recorded seismic noise increases during the day due to ground deformations induced by convective atmospheric vortices and ground-transferred wind-generated lander noise. Here we constrain properties of the crust beneath InSight, using signals from atmospheric vortices and from the hammering of InSight’s Heat Flow and Physical Properties (HP3) instrument, as well as the three largest Marsquakes detected as of September 2019. From receiver function analysis, we infer that the uppermost 8–11 km of the crust is highly altered and/or fractured. We measure the crustal diffusivity and intrinsic attenuation using multiscattering analysis and find that seismic attenuation is about three times larger than on the Moon, which suggests that the crust contains small amounts of volatiles.

12. Crustal Porosity of Lunar Impact Basins

    D. Wahl, M. A. Wieczorek, K. Wünnemann, and 1 more author

    Journal of Geophysical Research: Planets, 2020.

    abstract url

    Lateral variations in bulk density and porosity of the upper lunar highland crust are mapped using a high-resolution Gravity Recovery and Interior Laboratory (GRAIL) gravity field model and Lunar Reconnaissance Orbiter (LRO) derived topography. With a higher spatial resolution gravity model than previous studies, we focus on individual impact basins with diameters greater than 200 km. The bulk density of the upper few kilometers of the lunar crust is estimated by minimizing the correlation between the topography and Bouguer gravity at short wavelengths that are unaffected by lithospheric flexure. Porosity is then derived using estimates of the grain density obtained from remote sensing data of the surface composition. The near surface crust in proximity to many large basins is found to exhibit distinct radial porosity signatures. Low porosities are found in the basin centers within the peak ring, whereas high porosities are identified near and just exterior to the main rim. The larger basins exhibit a more pronounced porosity signature than the smaller basins. Though the number of basins investigated in this study is limited, younger basins appear to be associated with the largest amplitude variations in porosity. For basins with increasing age the magnitude of the porosity variations decreases.

2019

1. The Gravitational Signature of Martian Volcanoes

   A. Broquet, and M. A. Wieczorek

   Journal of Geophysical Research: Planets, 2019.

   abstract url

   By modeling the elastic flexure of the Martian lithosphere under imposed loads, we provide a systematic study of the old and low-relief volcanoes (>3.2 Ga, 0.5 to 7.4 km) and the younger and larger prominent constructs within the Tharsis and Elysium provinces (<3 Ga, 5.8 to 21.9 km). We fit the theoretical gravitational signal to observations in order to place constraints on 18 volcanic structures. Inverted parameters include the bulk density of the load, the elastic thickness required to support the volcanic edifice at the time it was emplaced (Te), the heat flow, the volume of extruded lava, and the ratio of volcanic products that form within (Vi) and above the preexisting surface (Ve). The bulk density of the volcanic structures is found to have a mean value of 3, 206 \textpm 190 kg/m3, which is representative of iron-rich basalts as sampled by the Martian basaltic meteorites. Te beneath the small volcanoes is found to be small, less than 15 km, which implies that the lithosphere was weak and hot when these volcanoes formed. Conversely, most large volcanoes display higher values of Te, which is consistent with the bulk of their emplacement occurring later in geologic history, when the elastic lithosphere was colder and thicker. Our estimates for the volumes of volcanic edifices are about 10 times larger than those that neglect the flexure of the lithosphere. Constraints on the magnitude of subsurface loads imply that the ratio Vi/Ve is generally 3:5, which is smaller than for the Hawaiian volcanoes on Earth.

2. Thickness of Lunar Mare Basalts: New Results Based on Modeling the Degradation of Partially Buried Craters

   Jun Du, Wenzhe Fa, Mark A. Wieczorek, and 3 more authors

   Journal of Geophysical Research: Planets, 2019.

   abstract url

   Partially buried craters on the Moon are those craters whose distal ejecta are covered by lava flows and where the crater rim crest still protrudes above the mare plain. Based on the difference in rim heights between a partially buried crater and an unburied crater, previous studies estimated the thicknesses of the lunar mare basalts. However, these studies did not consider the erosion of the crater rim height, which can result in an overestimate in the derived thickness. By using recent high-resolution topographic data, we report a basalt thickness estimation method based on numerically modeling the topographic degradation of partially buried craters. We identified 661 buried craters over the lunar surface, and their spatial distribution suggests a preferential occurrence along the mare-highland boundaries. An elevation model of fresh lunar craters was derived, and the topographic diffusion equation was used to model crater degradation. By modeling the formation, degradation, and flooding of partially buried craters, basalt thicknesses were estimated for 41 mare craters whose rims are completely exposed. The resulting mare basalt thicknesses vary from 33 to 455 m, with a median value of 105 m that is 95 m smaller than that derived when not considering crater degradation. The estimated eruption rate of lunar mare basalts is found to have peaked at 3.4 Ga and then decreased with time, indicating a progressive cooling of the lunar interior. As a by-product from the crater degradation model, our results suggest that the topographic diffusivity of lunar craters increases with diameter.

3. Lunar Seismology: An Update on Interior Structure Models

   Raphael F. Garcia, Amir Khan, Mélanie Drilleau, and 11 more authors

   Space Science Reviews, 2019.

   abstract url

   An international team of researchers gathered, with the support of the International Space Science Institute (ISSI), (1) to review seismological investigations of the lunar interior from the Apollo-era and up until the present and (2) to re-assess our level of knowledge and uncertainty on the interior structure of the Moon. A companion paper (Nunn et al. in Space Sci. Rev., submitted) reviews and discusses the Apollo lunar seismic data with the aim of creating a new reference seismic data set for future use by the community. In this study, we first review information pertinent to the interior of the Moon that has become available since the Apollo lunar landings, particularly in the past ten years, from orbiting spacecraft, continuing measurements, modeling studies, and laboratory experiments. Following this, we discuss and compare a set of recent published models of the lunar interior, including a detailed review of attenuation and scattering properties of the Moon. Common features and discrepancies between models and moonquake locations provide a first estimate of the error bars on the various seismic parameters. Eventually, to assess the influence of model parameterisation and error propagation on inverted seismic velocity models, an inversion test is presented where three different parameterisations are considered. For this purpose, we employ the travel time data set gathered in our companion paper (Nunn et al. in Space Sci. Rev., submitted). The error bars of the inverted seismic velocity models demonstrate that the Apollo lunar seismic data mainly constrain the upper- and mid-mantle structure to a depth of ∼1200 km. While variable, there is some indication for an upper mantle low-velocity zone (depth range 100–250 km), which is compatible with a temperature gradient around 1.7 ∘C/km. This upper mantle thermal gradient could be related to the presence of the thermally anomalous region known as the Procellarum Kreep Terrane, which contains a large amount of heat producing elements.

4. Density Distribution of Asteroid 25143 Itokawa Based on Smooth Terrain Shape

   Masanori Kanamaru, Sho Sasaki, and Mark Wieczorek

   Planetary and Space Science, 2019.

   abstract url

   The shape and internal structure of an asteroid is a result of its violent collisional history, and Asteroid 25143 Itokawa is an important target for investigating the internal structure and formation processes associated with rubble pile asteroids. The surface of Itokawa contains numerous boulders and is very rough, but 20% of the surface is covered with centimeter-sized gravels that are called ”smooth terrain”. These flat areas are associated with low areas of the gravitational potential and are considered to be formed by down-slope migration and accumulation of pebbles. We propose a method to constrain the interior density distribution of an asteroid by modeling the gravity field and fitting the smooth terrains to equi-potential surfaces. Density models that are composed of two distinct lobes best explain the data if the ”head” and ”body” have different densities of 2,450 kg/ m3 and 1,930 kg/m3, respectively. Alternatively, the two lobes could have comparable densities if there was a compressed ”neck” of higher density between the two. Three layer models that treat the densities of the head, neck and body separately are largely unconstrained. However, when using an independent estimate of the centerof-mass/center-of-figure offset as determined from modeling the YORP spin-up of Itokawa, our models require the density of the head to be larger than the global average.

5. SEIS: Insight’s Seismic Experiment for Internal Structure of Mars

   P. Lognonné, W. B. Banerdt, D. Giardini, and 177 more authors

   Space Science Reviews, 2019.

   abstract url

   By the end of 2018, 42 years after the landing of the two Viking seismometers on Mars, InSight will deploy onto Mars’ surface the SEIS (Seismic Experiment for Internal Structure) instrument; a six-axes seismometer equipped with both a long-period three-axes Very Broad Band (VBB) instrument and a three-axes short-period (SP) instrument. These six sensors will cover a broad range of the seismic bandwidth, from 0.01 Hz to 50 Hz, with possible extension to longer periods. Data will be transmitted in the form of three continuous VBB components at 2 sample per second (sps), an estimation of the short period energy content from the SP at 1 sps and a continuous compound VBB/SP vertical axis at 10 sps. The continuous streams will be augmented by requested event data with sample rates from 20 to 100 sps. SEIS will improve upon the existing resolution of Viking’s Mars seismic monitoring by a factor of ∼ 2500 at 1 Hz and ∼ 200 000 at 0.1 Hz. An additional major improvement is that, contrary to Viking, the seismometers will be deployed via a robotic arm directly onto Mars’ surface and will be protected against temperature and wind by highly efficient thermal and wind shielding. Based on existing knowledge of Mars, it is reasonable to infer a moment magnitude detection threshold of Mw ∼ 3at40\textopenbullet epicentral distance and a potential to detect several tens of quakes and about five impacts per year. In this paper, we first describe the science goals of the experiment and the rationale used to define its requirements. We then provide a detailed description of the hardware, from the sensors to the deployment system and associated performance, including transfer functions of the seismic sensors and temperature sensors. We conclude by describing the experiment ground segment, including data processing services, outreach and education networks and provide a description of the format to be used for future data distribution.

6. Pre-Mission InSights on the Interior of Mars

   Suzanne E. Smrekar, Philippe Lognonné, Tilman Spohn, and 30 more authors

   Space Science Reviews, 2019.

   abstract url

   The Interior exploration using Seismic Investigations, Geodesy, and Heat Transport (InSight) Mission will focus on Mars’ interior structure and evolution. The basic structure of crust, mantle, and core form soon after accretion. Understanding the early differentiation process on Mars and how it relates to bulk composition is key to improving our understanding of this process on rocky bodies in our solar system, as well as in other solar systems. Current knowledge of differentiation derives largely from the layers observed via seismology on the Moon. However, the Moon’s much smaller diameter make it a poor analog with respect to interior pressure and phase changes. In this paper we review the current knowledge of the thickness of the crust, the diameter and state of the core, seismic attenuation, heat flow, and interior composition. InSight will conduct the first seismic and heat flow measurements of Mars, as well as more precise geodesy. These data reduce uncertainty in crustal thickness, core size and state, heat flow, seismic activity and meteorite impact rates by a factor of 3–10\texttimes relative to previous estimates. Based on modeling of seismic wave propagation, we can further constrain interior temperature, composition, and the location of phase changes. By combining heat flow and a well constrained value of crustal thickness, we can estimate the distribution of heat producing elements between the crust and mantle. All of these quantities are key inputs to models of interior convection and thermal evolution that predict the processes that control subsurface temperature, rates of volcanism, plume distribution and stability, and convective state. Collectively these factors offer strong controls on the overall evolution of the geology and habitability of Mars.

7. Hydrostatic Interfaces in Bodies With Nonhydrostatic Lithospheres

   Mark A. Wieczorek, Mikael Beuthe, Attilio Rivoldini, and 1 more author

   Journal of Geophysical Research: Planets, 2019.

   abstract url

   Below the lithospheres of the terrestrial planets, dwarf planets, and moons, density interfaces adjust over geologic time to align with surfaces of constant gravitational potential. It is well known that the shape of such hydrostatic surfaces is controlled by the pseudo-rotational potential, tidal potential, and the induced potential of nonspherical density interfaces in the body. When a lithosphere is present, however, additional gravitational terms must be considered that arise from, for example, surface relief and crustal thickness variations. A first-order formalism is presented for calculating the shape of hydrostatic density interfaces beneath the lithosphere when the gravity field and surface shape of the body are known. Using an arbitrary discretized density profile, the shapes are obtained by solving a simple matrix equation. As examples, lithospheric gravity anomalies account for about 10% of the relief along hydrostatic interfaces in Mars, whereas for the Moon, the lithospheric gravity is the dominant contributor to the core shape. Spherical harmonic degree-1 mass anomalies in the lithosphere generate degree-1 relief along the core-mantle boundary, and for Mars and the Moon, the core is offset from the center of mass of the body by about 90 m. The moments of inertia of the core of these bodies are also misaligned with respect to the principal moments of the entire body. An improved crustal thickness map of Mars is constructed that accounts for gravity anomalies beneath the lithosphere, and the consequences of core relief on the Martian free core nutation are quantified.

2018

1. Ring Faults and Ring Dikes around the Orientale Basin on the Moon

   Jeffrey C. Andrews-Hanna, James W. Head, Brandon C. Johnson, and 6 more authors

   Icarus, 2018.

   abstract url

   The Orientale basin is the youngest and best-preserved multiring impact basin on the Moon, having experienced only modest modification by subsequent impacts and volcanism. Orientale is often treated as the type example of a multiring basin, with three prominent rings outside of the inner depression: the Inner Rook Montes, the Outer Rook Montes, and the Cordillera. Here we use gravity data from NASA’s Gravity Recovery and Interior Laboratory (GRAIL) mission to reveal the subsurface structure of Orientale and its ring system. Gradients of the gravity data reveal a continuous ring dike intruded into the Outer Rook along the plane of the fault associated with the ring scarp. The volume of this ring dike is ∼18 times greater than the volume of all extrusive mare deposits associated with the basin. The gravity gradient signature of the Cordillera ring indicates an offset along the fault across a shallow density interface, interpreted to be the base of the low-density ejecta blanket. Both gravity gradients and crustal thickness models indicate that the edge of the central cavity is shifted inward relative to the equivalent Inner Rook ring at the surface. Models of the deep basin structure show inflections along the crust–mantle interface at both the Outer Rook and Cordillera rings, indicating that the basin ring faults extend from the surface to at least the base of the crust. Fault dips range from 13^∘ to 22^∘ for the Cordillera fault in the northeastern quadrant, to 90^∘ for the Outer Rook in the northwestern quadrant. The fault dips for both outer rings are lowest in the northeast, possibly due to the effects of either the direction of projectile motion or regional gradients in pre-impact crustal thickness. Similar ring dikes and ring faults are observed around the majority of lunar basins.

2. Olivine-Bearing Lithologies on the Moon: Constraints on Origins and Transport Mechanisms from M3 Spectroscopy, Radiative Transfer Modeling, and GRAIL Crustal Thickness

   Laura M. Corley, Patrick J. McGovern, Georgiana Y. Kramer, and 8 more authors

   Icarus, 2018.

   abstract url

   High-resolution hyperspectral data from Chandrayaan-1’s Moon Mineralogy Mapper (M3) allow detection of olivine on the lunar surface. Olivine exposed at the surface may originate as mantle material or igneous products (intrusive or extrusive). Potential transport mechanisms include excavation of the mantle or lower crustal material by impacts that form basins and complex craters, differentiation of impact melt sheets, or magmatic emplacement of lavas, cumulates, or xenoliths. A sample of the lunar mantle, which has not been conclusively identified in the lunar sample collection, would yield fundamental new insights into the composition, structure, and evolution of the lunar interior. Olivine identified in remote spectral data is generally accepted to originate from the primary mantle, because abundant olivine is expected to exist in the mantle and lower crust, yet have sparse occurrences in the upper crust. In this study, we identified 111 M3 single-pixel spectra with characteristic absorption features consistent with olivine at Crisium, Nectaris, and Humorum basins and near the craters Roche and Tsiolkovsky. In an effort to determine the origins and transport mechanisms that led to these individual exposures, we estimated mineral abundances using radiative transfer modeling and examined crustal thickness estimates, topography and slope maps, and images from the Lunar Reconnaissance Orbiter Camera (LROC). At Crisium basin, where crustal thickness is near 0 km (Wieczorek et al., 2013), mantle olivine may have been exposed by basinforming impact and deposited on the rim. Picard crater, which is superposed on the floor of Crisium, also exhibits potential mantle olivine in its ejecta. Within Nectaris basin, olivine exposures are confined to the rims of small craters on the mare, which are inferred to excavate a layer of olivine-rich mare basalt. Olivine occurrences on the rim of Humorum basin, including those located on a graben, are likely to be cumulates of shallow intrusions that were transported magmatically to the surface. Near Roche crater, olivine may have originated in shallow dikes that reached the subsurface and were exposed by impacts. In addition to verifying both known and previously unidentified olivine exposures, our combined geophysical, spectral, and radiative transfer modeling investigation has allowed identification of both igneous and mantle-derived olivine.

3. Distribution of Radioactive Heat Sources and Thermal History of the Moon

   M. Laneuville, J. Taylor, and M. A. Wieczorek

   Journal of Geophysical Research: Planets, 2018.

   abstract url

   The initial distribution of heat sources in crustal and mantle reservoirs plays a major role in the thermal evolution of the Moon. We use new constraints on the thickness of the crust, the size of a nearside low in crustal magnetization, surface composition data from orbit, Apollo samples, and mass balance considerations to generate a set of plausible post magma ocean initial conditions. We then test those initial conditions using the 3-D thermochemical mantle convection code Gaia and compare with observables. Models that use Lunar Prospector gamma-ray spectrometer values of thorium throughout the highland crust cannot sustain long lasting volcanic activity, as low abundances of heat-producing elements are left in the mantle to keep an Earth-like bulk silicate composition. The low magnetic field intensities of the innermost Procellarum KREEP Terrane are consistent with a higher heat production than in the outermost portion and delayed cooling below the Curie temperature of iron metal until after 3.56 Ga when the dynamo field strength is known to have decreased by an order of magnitude. The distribution of crustal heat sources also influences the depth evolution of isotopic closure isotherms for a range of isotopic systems relevant to radiometric dating, which may be important for sample age estimation. Core crystallization can sustain a continuous dynamo for about 1 billion years, after which dynamo activity is potentially more episodic.

4. The Timeline of the Lunar Bombardment: Revisited

   A. Morbidelli, D. Nesvorny, V. Laurenz, and 5 more authors

   Icarus, 2018.

   abstract url

   The timeline of the lunar bombardment in the first Gy of Solar System history remains unclear. Basinforming impacts (e.g. Imbrium, Orientale), occurred 3.9–3.7 Gy ago, i.e. 600–800 My after the formation of the Moon itself. Many other basins formed before Imbrium, but their exact ages are not precisely known. There is an intense debate between two possible interpretations of the data: in the cataclysm scenario there was a surge in the impact rate approximately at the time of Imbrium formation, while in the accretion tail scenario the lunar bombardment declined since the era of planet formation and the latest basins formed in its tail-end. Here, we revisit the work of Morbidelli et al. (2012) that examined which scenario could be compatible with both the lunar crater record in the 3–4 Gy period and the abundance of highly siderophile elements (HSE) in the lunar mantle. We use updated numerical simulations of the fluxes of asteroids, comets and planetesimals leftover from the planet-formation process. Under the traditional assumption that the HSEs track the total amount of material accreted by the Moon since its formation, we conclude that only the cataclysm scenario can explain the data. The cataclysm should have started ∼ 3.95 Gy ago. However we also consider the possibility that HSEs are sequestered from the mantle of a planet during magma ocean crystallization, due to iron sulfide exsolution (O’Neil, 1991; Rubie et al., 2016). We show that this is likely true also for the Moon, if mantle overturn is taken into account. Based on the hypothesis that the lunar magma ocean crystallized about 10 0–150 My after Moon formation (Elkins-Tanton et al., 2011), and therefore that HSEs accumulated in the lunar mantle only after this timespan, we show that the bombardment in the 3–4 Gy period can be explained in the accretion tail scenario. This hypothesis would also explain why the Moon appears so depleted in HSEs relative to the Earth. We also extend our analysis of the cataclysm and accretion tail scenarios to the case of Mars. The accretion tail scenario requires a global resurfacing event on Mars ∼ 4.4 Gy ago, possibly associated with the formation of the Borealis basin, and it is consistent with the HSE budget of the planet. Moreover it implies that the Noachian and pre-Noachian terrains are ∼ 200 My older than usually considered.

5. The Thermal State and Interior Structure of Mars

   A.-C. Plesa, S. Padovan, N. Tosi, and 6 more authors

   Geophysical Research Letters, 2018.

   abstract url

   The present-day thermal state, interior structure, composition, and rheology of Mars can be constrained by comparing the results of thermal history calculations with geophysical, petrological, and geological observations. Using the largest-to-date set of 3-D thermal evolution models, we find that a limited set of models can satisfy all available constraints simultaneously. These models require a core radius strictly larger than 1,800 km, a crust with an average thickness between 48.8 and 87.1 km containing more than half of the planet’s bulk abundance of heat producing elements, and a dry mantle rheology. A strong pressure dependence of the viscosity leads to the formation of prominent mantle plumes producing melt underneath Tharsis up to the present time. Heat flow and core size estimates derived from the InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) mission will increase the set of constraining data and help to confine the range of admissible models.

6. Isostatic Compensation of the Lunar Highlands: Isostatic Compensation of the Lunar Highlands

   Michael M. Sori, Peter B. James, Brandon C. Johnson, and 4 more authors

   Journal of Geophysical Research: Planets, 2018.

   abstract url

   The lunar highlands are isostatically compensated at large horizontal scales, but the specific compensation mechanism has been difficult to identify. With topographic data from the Lunar Orbiter Laser Altimeter and gravity data from the Gravity Recovery and Interior Laboratory, we investigate support of highland topography. Poor correlation between crustal density and elevation shows that Pratt compensation is not important in the highlands. Using spectrally weighted admittance, we compared observed values of geoid-to-topography ratio (GTR) with those predicted by isostatic models. Observed GTRs are 25.8+–57..75 m/km for the nearside highlands and 39.3+–65..27 m/km for the farside highlands. These values are not consistent with flexural compensation of long-wavelength topography or Airy isostasy defined under an assumption of equal mass in crustal columns. Instead, the observed GTR values are consistent with models of Airy compensation in which isostasy is defined under a requirement of equal pressures at equipotential surfaces at depth. The gravity and topography data thus reveal that long-wavelength topography on the Moon is most likely compensated by variations in crustal thickness, implying that highland topography formed early in lunar history before the development of a thick elastic lithosphere.

7. SHTools: Tools for Working with Spherical Harmonics

   Mark A. Wieczorek, and Matthias Meschede

   Geochemistry, Geophysics, Geosystems, 2018.

   abstract url

   Geophysical analyses are often performed in spherical geometry and require the use of spherical harmonic functions to express observables or physical quantities. When expanded to high degree, the accuracy and speed of the spherical harmonic transforms and reconstructions are of paramount importance. SHTools is a time and user-tested open-source archive of both Fortran 95 and Python routines for performing spherical harmonic analyses. The routines support all spherical-harmonic normalization conventions used in the geosciences, including 4p-normalized, Schmidt seminormalized, orthonormalized, and unnormalized harmonics, along with the option of employing the Condon-Shortley phase factor of \dh21\THm. Data on the sphere can be sampled on a variety of grid formats, including equally spaced cylindrical grids and grids appropriate for integration by Gauss-Legendre quadrature. The spherical-harmonic transforms are proven to be fast and accurate for spherical harmonic degrees up to 2800. Several tools are provided for the geoscientist, including routines for performing localized spectral analyses and basic operations related to global gravity and magnetic fields. In the Python environment, operations are very simple to perform as a result of three class structures that encompass all operations on grids, spherical harmonic coefficients, and spatiospectral localization windows. SHTools is released under the unrestrictive BSD 3-clause license.

8. Strength, Depth, and Geometry of Magnetic Sources in the Crust of the Moon From Localized Power Spectrum Analysis

   Mark A. Wieczorek

   Journal of Geophysical Research: Planets, 2018.

   abstract url

   Spacecraft observations show that weak magnetic fields of crustal origin are ubiquitous across the surface of the Moon. To investigate the origin of these magnetic anomalies, a model was developed for the magnetic power spectrum that consists of ensembles of randomly magnetized sills or prisms. Localized spectrum analyses constrained how the parameters of this model vary with position, including the size of the sources, a quantity proportional to their mean-squared dipole moment, and the depth to the top and bottom of the magnetized region. The depth to the top of the magnetized region varies from the surface to about 25 km. The magnetic carriers in the deep crust likely formed at the same time as the crust itself, implying that a core-generated dynamo field must have existed when the crust was cooling during the first 100 Myr of lunar evolution. The parameter related to the strength of magnetization shows the existence of a prominent region on the nearside hemisphere that is largely unmagnetized and that correlates with a region of extremely low surface field strengths. This region lies entirely within a geological province that is highly enriched in heat-producing elements (the Procellarum KREEP Terrane), suggesting that this region escaped being magnetized because of prolonged high crustal temperatures. The nearside magnetic low may be representative of the size of that portion of the crust that is highly enriched in heat-producing elements, which is almost one third the size of the Procellarum KREEP Terrane based on surface thorium abundances.

2017

1. Fundamental Relations of Mineral Specific Magnetic Carriers for Paleointensity Determination

   G. Kletetschka, and M.A. Wieczorek

   Physics of the Earth and Planetary Interiors, 2017.

   abstract url

   A fundamental linear relationship exists between the efficiency of thermoremanent magnetization measured at room temperature and the magnitude of the ambient field at the time of acquisition. The magnetic efficiency (the ratio of thermoremanent magnetization to saturation remanent magnetization) multiplied by the saturation magnetization is proportional to the magnetizing field, where the proportionality constant is independent of magnetic mineralogy and domain state. The empirical constant for this equation was determined using existing experimental data of single domain through pseudosingle domain to multidomain states of iron, meteoritic iron, magnetite, maghemite, pyrrhotite, and hematite. We show that the acquired magnetic efficiency is closely related to two types of demagnetizing fields that act as barriers against domain wall pinning during magnetic acquisition. The first relates to the saturation magnetization that is derived from the distribution of Bohr magnetons within the crystal lattice, and the second originates from grain shape. Theoretical considerations imply a factor of two difference in the magnetic efficiency acquired during laboratory and geologic timescales. This equation reveals that troilite may be a potentially important magnetic carrier for extraterrestrial magnetism. Using magnetic scanning techniques, our relationship allows for estimating the paleointensity from samples that contain more than one magnetic species.

2. Iron Abundances in Lunar Impact Basin Melt Sheets From Orbital Magnetic Field Data

   Joana S. Oliveira, Mark A. Wieczorek, and Gunther Kletetschka

   Journal of Geophysical Research: Planets, 2017.

   abstract url

   Magnetic field data acquired from orbit shows that the Moon possesses many magnetic anomalies. Though most of these are not associated with known geologic structures, some are found within large impact basins within the interior peak ring. The primary magnetic carrier in lunar rocks is metallic iron, but indigenous lunar rocks are metal poor and cannot account easily for the observed field strengths. The projectiles that formed the largest impact basins must have contained a significant quantity of metallic iron, and a portion of this iron would have been retained on the Moon’s surface within the impact melt sheet. Here we use orbital magnetic field data to invert for the magnetization within large impact basins using the assumption that the crust is unidirectionally magnetized. We develop a technique based on laboratory thermoremanent magnetization acquisition to quantify the relationship between the strength of the magnetic field at the time the rock cooled and the abundance of metal in the rock. If we assume that the magnetized portion of the impact melt sheet is 1 km thick, we find average abundances of metallic iron ranging from 0.11% to 0.45 wt %, with an uncertainty of a factor of about 3. This abundance is consistent with the metallic iron abundances in sampled lunar impact melts and the abundance of projectile contamination in terrestrial impact melts. These results help constrain the composition of the projectile, the impact process, and the time evolution of the lunar dynamo.

3. Testing the Axial Dipole Hypothesis for the Moon by Modeling the Direction of Crustal Magnetization

   J. S. Oliveira, and M. A. Wieczorek

   Journal of Geophysical Research: Planets, 2017.

   abstract url

   Orbital magnetic field data show that portions of the Moon’s crust are strongly magnetized, and paleomagnetic data of lunar samples suggest that Earth strength magnetic fields could have existed during the first several hundred million years of lunar history. The origin of the fields that magnetized the crust are not understood and could be the result of either a long-lived core-generated dynamo or transient fields associated with large impact events. Core dynamo models usually predict that the field would be predominantly dipolar, with the dipole axis aligned with the rotation axis. We test this hypothesis by modeling the direction of crustal magnetization using a global magnetic field model of the Moon derived from Lunar Prospector and Kaguya magnetometer data. We make use of a model that assumes that the crust is unidirectionally magnetized. The intensity of magnetization can vary with the crust, and the best fitting direction of magnetization is obtained from a nonnegative least squares inversion. From the best fitting magnetization direction we obtain the corresponding north magnetic pole predicted by an internal dipolar field. Some of the obtained paleopoles are associated with the current geographic poles, while other well-constrained anomalies have paleopoles at equatorial latitudes, preferentially at 90∘ east and west longitudes. One plausible hypothesis for this distribution of paleopoles is that the Moon possessed a long-lived dipolar field but that the dipole was not aligned with the rotation axis as a result of large-scale heat flow heterogeneities at the core-mantle boundary.

4. Planned Products of the Mars Structure Service for the InSight Mission to Mars

   Mark P. Panning, Philippe Lognonné, W. Bruce Banerdt, and 33 more authors

   Space Science Reviews, 2017.

   abstract url

   The InSight lander will deliver geophysical instruments to Mars in 2018, including seismometers installed directly on the surface (Seismic Experiment for Interior Structure, SEIS). Routine operations will be split into two services, the Mars Structure Service (MSS) and Marsquake Service (MQS), which will be responsible, respectively, for defining the structure models and seismicity catalogs from the mission. The MSS will deliver a series of products before the landing, during the operations, and finally to the Planetary Data System (PDS) archive. Prior to the mission, we assembled a suite of a priori models of Mars, based on estimates of bulk composition and thermal profiles. Initial models during the mission will rely on modeling surface waves and impact-generated body waves independent of prior knowledge of structure. Later modeling will include simultaneous inversion of seismic observations for source and structural parameters. We use Bayesian inversion techniques to obtain robust probability distribution functions of interior structure parameters. Shallow structure will be characterized using the hammering of the heatflow probe mole, as well as measurements of surface wave ellipticity. Crustal scale structure will be constrained by measurements of receiver function and broadband Rayleigh wave ellipticity measurements. Core interacting body wave phases should be observable above modeled martian noise levels, allowing us to constrain deep structure. Normal modes of Mars should also be observable and can be used to estimate the globally averaged 1D structure, while combination with results from the InSight radio science mission and orbital observations will allow for constraint of deeper structure.

2016

1. The Forced Precession of the Moon’s Inner Core

   Mathieu Dumberry, and Mark A. Wieczorek

   Journal of Geophysical Research: Planets, 2016.

   abstract url

   The tilt angle of the 18.6 year precession of the Moon’s solid inner core is unknown, but it is set by a balance between gravitational and pressure torques acting on its elliptical figure. We show here that to first order, the angle of precession of the inner core of a planetary body is determined by the frequency of the free inner core nutation, ωficn, relative to the precession frequency, \textohmp. If \textbarωficn\textbar ≪ \textbar\textohmp\textbar, the inner core is blind to the gravitational influence of the mantle. If \textbarωficn\textbar ≫ \textbar\textohmp\textbar, the inner core is gravitationally locked to the mantle and is nearly aligned with it. If ωficn ≈ \textohmp, large inner core tilt angles can result from resonant excitation. Viscous inner core relaxation and electromagnetic coupling can attenuate large tilt angles. For the specific case of the Moon, we show that ωficn is to within a factor of 2 of \textohmp = 2\pi/18.6 yr-1. For a rigid inner core, this implies a tilt of 2 to 5∘ with respect to the mantle, and larger if ωficn is very close to \textohmp. More modest tilt angles between 0 and 0.5∘ result if viscous relaxation within the inner core occurs on a timescale of one lunar day. Predictions from our model may be used in an attempt to detect the gravity signal resulting from a tilted inner core, to determine the past history of the inner core tilt angle, and to assess models of dynamo generation powered by differential rotation at the core-mantle and inner core boundaries.

2. Thicknesses of Mare Basalts on the Moon from Gravity and Topography

   Shengxia Gong, Mark A. Wieczorek, Francis Nimmo, and 5 more authors

   Journal of Geophysical Research: Planets, 2016.

   abstract url

   A new method of determining the thickness of mare basalts on the Moon is introduced that is made possible by high-resolution gravity data acquired from NASA’s Gravity Recovery and Interior Laboratory (GRAIL) mission. Using a localized multitaper spherical-harmonic analysis, an effective density spectrum is calculated that provides an estimate of the average crustal density as a function of spherical harmonic degree. By comparing the observed effective density spectrum with one generated from a theoretical model, the thickness of mare basalts can be constrained. We assume that the grain density of the basalts is known from remote sensing data and petrologic considerations, we assign a constant porosity to the basalts, and we let both the thickness of the basalts and the density of the underlying crust vary. Using this method, the total thickness of basalts was estimated on the nearside hemisphere, yielding an average of 0.74 km with 1σ upper and lower bounds of 1.62 km and 100 m, respectively. The region of Marius Hills, which is a long-lived volcanic complex, is found to have the thickest basalts, with an average of 2.86 km and 1σ limits of 3.65 and 1.02 km, respectively. The crust beneath the Mare Imbrium basalts is found to have an atypically high density of about 3000 kg m-3 that we interpret as representing a mafic, unfractured, impact melt sheet.

3. Formation of the Orientale Lunar Multiring Basin

   B. C. Johnson, D. M. Blair, G. S. Collins, and 11 more authors

   Science, 2016.

   abstract url

   Multiring basins, large impact craters characterized by multiple concentric topographic rings, dominate the stratigraphy, tectonics, and crustal structure of the Moon. Using a hydrocode, we simulated the formation of the Orientale multiring basin, producing a subsurface structure consistent with high-resolution gravity data from the Gravity Recovery and Interior Laboratory (GRAIL) spacecraft. The simulated impact produced a transient crater, ~390 kilometers in diameter, that was not maintained because of subsequent gravitational collapse. Our simulations indicate that the flow of warm weak material at depth was crucial to the formation of the basin’s outer rings, which are large normal faults that formed at different times during the collapse stage. The key parameters controlling ring location and spacing are impactor diameter and lunar thermal gradients.

4. GRAIL, LLR, and LOLA Constraints on the Interior Structure of the Moon

   Isamu Matsuyama, Francis Nimmo, James T. Keane, and 5 more authors

   Geophysical Research Letters, 2016.

   abstract url

   The interior structure of the Moon is constrained by its mass, moment of inertia, and k2 and h2 tidal Love numbers. We infer the likely radius, density, and (elastic limit) rigidity of all interior layers by solving the inverse problem using these observational constraints assuming spherical symmetry. Our results do not favor the presence of a low rigidity transition layer between a liquid outer core and mantle. If a transition layer exists, its rigidity is constrained to 43+-296 GPa, with a preference for the high rigidity values. Therefore, if a transition layer exists, it is more likely to have a rigidity similar to that of the mantle (∼70 GPa). The total (solid and liquid) core mass fraction relative to the lunar mass is constrained to 0.0098+-00..00006964 and 0.0198+-00..00002469 for interior structures with and without a transition layer, respectively, narrowing the range of possible giant impact formation scenarios.

5. Subsurface Morphology and Scaling of Lunar Impact Basins: Morphology and Scaling of Lunar Basins

   K. Miljković, G. S. Collins, M. A. Wieczorek, and 4 more authors

   Journal of Geophysical Research: Planets, 2016.

   abstract url

   Impact bombardment during the first billion years after the formation of the Moon produced at least several tens of basins. The Gravity Recovery and Interior Laboratory (GRAIL) mission mapped the gravity field of these impact structures at significantly higher spatial resolution than previous missions, allowing for detailed subsurface and morphological analyses to be made across the entire globe. GRAIL-derived crustal thickness maps were used to define the regions of crustal thinning observed in centers of lunar impact basins, which represents a less unambiguous measure of a basin size than those based on topographic features. The formation of lunar impact basins was modeled numerically by using the iSALE-2D hydrocode, with a large range of impact and target conditions typical for the first billion years of lunar evolution. In the investigated range of impactor and target conditions, the target temperature had the dominant effect on the basin subsurface morphology. Model results were also used to update current impact scaling relationships applicable to the lunar setting (based on assumed target temperature). Our new temperature-dependent impact-scaling relationships provide estimates of impact conditions and transient crater diameters for the majority of impact basins mapped by GRAIL. As the formation of lunar impact basins is associated with the first ~700 Myr of the solar system evolution when the impact flux was considerably larger than the present day, our revised impact scaling relationships can aid further analyses and understanding of the extent of impact bombardment on the Moon and terrestrial planets in the early solar system.

6. How Large Are Present-Day Heat Flux Variations across the Surface of Mars?

   A.-C. Plesa, M. Grott, N. Tosi, and 3 more authors

   Journal of Geophysical Research: Planets, 2016.

   abstract url

   The first in situ Martian heat flux measurement to be carried out by the InSight Discovery-class mission will provide an important baseline to constrain the present-day heat budget of the planet and, in turn, the thermochemical evolution of its interior. In this study, we estimate the magnitude of surface heat flux heterogeneities in order to assess how the heat flux at the InSight landing site relates to the average heat flux of Mars. To this end, we model the thermal evolution of Mars in a 3-D spherical geometry and investigate the resulting surface spatial variations of heat flux at the present day. Our models assume a fixed crust with a variable thickness as inferred from gravity and topography data and with radiogenic heat sources as obtained from gamma ray measurements of the surface. We test several mantle parameters and show that the present-day surface heat flux pattern is dominated by the imposed crustal structure. The largest surface heat flux peak-to peak variations lie between 17.2 and 49.9 mW m-2, with the highest values being associated with the occurrence of prominent mantle plumes. However, strong spatial variations introduced by such plumes remain narrowly confined to a few geographical regions and are unlikely to bias the InSight heat flux measurement. We estimated that the average surface heat flux varies between 23.2 and 27.3 mW m-2, while at the InSight location it lies between 18.8 and 24.2 mW m-2. In most models, elastic lithosphere thickness values exceed 250 km at the north pole, while the south pole values lie well above 110 km.

7. SHTOOLS: Version 4.0

   M. A. Wieczorek, M. Meschede, I. Oshchepkov, and 2 more authors

   Zenodo, 2016.

   url
8. Gravity Field of the Orientale Basin from the Gravity Recovery and Interior Laboratory Mission

   M. T. Zuber, D. E. Smith, G. A. Neumann, and 25 more authors

   Science, 2016.

   abstract url

   The Orientale basin is the youngest and best-preserved major impact structure on the Moon. We used the Gravity Recovery and Interior Laboratory (GRAIL) spacecraft to investigate the gravitational field of Orientale at 3- to 5-kilometer (km) horizontal resolution. A volume of at least (3.4 \textpm 0.2) \texttimes 106 km3 of crustal material was removed and redistributed during basin formation. There is no preserved evidence of the transient crater that would reveal the basin’s maximum volume, but its diameter may now be inferred to be between 320 and 460 km. The gravity field resolves distinctive structures of Orientale’s three rings and suggests the presence of faults associated with the outer two that penetrate to the mantle. The crustal structure of Orientale provides constraints on the formation of multiring basins.

2015

1. Excavation of the Lunar Mantle by Basin-Forming Impact Events on the Moon

   Katarina Miljković, Mark A. Wieczorek, Gareth S. Collins, and 3 more authors

   Earth and Planetary Science Letters, 2015.

   abstract url

   Global maps of crustal thickness on the Moon, derived from gravity measurements obtained by NASA’s Gravity Recovery and Interior Laboratory (GRAIL) mission, have shown that the lunar crust is thinner than previously thought. Hyperspectral data obtained by the Kaguya mission have also documented areas rich in olivine that have been interpreted as material excavated from the mantle by some of the largest lunar impact events. Numerical simulations were performed with the iSALE-2D hydrocode to investigate the conditions under which mantle material may have been excavated during large impact events and where such material should be found. The results show that excavation of the mantle could have occurred during formation of the several largest impact basins on the nearside hemisphere as well as the Moscoviense basin on the farside hemisphere. Even though large areas in the central portions of these basins were later covered by mare basaltic lava flows, surficial lunar mantle deposits are predicted in areas external to these maria. Our results support the interpretation that the high olivine abundances detected by the Kaguya spacecraft could indeed be derived from the lunar mantle.

2. Lunar Impact Basins Revealed by Gravity Recovery and Interior Laboratory Measurements

   Gregory A. Neumann, Maria T. Zuber, Mark A. Wieczorek, and 19 more authors

   Science Advances, 2015.

   abstract url

   Observations from the Gravity Recovery and Interior Laboratory (GRAIL) mission indicate a marked change in the gravitational signature of lunar impact structures at the morphological transition, with increasing diameter, from complex craters to peak-ring basins. At crater diameters larger than ~200 km, a central positive Bouguer anomaly is seen within the innermost peak ring, and an annular negative Bouguer anomaly extends outward from this ring to the outer topographic rim crest. These observations demonstrate that basin-forming impacts remove crustal materials from within the peak ring and thicken the crust between the peak ring and the outer rim crest. A correlation between the diameter of the central Bouguer gravity high and the outer topographic ring diameter for well-preserved basins enables the identification and characterization of basins for which topographic signatures have been obscured by superposed cratering and volcanism. The GRAIL inventory of lunar basins improves upon earlier lists that differed in their totals by more than a factor of 2. The size-frequency distributions of basins on the nearside and farside hemispheres of the Moon differ substantially; the nearside hosts more basins larger than 350 km in diameter, whereas the farside has more smaller basins. Hemispherical differences in target properties, including temperature and porosity, are likely to have contributed to these different distributions. Better understanding of the factors that control basin size will help to constrain models of the original impactor population.

3. Thickness of the Crust of Mercury from Geoid-to-Topography Ratios

   Sebastiano Padovan, Mark A. Wieczorek, Jean-Luc Margot, and 2 more authors

   Geophysical Research Letters, 2015.

   abstract url

   To gain insight into the thickness of the crust of Mercury, we use gravity and topography data acquired by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging spacecraft to calculate geoid-to-topography ratios over the northern hemisphere of the planet. For an Airy model for isostatic compensation of variations in topography, we infer an average crustal thickness of 35 \textpm 18 km. Combined with the value of the radius of the core of Mercury, this crustal thickness implies that Mercury had the highest efficiency of crustal production among the terrestrial planets. From the measured abundance of heat-producing elements on the surface, we calculate that the heat production in the mantle from long-lived radioactive elements at 4.45 Ga was greater than 5.4 \texttimes 10-12 W/kg. By analogy with the Moon, the relatively thin crust of Mercury allows for the possibility that major impact events, such as the one that formed the Caloris basin, excavated material from Mercury’s mantle.

4. The Fractured Moon: Production and Saturation of Porosity in the Lunar Highlands from Impact Cratering

   Jason M. Soderblom, Alexander J. Evans, Brandon C. Johnson, and 14 more authors

   Geophysical Research Letters, 2015.

   abstract url

   We have analyzed the Bouguer anomaly (BA) of ~1200 complex craters in the lunar highlands from Gravity Recovery and Interior Laboratory observations. The BA of these craters is generally negative, though positive BA values are observed, particularly for smaller craters. Crater BA values scale inversely with crater diameter, quantifying how larger impacts produce more extensive fracturing and dilatant bulking. The Bouguer anomaly of craters larger than 93\thÀ1497 km in diameter is independent of crater size, indicating that there is a limiting depth to impact-generated porosity, presumably from pore collapse associated with either overburden pressure or viscous flow. Impact-generated porosity of the bulk lunar crust is likely in a state of equilibrium for craters smaller than ~30 km in diameter, consistent with an ~8 km thick lunar megaregolith, whereas the gravity signature of larger craters is still preserved and provides new insight into the cratering record of even the oldest lunar surfaces.

5. Gravitational Signatures of Lunar Floor-Fractured Craters

   Clément Thorey, Chloé Michaut, and Mark Wieczorek

   Earth and Planetary Science Letters, 2015.

   abstract url

   Lunar floor-fractured craters are impact craters characterized by distinctive shallow floors crossed by important networks of fractures. Different scenarios have been proposed to explain their formations but recent studies showed that the intrusion of magma at depth below the crater floor is the most plausible explanation. The intrusion of dense magma within the light upper-most part of the lunar crust should have left a positive signature in the gravity field. This study takes advantage of the unprecedented resolution of the lunar gravity field obtained from the NASA’s Gravity Recovery and Interior Laboratory (GRAIL) mission, in combination with topographic data obtained from the Lunar Orbiter Laser Altimeter (LOLA) instrument, to investigate the gravitational signatures of both normal and floor-fractured craters. Despite the large variability in their gravitational signatures, the floor-fractured and normal craters in the Highlands show significant differences: the gravitational anomalies are significantly larger at floorfractured craters. The anomaly amplitudes for floor-fractured craters are in agreement with synthetic gravity anomalies based on the predicted intrusion shapes from a theoretical flow model. Our results are consistent with magmatic intrusions intruding a crust characterized by a 12% porosity and where the intrusion has no porosity. Similar studies have been carried out in the lunar maria and South Pole–Aikten basin. Although the average gravitational signature of floor-fractured craters is larger than at normal craters in these regions, they cannot be distinguished statistically due to the small number of craters and the large variability of the anomalies. In general, a better characterization of the signal due solely to the initial impact crater is needed to isolate the magmatic intrusion signal and characterize the density contrast between the magma and crust.

6. Mercury’s Low-degree Geoid and Topography Controlled by Insolation-driven Elastic Deformation

   N. Tosi, O. Čadek, M. Běhounková, and 6 more authors

   Geophysical Research Letters, 2015.

   abstract url

   Mercury experiences an uneven insolation that leads to significant latitudinal and longitudinal variations of its surface temperature. These variations, which are predominantly of spherical harmonic degrees 2 and 4, propagate to depth, imposing a long-wavelength thermal perturbation throughout the mantle. We computed the accompanying density distribution and used it to calculate the mechanical and gravitational response of a spherical elastic shell overlying a quasi-hydrostatic mantle. We then compared the resulting geoid and surface deformation at degrees 2 and 4 with Mercury’s geoid and topography derived from the MErcury, Surface, Space ENvironment, GEochemistry, and Ranging spacecraft. More than 95% of the data can be accounted for if the thickness of the elastic lithosphere were between 110 and 180 km when the thermal anomaly was imposed. The obtained elastic thickness implies that Mercury became locked into its present 3:2 spin orbit resonance later than about 1 Gyr after planetary formation.

7. Gravity and Topography of the Terrestrial Planets

   M. A. Wieczorek

   In Treatise on Geophysics (Second Edition), 2015.

   abstract url

   This chapter reviews our current knowledge of the gravity and topography of the terrestrial planets and describes the methods that are used to analyze these data. A general review of the mathematical formalism that is used in describing gravity and topography is first given. Next, the basic properties of Earth, Venus, Mars, Mercury, and the Moon are characterized. Following this, the relationship between gravity and topography is quantified, and techniques by which geophysical parameters can be constrained are detailed. Analysis methods include crustal thickness modeling, geoid/topography ratios, spectral admittance and correlation functions, and localized spectral analysis and wavelet techniques. Finally, the major results that have been obtained by modeling the gravity and topography of Earth, Venus, Mars, Mercury, and the Moon are summarized.

2014

1. Petrological Constraints on the Density of the Martian Crust

   David Baratoux, Henri Samuel, Chloé Michaut, and 5 more authors

   Journal of Geophysical Research: Planets, 2014.

   abstract url

   New insights into the chemistry of the Martian crust have been made available since the derivation of crustal thickness maps from Mars Global Surveyor gravity and topography data that used a conservative range of density values (2700–3100 kg/m3). A new range of crustal density values is calculated from the major element chemistry of Martian meteorites (3100–3700 kg/m3), igneous rocks at Gusev crater (3100–3600 kg/m3) and from the surface concentration of Fe, Al, Ca, Si, and K measured by the Gamma-Ray Spectrometer on board Mars Odyssey (3250–3450 kg/m3). In addition, the density of mineral assemblages resulting from low-pressure crystallization of primary melts of the primitive mantle are estimated for plausible conditions of partial melting corresponding to the Noachian to Amazonian periods (3100–3300 kg/m3). Despite the differences between these approaches, the results are all consistent with an average density above 3100 kg/m3 for those materials that are close to the surface. The density may be compatible with the measured mass of Mars and the moment of inertia factor, but only if the average crustal thickness is thicker than previously thought (approaching 100 km). A thicker crust implies that crustal delamination and recycling could be possible and may even control its thickness, globally or locally. Alternatively, and considering that geoid-to-topography ratios argue against such a thick crust for the highlands, our results suggest the existence of a buried felsic or anorthositic component in the southern hemisphere of Mars.

2. GRAIL Gravity Constraints on the Vertical and Lateral Density Structure of the Lunar Crust

   Jonathan Besserer, Francis Nimmo, Mark A. Wieczorek, and 6 more authors

   Geophysical Research Letters, 2014.

   abstract url

   We analyzed data from the Gravity Recovery and Interior Laboratory (GRAIL) mission using a localized admittance approach to map out spatial variations in the vertical density structure of the lunar crust. Mare regions are characterized by a distinct decrease in density with depth, while the farside is characterized by an increase in density with depth at an average gradient of ∼35 kg m-3 km-1 and typical surface porosities of at least 20%. The Apollo 12 and 14 landing site region has a similar density structure to the farside, permitting a comparison with seismic velocity profiles. The interior of the South Pole-Aitken (SP-A) impact basin appears distinct with a near-surface low-density (porous) layer 2–3 times thinner than the rest of the farside. This result suggests that redistribution of material during the large SP-A impact likely played a major role in sculpting the lunar crust.

3. The Formation of Lunar Mascon Basins from Impact to Contemporary Form: Formation of Lunar Mascon Basins

   Andrew M. Freed, Brandon C. Johnson, David M. Blair, and 6 more authors

   Journal of Geophysical Research: Planets, 2014.

   abstract url

   Positive free-air gravity anomalies associated with large lunar impact basins represent a superisostatic mass concentration or “mascon.” High-resolution lunar gravity data from the Gravity Recovery and Interior Laboratory spacecraft reveal that these mascons are part of a bulls-eye pattern in which the central positive anomaly is surrounded by an annulus of negative anomalies, which in turn is surrounded by an outer annulus of positive anomalies. To understand the origin of this gravity pattern, we modeled numerically the entire evolution of basin formation from impact to contemporary form. With a hydrocode, we simulated impact excavation and collapse and show that during the major basin-forming era, the preimpact crust and mantle were sufficiently weak to enable a crustal cap to flow back over and cover the mantle exposed by the impact within hours. With hydrocode results as initial conditions, we simulated subsequent cooling and viscoelastic relaxation of topography using a finite element model, focusing on the mare-free Freundlich-Sharonov and mare-infilled Humorum basins. By constraining these models with measured free-air and Bouguer gravity anomalies as well as surface topography, we show that lunar basins evolve by isostatic adjustment from an initially subisostatic state following the collapse stage. The key to the development of a superisostatic inner basin center is its mechanical coupling to the outer basin that rises in response to subisostatic stresses, enabling the inner basin to rise above isostatic equilibrium. Our calculations relate basin size to impactor diameter and velocity, and they constrain the preimpact lunar thermal structure, crustal thickness, viscoelastic rheology, and, for the Humorum basin, the thickness of its postimpact mare fill.

4. A Long-Lived Lunar Dynamo Powered by Core Crystallization

   M. Laneuville, M.A. Wieczorek, D. Breuer, and 3 more authors

   Earth and Planetary Science Letters, 2014.

   abstract url

   The Moon does not possess an internally generated magnetic field at the present day, but extensive evidence shows that such a field existed between at least 4.2 and 3.56 Ga ago. The existence of a metallic lunar core is now firmly established, and we investigate the influence of inner core growth on generating a lunar core dynamo. We couple the results of a 3-D spherical thermochemical convection model of the lunar mantle to a 1-D thermodynamic model of its core. The energy and entropy budget of the core are computed to determine the inner core growth rate and its efficiency to power a dynamo. Sulfur is considered to be the main alloying element and we investigate how different sulfur abundances and initial core temperatures affect the model outcomes. For reasonable initial conditions, a solid inner core between 100 and 200 km is always produced. During its growth, a surface magnetic field of about 0.3 μT is generated and is predicted to last several billion years. Though most simulations predict the existence of a core dynamo at the present day, one way to stop magnetic field generation when the inner core is growing is by a transition between a bottom–up and top–down core crystallization scheme when the sulfur content becomes high enough in the outer core. According to this hypothesis, a model with about 6 to 8wt.%sulfur in the core would produce a 120–160 km inner core and explain the timing of the lunar dynamo as constrained by paleomagnetic data.

5. Effect of Ray and Speed Perturbations on Ionospheric Tomography by Over-the-Horizon Radar: A New Method

   Corinna Roy, Giovanni Occhipinti, Lapo Boschi, and 2 more authors

   Journal of Geophysical Research: Space Physics, 2014.

   abstract url

   Most recent methods in ionospheric tomography are based on the inversion of the total electron content measured by ground-based GPS receivers. As a consequence of the high frequency of the GPS signal and the absence of horizontal raypaths, the electron density structure is mainly reconstructed in the F2 region (300 km), where the ionosphere reaches the maximum of ionization, and is not sensitive to the lower ionospheric structure. We propose here a new tomographic method of the lower ionosphere, based on the full inversion of over-the-horizon (OTH) radar data. Previous studies using OTH radar for ionospheric tomography inverted only the leading edge echo curve of backscatter ionograms. The major advantage of our methodology is taking into account, numerically and jointly, the effect that the electron density perturbations induce not only in the speed of electromagnetic waves but also on the raypath geometry. This last point is extremely critical for OTH radar inversions as the emitted signal propagates through the ionosphere between a fixed starting point (the radar) and an unknown end point on the Earth surface where the signal is backscattered. We detail our ionospheric tomography method with the aid of benchmark tests. Having proved the necessity to take into account both effects simultaneously, we apply our method to real data. This is the first time that the effect of the raypath deflection has been quantified and that the ionospheric plasma density has been estimated over the entirety of Europe with an OTH radar.

6. Lunar Bulk Chemical Composition: A Post-Gravity Recovery and Interior Laboratory Reassessment

   G. Jeffrey Taylor, and Mark A. Wieczorek

   Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 2014.

   abstract url

   New estimates of the thickness of the lunar highlands crust based on data from the Gravity Recovery and Interior Laboratory mission, allow us to reassess the abundances of refractory elements in the Moon. Previous estimates of the Moon fall into two distinct groups: earthlike and a 50% enrichment in the Moon compared with the Earth. Revised crustal thicknesses and compositional information from remote sensing and lunar samples indicate that the crust contributes 1.13–1.85 wt% Al203 to the bulk Moon abundance. Mare basalt Al203 concentrations (8–10 wt%) and Al203 partitioning behaviour between melt and pyroxene during partial melting indicate mantle Al203 concentration in the range 1.3–3.1 wt%, depending on the relative amounts of pyroxene and olivine. Using crustal and mantle mass fractions, we show that that the Moon and the Earth most likely have the same (within 20%) concentrations of refractory elements. This allows us to use correlations between pairs of refractory and volatile elements to confirm that lunar abundances of moderately volatile elements such as K, Rb and Cs are depleted by 75% in the Moon compared with the Earth and that highly volatile elements, such as Tl and Cd, are depleted by 99%. The earthlike refractory abundances and depleted volatile abundances are strong constraints on lunar formation processes.

7. Lunar Interior Properties from the GRAIL Mission: Lunar Interior Properties

   James G. Williams, Alexander S. Konopliv, Dale H. Boggs, and 22 more authors

   Journal of Geophysical Research: Planets, 2014.

   abstract url

   The Gravity Recovery and Interior Laboratory (GRAIL) mission has sampled lunar gravity with unprecedented accuracy and resolution. The lunar GM, the product of the gravitational constant G and the mass M, is very well determined. However, uncertainties in the mass and mean density, 3345.56 \textpm 0.40 kg/m3, are limited by the accuracy of G. Values of the spherical harmonic degree-2 gravity coefficients J2 and C22, as well as the Love number k2 describing lunar degree-2 elastic response to tidal forces, come from two independent analyses of the 3 month GRAIL Primary Mission data at the Jet Propulsion Laboratory and the Goddard Space Flight Center. The two k2 determinations, with uncertainties of ~1%, differ by 1%; the average value is 0.02416 \textpm 0.00022 at a 1 month period with reference radius R = 1738 km. Lunar laser ranging (LLR) data analysis determines (C À A)/B and (B À A)/C, where A < B < C are the principal moments of inertia; the flattening of the fluid outer core; the dissipation at its solid boundaries; and the monthly tidal dissipation Q = 37.5 \textpm 4. The moment of inertia computation combines the GRAIL-determined J2 and C22 with LLR-derived (C À A)/B and (B À A)/C. The normalized mean moment of inertia of the solid Moon is Is/MR2 = 0.392728 \textpm 0.000012. Matching the density, moment, and Love number, calculated models have a fluid outer core with radius of 200–380 km, a solid inner core with radius of 0–280 km and mass fraction of 0–1%, and a deep mantle zone of low seismic shear velocity. The mass fraction of the combined inner and outer core is ≤1.5%.

2013

1. Ancient Igneous Intrusions and Early Expansion of the Moon Revealed by GRAIL Gravity Gradiometry

   Jeffrey C. Andrews-Hanna, Sami W. Asmar, James W. Head, and 16 more authors

   Science, 2013.

   abstract url

   The earliest history of the Moon is poorly preserved in the surface geologic record due to the high flux of impactors, but aspects of that history may be preserved in subsurface structures. Application of gravity gradiometry to observations by the Gravity Recovery and Interior Laboratory (GRAIL) mission results in the identification of a population of linear gravity anomalies with lengths of hundreds of kilometers. Inversion of the gravity anomalies indicates elongated positive-density anomalies that are interpreted to be ancient vertical tabular intrusions or dikes formed by magmatism in combination with extension of the lithosphere. Crosscutting relationships support a pre-Nectarian to Nectarian age, preceding the end of the heavy bombardment of the Moon. The distribution, orientation, and dimensions of the intrusions indicate a globally isotropic extensional stress state arising from an increase in the Moon’s radius by 0.6 to 4.9 kilometers early in lunar history, consistent with predictions of thermal models.

2. Le Magnétisme de la Lune

   David Cébron, Michael Le Bars, and Mark Wieczorek

   Le Magnétisme de la Lune / David Cébron, 2013.

   abstracturl

   Les échantillons de roches lunaires rapportés par les missions Apollo indiquent que la Lune présentait, dans le passé, un champ magnétique global. Son origine semble très différente de celle du champ magnétique terrestre.

3. Asymmetric Thermal Evolution of the Moon

   M. Laneuville, M. A. Wieczorek, D. Breuer, and 1 more author

   Journal of Geophysical Research: Planets, 2013.

   abstract url

   The Moon possesses a clear dichotomy in geological processes between the nearside and farside hemispheres. The most pronounced expressions of this dichotomy are the strong concentration of radioactive heat sources on the nearside in a region known as the Procellarum KREEP Terrane (PKT) and the mare basaltic lava flows that erupted in or adjacent to this terrane. We model the thermochemical evolution of the Moon using a 3-D spherical thermochemical convection code in order to assess the consequences of a layer enriched in heat sources below the PKT on the Moon’s global evolution. We find that in addition to localizing most of the melt production on the nearside, such an enriched concentration of heat sources in the PKT crust has an influence down to the core-mantle boundary and leaves a present-day temperature anomaly within the nearside mantle. Moderate gravitational and topographic anomalies that are predicted in the PKT, but not observed, may be masked either by crustal thinning or gravitational anomalies from dense material in the underlying mantle. Our models also predict crystallization of an inner core for sulfur concentrations less than 6 wt %.

4. The Origin of Lunar Mascon Basins

   H. J. Melosh, Andrew M. Freed, Brandon C. Johnson, and 8 more authors

   Science, 2013.

   abstract url

   High-resolution gravity data from the Gravity Recovery and Interior Laboratory spacecraft have clarified the origin of lunar mass concentrations (mascons). Free-air gravity anomalies over lunar impact basins display bull’s-eye patterns consisting of a central positive (mascon) anomaly, a surrounding negative collar, and a positive outer annulus. We show that this pattern results from impact basin excavation and collapse followed by isostatic adjustment and cooling and contraction of a voluminous melt pool. We used a hydrocode to simulate the impact and a self-consistent finite-element model to simulate the subsequent viscoelastic relaxation and cooling. The primary parameters controlling the modeled gravity signatures of mascon basins are the impactor energy, the lunar thermal gradient at the time of impact, the crustal thickness, and the extent of volcanic fill.

5. Asymmetric Distribution of Lunar Impact Basins Caused by Variations in Target Properties

   Katarina Miljković, Mark A. Wieczorek, Gareth S. Collins, and 7 more authors

   Science, 2013.

   abstract url

   Maps of crustal thickness derived from NASA’s Gravity Recovery and Interior Laboratory (GRAIL) mission revealed more large impact basins on the nearside hemisphere of the Moon than on its farside. The enrichment in heat-producing elements and prolonged volcanic activity on the lunar nearside hemisphere indicate that the temperature of the nearside crust and upper mantle was hotter than that of the farside at the time of basin formation. Using the iSALE-2D hydrocode to model impact basin formation, we found that impacts on the hotter nearside would have formed basins with up to twice the diameter of similar impacts on the cooler farside hemisphere. The size distribution of lunar impact basins is thus not representative of the earliest inner solar system impact bombardment.

6. The Crust of the Moon as Seen by GRAIL

   Mark A. Wieczorek, Gregory A. Neumann, Francis Nimmo, and 13 more authors

   Science, 2013.

   abstract url

   High-resolution gravity data obtained from the dual Gravity Recovery and Interior Laboratory (GRAIL) spacecraft show that the bulk density of the Moon’s highlands crust is 2550 kilograms per cubic meter, substantially lower than generally assumed. When combined with remote sensing and sample data, this density implies an average crustal porosity of 12% to depths of at least a few kilometers. Lateral variations in crustal porosity correlate with the largest impact basins, whereas lateral variations in crustal density correlate with crustal composition. The low-bulk crustal density allows construction of a global crustal thickness model that satisfies the Apollo seismic constraints, and with an average crustal thickness between 34 and 43 kilometers, the bulk refractory element composition of the Moon is not required to be enriched with respect to that of Earth.

7. Gravity Field of the Moon from the Gravity Recovery and Interior Laboratory (GRAIL) Mission

   Maria T. Zuber, David E. Smith, Michael M. Watkins, and 14 more authors

   Science, 2013.

   abstract url

   Spacecraft-to-spacecraft tracking observations from the Gravity Recovery and Interior Laboratory (GRAIL) have been used to construct a gravitational field of the Moon to spherical harmonic degree and order 420. The GRAIL field reveals features not previously resolved, including tectonic structures, volcanic landforms, basin rings, crater central peaks, and numerous simple craters. From degrees 80 through 300, over 98% of the gravitational signature is associated with topography, a result that reflects the preservation of crater relief in highly fractured crust. The remaining 2% represents fine details of subsurface structure not previously resolved. GRAIL elucidates the role of impact bombardment in homogenizing the distribution of shallow density anomalies on terrestrial planetary bodies.

2012

1. Scientific Preparations for Lunar Exploration

   James D. Carpenter, Ian A. Crawford, Charles Cockell, and 3 more authors

   Planetary and Space Science, 2012.

   url
2. Back to the Moon: The Scientific Rationale for Resuming Lunar Surface Exploration

   I.A. Crawford, M. Anand, C.S. Cockell, and 4 more authors

   Planetary and Space Science, 2012.

   abstract url

   The lunar geological record has much to tell us about the earliest history of the Solar System, the origin and evolution of the Earth–Moon system, the geological evolution of rocky planets, and the near-Earth cosmic environment throughout Solar System history. In addition, the lunar surface offers outstanding opportunities for research in astronomy, astrobiology, fundamental physics, life sciences and human physiology and medicine. This paper provides an interdisciplinary review of outstanding lunar science objectives in all of these different areas. It is concluded that addressing them satisfactorily will require an end to the 40-year hiatus of lunar surface exploration, and the placing of new scientific instruments on, and the return of additional samples from, the surface of the Moon. Some of these objectives can be achieved robotically (e.g., through targeted sample return, the deployment of geophysical networks, and the placing of antennas on the lunar surface to form radio telescopes). However, in the longer term, most of these scientific objectives would benefit significantly from renewed human operations on the lunar surface. For these reasons it is highly desirable that current plans for renewed robotic surface exploration of the Moon are developed in the context of a future human lunar exploration programme, such as that proposed by the recently formulated Global Exploration Roadmap.

3. Regolith Thickness over the Lunar Nearside: Results from Earth-based 70-Cm Arecibo Radar Observations

   Wenzhe Fa, and Mark A. Wieczorek

   Icarus, 2012.

   abstract url

   The inversion of regolith thickness over the nearside hemisphere of the Moon from newly acquired Earthbased 70-cm Arecibo radar data is investigated using a quantitative radar scattering model. The radar scattering model takes into account scattering from both the lunar surface and buried rocks in the lunar regolith, and three parameters are critically important in predicting the radar backscattering coefficient: the dielectric constant of the lunar regolith, the surface roughness, and the size and abundance of subsurface rocks. The measured dielectric properties of the Apollo regolith samples at 450 MHz are re-analyzed, and an improved relation among the complex dielectric constant, bulk density and regolith composition is obtained. The complex dielectric constant of the lunar regolith is estimated globally from this relation using the regolith composition derived from Lunar Prospector gamma-ray spectrometer data. To constrain the lunar surface roughness and abundance of subsurface rocks from radar data, nine regions are selected as calibration sites where the regolith thickness has been estimated using independent analysis techniques. For these sites, scattering from the lunar surface and buried rocks cannot be perfectly distinguished, and a tradeoff relationship exists between the size and abundance of buried rocks and surface roughness. Using these tradeoff relations as guidelines for globally representative parameters, the regolith thickness of four regions over the lunar nearside is inverted, and the inversion uncertainties caused by calibration errors of the radar data and model input parameters are analyzed. The regolith thickness of the maria is generally smaller than that of highlands, and older surfaces have thicker regolith thicknesses. Our approach cannot be applied to regions where the surface roughness is very high, such as with young rocky craters and regions in the highly rugged highlands.

4. Density and Lithospheric Structure at Tyrrhena Patera, Mars, from Gravity and Topography Data

   M. Grott, and M.A. Wieczorek

   Icarus, 2012.

   abstract url

   The Tyrrhena Patera highland volcano, Mars, is associated with a relatively well localized gravity anomaly and we have carried out a localized admittance analysis in the region to constrain the density of the volcanic load, the load thickness, and the elastic thickness at the time of load emplacement. The employed admittance model considers loading of an initially spherical surface, and surface as well as subsurface loading is taken into account. Our results indicate that the gravity and topography data available at Tyrrhena Patera is consistent with the absence of subsurface loading, but the presence of a small subsurface load cannot be ruled out. We obtain minimum load densities of 2960 kg mÀ3, minimum load thicknesses of 5 km, and minimum load volumes of 0.6 Â 106 km3. Photogeological evidence suggests that pyroclastic deposits make up at most 30% of this volume, such that the bulk of Tyrrhena Patera is likely composed of competent basalt. Best fitting model parameters are a load density of 3343 kg mÀ3, a load thickness of 10.8 km, and a load volume of 1.7 Â 106 km3. These relatively large load densities indicate that lava compositions are comparable to those at other martian volcanoes, and densities are comparable to those of the martian meteorites. The elastic thickness in the region is constrained to be smaller than 27.5 km at the time of loading, indicating surface heat flows in excess of 24 mW mÀ2.

5. Density and Porosity of the Lunar Crust from Gravity and Topography

   Qian Huang, and Mark A. Wieczorek

   Journal of Geophysical Research: Planets, 2012.

   abstract url

   Newly obtained gravity and topography data of the Moon, combined with a lithospheric flexure model that considers both surface and subsurface loading, are used to place constraints on the density of the upper crust from a localized spectral admittance analysis. Subsurface loads are found to be relatively unimportant in the highlands, and when subsurface loads are neglected, the best fitting bulk densities for a number of highland regions are found to vary from 2590 to 2870 kg m 3, with a mean value of 2691 kg m 3. Crustal rock densities estimated from geochemical considerations and global iron and titanium abundances imply somewhat greater densities, which we interpret as porosity affecting the gravity-derived bulk density estimates. The average porosity in the upper few kilometers of crust is calculated to be about 7.7%, which is consistent with porosity estimates of impact-fractured meteorites and terrestrial impact craters.

6. Geology, Geochemistry, and Geophysics of the Moon: Status of Current Understanding

   R. Jaumann, H. Hiesinger, M. Anand, and 18 more authors

   Planet. Space Sci., 2012.

   abstract url

   The Moon is key to understanding both Earth and our Solar System in terms of planetary processes and has been a witness of the Solar System history for more than 4.5 Ga. Building on earlier telescopic observations, our knowledge about the Moon was transformed by the wealth of information provided by Apollo and other space missions. These demonstrated the value of the Moon for understanding the fundamental processes that drive planetary formation and evolution. The Moon was understood as an inert body with its geology mainly restricted to impact and volcanism with associated tectonics, and a relative simple composition. Unlike Earth, an absence of plate tectonics has preserved a well-defined accretion and geological evolution record. However recent missions to the Moon show that this traditional view of the lunar surface is certainly an over simplification. For example, although it has long been suspected that ice might be preserved in cold traps at the lunar poles, recent results also indicate the formation and retention of OHÀ and H2O outside of polar regions. These volatiles are likely to be formed as a result of hydration processes operating at the lunar surface including the production of H2O and OH by solar wind protons interacting with oxygen-rich rock surfaces produced during micrometeorite impact on lunar soil particles. Moreover, on the basis of Lunar Prospector gamma-ray data, the lunar crust and underlying mantle has been found to be divided into distinct terranes that possess unique geochemical, geophysical, and geological characteristics. The concentration of heat producing elements on the nearside hemisphere of the Moon in the Procellarum KREEP Terrane has apparently led to the nearside being more volcanically active than the farside. Recent dating of basalts has shown that lunar volcanism was active for almost 3 Ga, starting at about 3.9–4.0 Ga and ceasing at $ 1.2 Ga. A recent re-processing of the seismic data supports the presence of a partially molten layer at the base of the mantle and shows not only the presence of a 330 km liquid core, but also a small solid inner core. Today, the Moon does not have a dynamo-generated magnetic field like that of the Earth. However, remnant magnetization of the lunar crust and the paleomagnetic record of some lunar samples suggest that magnetization was acquired, possibly from an intrinsic magnetic field caused by an early lunar core dynamo. In summary, the Moon is a complex differentiated planetary object and much remains to be explored and discovered, especially regarding the origin of the Moon, the history of the Earth–Moon system, and processes that have operated in the inner Solar System over the last 4.5 Ga. Returning to the Moon is therefore the critical next stepping-stone to further exploration and understanding of our planetary neighborhood.

7. Farside Explorer: Unique Science from a Mission to the Farside of the Moon

   David Mimoun, Mark A. Wieczorek, Leon Alkalai, and 27 more authors

   Experimental Astronomy, 2012.

   abstract url

   Farside Explorer is a proposed Cosmic Vision medium-size mission to the farside of the Moon consisting of two landers and an instrumented relay satellite. The farside of the Moon is a unique scientific platform in that it is shielded from terrestrial radio-frequency interference, it recorded the primary differentiation and evolution of the Moon, it can be continuously monitored from the Earth–Moon L2 Lagrange point, and there is a complete lack of reflected solar illumination from the Earth. Farside Explorer will exploit these properties and make the first radio-astronomy measurements from the most radio-quiet region of near-Earth space, determine the internal structure and thermal evolution of the Moon, from crust to core, and quantify impact hazards in near-Earth space by the measurement of flashes generated by impact events. The Farside Explorer flight system includes two identical solarpowered landers and a science/telecommunications relay satellite to be placed in a halo orbit about the Earth–Moon L2 Lagrange point. One lander would explore the largest and oldest recognized impact basin in the Solar System—the South Pole–Aitken basin—and the other would investigate the primordial highlands crust. Radio astronomy, geophysical, and geochemical instruments would be deployed on the surface, and the relay satellite would continuously monitor the surface for impact events.

8. Lunar Net—a Proposal in Response to an ESA M3 Call in 2010 for a Medium Sized Mission

   Alan Smith, I. A. Crawford, Robert Anthony Gowen, and 58 more authors

   Experimental Astronomy, 2012.

   abstract url

   Emplacement of four or more kinetic penetrators geographically distributed over the lunar surface can enable a broad range of scientific exploration objectives of high priority and provide significant synergy with planned orbital missions. Whilst past landed missions achieved a great deal, they have not included a far-side lander, or investigation of the lunar interior apart from a very small area on the near side. Though the LCROSS mission detected water from a permanently shadowed polar crater, there remains in-situ confirmation, knowledge of concentration levels, and detailed identification of potential organic chemistry of astrobiology interest. The planned investigations will also address issues relating to the origin and evolution of the Earth–Moon system and other Solar System planetary bodies. Manned missions would be enhanced with use of water as a potential in-situ resource; knowledge of potential risks from damaging surface Moonquakes, and exploitation of lunar regolith for radiation shielding. LunarNet is an evolution of the 2007 LunarEX proposal to ESA (European Space Agency) which draws on recent significant advances in mission definition and feasibility. In particular, the successful Pendine full-scale impact trials have proved impact survivability for many of the key technology items, and a penetrator system study has greatly improved the definition of descent systems, detailed penetrator designs, and required resources. LunarNet is hereby proposed as an exciting stand-alone mission, though is also well suited in whole or in-part to contribute to the jigsaw of upcoming lunar missions, including that of a significant element to the ILN (International Lunar Network).

9. The Chandrayaan-1 X-ray Spectrometer: First Results

   S.Z. Weider, B.J. Kellett, B.M. Swinyard, and 30 more authors

   Planetary and Space Science, 2012.

   abstract url

   We present X-ray fluorescence observations of the lunar surface, made by the Chandrayaan-1 X-ray Spectrometer during two solar flare events early in the mission (12th December 2008 and 10th January 2009). Modelling of the X-ray spectra with an abundance algorithm allows quantitative estimates of the MgO/SiO2 and Al2O3/SiO2 ratios to be made for the two regions, which are in mainly basaltic areas of the lunar nearside. One of these ground tracks includes the Apollo 14 landing site on the Fra Mauro Formation. Within the 1s errors provided, the results are inside the range of basaltic samples from the Apollo and Luna collections. The Apollo 14 soil composition is in agreement with the results from the January flare at the 1s uncertainty level. Discrepancies are observed between our results and compositions derived for the same areas by the Lunar Prospector gamma-ray spectrometer; some possible reasons for this are discussed.

10. Mercury’s Spin–Orbit Resonance Explained by Initial Retrograde and Subsequent Synchronous Rotation

    Mark A. Wieczorek, Alexandre C. M. Correia, Mathieu Le Feuvre, and 2 more authors

    Nature Geoscience, 2012.

    abstract url

    The planet Mercury rotates three times about its spin axis for every two orbits about the Sun1,2, in a 3/2 spin–orbit resonance. This unique state has been explained by an initial rapid prograde rotation, which was then decelerated by tidal torques to the present resonance3–6. When friction at the core–mantle boundary is accounted for, capture into the 3/2 resonance occurs with a probability of only 26%, whereas the most likely outcome is capture into one of the higher-order resonances7. Here we use a numerical model of Mercury’s rotational evolution to investigate the consequences of an initial retrograde rotation of Mercury. We find that in this case, the planet would be captured into synchronous rotation, with one hemisphere always facing the Sun, with a probability of 68%. Strong lateral variations in the impact cratering rate would have existed, consistent with the observed distribution of large impact basins. Escape from this highly stable resonance can be initiated by the momentum imparted by large, basin-forming impact events8–10, and subsequent capture into the 3/2 resonance is likely. During synchronous rotation, substantial quantities of volatile deposits would have accumulated on the hemisphere facing away from the Sun, potentially explaining the existence of sublimation hollows on Mercury’s surface11.

11. An Impactor Origin for Lunar Magnetic Anomalies

    Mark A. Wieczorek, Benjamin P. Weiss, and Sarah T. Stewart

    Science, 2012.

    abstract url

    The Moon possesses strong magnetic anomalies that are enigmatic given the weak magnetism of lunar rocks. We show that the most prominent grouping of anomalies can be explained by highly magnetic extralunar materials from the projectile that formed the largest and oldest impact crater on the Moon: the South Pole–Aitken basin. The distribution of projectile materials from a model oblique impact coincides with the distribution of magnetic anomalies surrounding this basin, and the magnetic properties of these materials can account for the intensity of the observed anomalies if they were magnetized in a core dynamo field. Distal ejecta from this event can explain the origin of isolated magnetic anomalies far from this basin.

2011

1. Modeling Polarimetric Radar Scattering from the Lunar Surface: Study on the Effect of Physical Properties of the Regolith Layer

   Wenzhe Fa, Mark A. Wieczorek, and Essam Heggy

   Journal of Geophysical Research Planets, 2011.

   abstract url

   A theoretical model for radar scattering from the lunar regolith using the vector radiative transfer theory for random media has been developed in order to aid in the interpretation of Mini-SAR data from the Chandrayaan-1 and Lunar Reconnaissance Orbiter missions. The lunar regolith is represented as a homogeneous fine-grained layer with rough upper and lower parallel interfaces that possesses embedded inclusions with a different dielectric constant. Our model considers five scattering mechanisms in the regolith layer: diffuse scattering from both the surface and subsurface, volume scattering from buried inclusions, and the interactions of scattering between buried inclusions and the rough interfaces (both the lunar surface and subsurface). Multiple scattering between buried inclusions and coherent backscatter opposite effect are not considered in the current model. The modeled radar scattering coefficients are validated using numerical finite difference time domain simulations and are compared with incident angle–averaged Earth-based radar observations of the Moon. Both polarized and depolarized radar backscattering coefficients and the circular polarization ratio (CPR) are calculated as a function of incidence angle, regolith thickness, surface and subsurface roughness, surface slope, abundance and shape of buried rocks, and the FeO+TiO2 content of the regolith. Simulation results show that the polarized (opposite sense) radar echo strength at S and X bands is mostly dominated by scattering from the rough surface and buried rocks, while the depolarized (same sense) radar echo strength is dominated by scattering from buried rocks or ice inclusions. Finally, to explore the expected polarimetric signature of ice in the polar permanently shadowed areas, four parametric regolith models are considered and the possibility of detecting diffuse ice inclusions by the CPR is addressed. Our study suggests that detection of ice inclusions at the lunar poles using solely the CPR will be difficult given the small dielectric contrast between the regolith and ice.

2. An Impact-Driven Dynamo for the Early Moon

   M. Le Bars, M. A. Wieczorek, Ö. Karatekin, and 2 more authors

   Nature, 2011.

   abstract url

   The origin of lunar magnetic anomalies1–5 remains unresolved after their discovery more than four decades ago. A commonly invoked hypothesis is that the Moon might once have possessed a thermally driven core dynamo3, but this theory is problematical given the small size of the core and the required surface magnetic field strengths6. An alternative hypothesis is that impact events might have amplified ambient fields near the antipodes of the largest basins7, but many magnetic anomalies exist that are not associated with basin antipodes. Here we propose a new model for magnetic field generation, in which dynamo action comes from impact-induced changes in the Moon’s rotation rate. Basinforming impact events are energetic enough to have unlocked the Moon from synchronous rotation8, and we demonstrate that the subsequent large-scale fluid flows in the core, excited by the tidal distortion of the core–mantle boundary9, could have powered a lunar dynamo. Predicted surface magnetic field strengths are on the order of several microteslas, consistent with palaeomagnetic measurements5, and the duration of these fields is sufficient to explain the central magnetic anomalies associated with several large impact basins.

3. Nonuniform Cratering of the Moon and a Revised Crater Chronology of the Inner Solar System

   Mathieu Le Feuvre, and Mark A. Wieczorek

   Icarus, 2011.

   abstract url

   We model the cratering of the Moon and terrestrial planets from the present knowledge of the orbital and size distribution of asteroids and comets in the inner Solar System, in order to refine the crater chronology method. Impact occurrences, locations, velocities and incidence angles are calculated semi-analytically, and scaling laws are used to convert impactor sizes into crater sizes. Our approach is generalizable to other moons or planets. The lunar cratering rate varies with both latitude and longitude: with respect to the global average, it is about 25% lower at (\textpm65^∘N, 90^∘E) and larger by the same amount at the apex of motion (0^∘N, 90^∘W) for the present Earth–Moon separation. The measured size-frequency distributions of lunar craters are reconciled with the observed population of near-Earth objects under the assumption that craters smaller than a few kilometers in diameter form in a porous megaregolith. Varying depths of this megaregolith between the mare and highlands is a plausible partial explanation for differences in previously reported measured size-frequency distributions. We give a revised analytical relationship between the number of craters and the age of a lunar surface. For the inner planets, expected size-frequency crater distributions are calculated that account for differences in impact conditions, and the age of a few key geologic units is given. We estimate the Orientale and Caloris basins to be 3.73 Ga old, and the surface of Venus to be 240 Ma old. The terrestrial cratering record is consistent with the revised chronology and a constant impact rate over the last 400 Ma. Better knowledge of the orbital dynamics, crater scaling laws and megaregolith properties are needed to confidently assess the net uncertainty of the model ages that result from the combination of numerous steps, from the observation of asteroids to the formation of craters. Our model may be inaccurate for periods prior to 3.5 Ga because of a different impactor population, or for craters smaller than a few kilometers on Mars and Mercury, due to the presence of subsurface ice and to the abundance of large secondaries, respectively. Standard parameter values allow for the first time to naturally reproduce both the size distribution and absolute number of lunar craters up to 3.5 Ga ago, and give self-consistent estimates of the planetary cratering rates relative to the Moon.

4. Lunar X-ray Fluorescence Observations by the Chandrayaan-1 X-ray Spectrometer (C1XS): Results from the Nearside Southern Highlands

   S. Narendranath, P.S. Athiray, P. Sreekumar, and 18 more authors

   Icarus, 2011.

   abstract url

   The Chandrayaan-1 X-ray Spectrometer (C1XS) flown on-board the first Indian lunar mission Chandrayaan-1, measured X-ray fluorescence spectra during several episodes of solar flares during its operational period of $9 months. The accompanying X-ray Solar Monitor (XSM) provided simultaneous spectra of solar X-rays incident on the Moon which are essential to derive elemental chemistry. In this paper, we present the surface abundances of Mg, Al, Si, Ca and Fe, derived from C1XS data for a highland region on the southern nearside of the Moon. Analysis techniques are described in detail including absolute X-ray line flux derivation and conversion into elemental abundance. The results are consistent with a composition rich in plagioclase with a slight mafic mineral enhancement and a Ca/Al ratio that is significantly lower than measured in lunar returned samples. We suggest various possible scenarios to explain the deviations.

2010

1. Structure and Formation of the Lunar Farside Highlands

   Ian Garrick-Bethell, Francis Nimmo, and Mark A. Wieczorek

   Science, 2010.

   abstract url

   The formation of the lunar farside highlands has long been an open problem in lunar science. We show that much of the topography and crustal thickness in this terrain can be described by a degree-2 harmonic. No other portion of the Moon exhibits comparable degree-2 structure. The quantified structure of the farside highlands unites them with the nearside and suggests a relation between lunar crustal structure, nearside volcanism, and heat-producing elements. The farside topography cannot be explained by a frozen-in tidal bulge. However, the farside crustal thickness and the topography it produces may have been caused by spatial variations in tidal heating when the ancient crust was decoupled from the mantle by a liquid magma ocean, similar to Europa’s present ice shell.

2009

1. Compositional Variations of the Lunar Crust: Results from Radiative Transfer Modeling of Central Peak Spectra

   J. T. S. Cahill, P. G. Lucey, and M. A. Wieczorek

   Journal of Geophysical Research Planets, 2009.

   abstract url

   We present model mineralogy of impact crater central peaks combined with crustal thickness and crater central peak depth of origin models to report multiple perspectives of lunar crustal composition with depth. Here we report the analyses of 55 impact crater central peaks and how their compositions directly relate to the lunar highlands sample suite. A radiative transfer model is used to analyze Clementine visible plus near-infrared spectra to place compositional constraints on these central peak materials. Central peaks analyzed are dominantly magnesian- and plagioclase-poor; strong compositional similarities to lunar Mg-suite materials are evident. Relative to crustal thickness estimates, central peak mineralogy becomes more plagioclase-rich as the crust thickens. Relative to the crust-mantle boundary, the origin of peaks with dominantly mafic mineralogy are confined to the lower crust and primarily within the South-Pole Aitken and Procellarum KREEP Terranes (PKT); additionally, central peaks with anorthositic mineralogy (>60 vol % plagioclase) are transported to the surface from all depths in the crustal column and confined to the Feldspathic Highlands Terrane (FHT). The discovery of mafic and magnesian materials, consistent with Mg-suite rocks of the sample collection, in all lunar terranes suggests that the process and sources that give rise to these types of rocks is not unique to the PKT and not necessarily dependent on incompatible elements for formation. The identification of ferroan and magnesian anorthositic material near the crust-mantle boundary of the FHT is also inconsistent with an increasing mafic/feldspar ratio and Mg’ with depth in the crust.

2. The Scientific Rationale for the C1XS X-ray Spectrometer on India’s Chandrayaan-1 Mission to the Moon

   I.A. Crawford, K.H. Joy, B.J. Kellett, and 23 more authors

   Planetary and Space Science, 2009.

   abstract url

   The UK-built Chandrayaan-1 X-ray Spectrometer (C1XS) will fly as an ESA instrument on India’s Chandrayaan-1 mission to the Moon, launched in October 2008. C1XS builds on experience gained with the earlier D-CIXS instrument on SMART-1, but will be a scientifically much more capable instrument. Here we describe the scientific objectives of this instrument, which include mapping the abundances of the major rock-forming elements (principally Mg, Al, Si, Ti, Ca and Fe) in the lunar crust. These data will aid in determining whether regional compositional differences (e.g., the Mg/Fe ratio) are consistent with models of lunar crustal evolution. C1XS data will also permit geochemical studies of smaller scale features, such as the ejecta blankets and central peaks of large impact craters, and individual lava flows and pyroclastic deposits. These objectives all bear on important, and currently unresolved, questions in lunar science, including the structure and evolution of any primordial magma ocean, as revealed by vertical and lateral geochemical variations in the crust, and the composition of the lunar mantle, which will further constrain theories of the Moon’s origin, thermal history and internal structure.

3. The C1XS X-ray Spectrometer on Chandrayaan-1

   M. Grande, B.J. Maddison, C.J. Howe, and 27 more authors

   Planetary and Space Science, 2009.

   abstract url

   The Chandrayaan-1 X-ray Spectrometer (C1XS) is a compact X-ray spectrometer for the Indian Space Research Organisation (ISRO) Chandrayaan-1 lunar mission. It exploits heritage from the D-CIXS instrument on ESA’s SMART-1 mission. As a result of detailed developments to all aspects of the design, its performance as measured in the laboratory greatly surpasses that of D-CIXS. In comparison with SMART-1, Chandrayaan-1 is a science-oriented rather than a technology mission, leading to far more favourable conditions for science measurements. C1XS is designed to measure absolute and relative abundances of major rock-forming elements (principally Mg, Al, Si, Ca and Fe) in the lunar crust with spatial resolution p25 FWHM km, and to achieve relative elemental abundances of better than 10%.

4. LunarEX—a Proposal to Cosmic Vision

   A. Smith, I. A. Crawford, R. A. Gowen, and 36 more authors

   Experimental Astronomy, 2009.

   abstract url

   While the surface missions to the Moon of the 1970s achieved a great deal, scientifically much was also left unresolved. The recent plethora of lunar missions (flown or proposed) reflects a resurgence in interest in the Moon, not only in its own right, but also as a record of the early solar system including the formation of the Earth. Results from recent orbiter missions have shown evidence of ice or at least hydrogen within shadowed craters at the lunar poles.

5. X-Ray Fluorescence Observations of the Moon by SMART-1/D-CIXS and the First Detection of Ti Kα from the Lunar Surface

   B.M. Swinyard, K.H. Joy, B.J. Kellett, and 9 more authors

   Planetary and Space Science, 2009.

   abstract url

   The demonstration of a compact imaging X-ray spectrometer (D-CIXS), which flew on ESA’s SMART-1 mission to the Moon (Racca et al., 2001; Foing et al., 2006), was designed to test innovative new technologies for orbital X-ray fluorescence spectroscopy. D-CIXS conducted observations of the lunar surface from January 2005 until SMART-1 impacted the Moon in September 2006. Here, we present scientific observations made during two solar flare events and show the first detection of Titanium Ka from the lunar surface. We discuss the geological implications of these results. We also discuss how experience from D-CIXS has aided the design of a similar instrument (Chandrayaan-1 X-ray Spectrometer (C1XS)) that was launched on the 22nd October 2008 on India’s Chandrayaan-1 mission to the Moon.

6. Did a Large Impact Reorient the Moon?

   Mark A. Wieczorek, and Mathieu Le Feuvre

   Icarus, 2009.

   abstract url

   The Moon is currently locked in a spin–orbit resonance of synchronous rotation, of which one consequence is that more impacts should occur near the Moon’s apex of motion (0\textopenbullet N, 90\textopenbullet W) than near its antapex of motion (0\textopenbullet N, 90\textopenbullet E). Several of the largest lunar impact basins could have temporarily unlocked the Moon from synchronous rotation, and after the re-establishment of this state the Moon would have been left in either its initial orientation, or one that was rotated 180\textopenbullet about its spin axis. We show that there is less than a 2% probability that the oldest lunar impact basins are randomly distributed across the lunar surface. Furthermore, these basins are preferentially located near the Moon’s antapex of motion, and this configuration has less than a 0.3% probability of occurring by chance. We postulate that the current “near side” of the Moon was in fact its “far side” when the oldest basins formed. One basin with the required size and temporal characteristics to account for a 180\textopenbullet reorientation is the Smythii basin.

7. The Interior Structure of the Moon: What Does Geophysics Have to Say?

   M. A. Wieczorek

   Elements, 2009.

   abstract url

   Geophysical data obtained from orbit and surface stations show that the Moon is a differentiated body possessing a crust, mantle, and core. The crust is on average about 40 km thick, and impact events with asteroids and comets have excavated materials to great depths within the crust. Moonquakes that are correlated in time with Earth-raised tides occur about halfway to the center of the Moon and suggest that the deepest portion of the mantle might be partially molten. The lunar core is relatively small in comparison with the cores of the terrestrial planets, with a size less than one-quarter of the Moon’s radius.

2008

1. Nonuniform Cratering of the Terrestrial Planets

   M. Le Feuvre, and M. A. Wieczorek

   Icarus, 2008.

   abstract url

   We estimate the impact flux and cratering rate as a function of latitude on the terrestrial planets using a model distribution of planet crossing asteroids and comets [Bottke, W.F., Morbidelli, A., Jedicke, R., Petit, J.-M., Levison, H.F., Michel, P., Metcalfe, T.S., 2002. Icarus 156, 399–433]. After determining the planetary impact probabilities as a function of the relative encounter velocity and encounter inclination, the impact positions are calculated analytically, assuming the projectiles follow hyperbolic paths during the encounter phase. As the source of projectiles is not isotropic, latitudinal variations of the impact flux are predicted: the calculated ratio between the pole and equator is 1.05 for Mercury, 1.00 for Venus, 0.96 for the Earth, 0.90 for the Moon, and 1.14 for Mars over its long-term obliquity variation history. By taking into account the latitudinal dependence of the impact velocity and impact angle, and by using a crater scaling law that depends on the vertical component of the impact velocity, the latitudinal variations of the cratering rate (the number of craters with a given size formed per unit time and unit area) is in general enhanced. With respect to the equator, the polar cratering rate is about 30% larger on Mars and 10% on Mercury, whereas it is 10% less on the Earth and 20% less on the Moon. The cratering rate is found to be uniform on Venus. The relative global impact fluxes on Mercury, Venus, the Earth and Mars are calculated with respect to the Moon, and we find values of 1.9, 1.8, 1.6, and 2.8, respectively. Our results show that the relative shape of the crater size-frequency distribution does not noticeably depend upon latitude for any of the terrestrial bodies in this study. Nevertheless, by neglecting the expected latitudinal variations of the cratering rate, systematic errors of 20–30% in the age of planetary surfaces could exist between equatorial and polar regions when using the crater chronology method.

2. Constraints on the Composition of the Martian South Polar Cap from Gravity and Topography

   M Wieczorek

   Icarus, 2008.

   abstract url

   The polar caps of Mars have long been acknowledged to be composed of unknown proportions of water ice, solid CO2 (dry ice), and dust. Gravity and topography data are here analyzed over the southern cap to place constraints on its density, and hence composition. Using a localized spectral analysis combined with a lithospheric flexure model of ice cap loading, the best fit density of the volatile-rich south polar layered deposits is found to be 1271 kg m-3 with 1-σ limits of 1166 and 1391 kg m-3. The best fit elastic thickness of this geologically young deposit is 140 km, though any value greater than 102 km can fit the observations. The best fit density implies that about 55% dry ice by volume could be sequestered in these deposits if they were completely dust free. Alternatively, if these deposits were completely free of solid CO2, the dust content would be constrained to lie between about 14 and 28% by volume. The bulk thermal conductivity of the polar cap is not significantly affected by these maximum allowable concentrations of dust. However, even if a moderate quantity of solid CO2 were present as horizontal layers, the bulk thermal conductivity of the polar cap would be significantly reduced. Reasonable estimates of the present day heat flow of Mars predict that dry ice beneath the thicker portions of the south polar cap would have melted. Depending on the quantity of solid CO2 in these deposits today, it is even possible that water ice could melt where the cap is thickest. If independent estimates for either the dust or CO2 content of the south polar cap could be obtained, and if radar sounding data could determine whether this polar cap is presently experiencing basal melting or not, it would be possible to use these observations to place tight constraints on the present day heat flow of Mars.

2007

1. The D-CIXS X-ray Spectrometer on the SMART-1 Mission to the Moon—First Results

   M. Grande, B.J. Kellett, C. Howe, and 28 more authors

   Planet. Space Sci., 2007.

   abstract url

   The SMART-1 mission has recently arrived at the Moon. Its payload includes D-CIXS, a compact X-ray spectrometer. SMART-1 is a technology evaluation mission, and D-CIXS is the first of a new generation of planetary X-ray spectrometers. Novel technologies enable new capabilities for measuring the fluorescent yield of a planetary surface or atmosphere which is illuminated by solar X-rays. During the extended SMART-1 cruise phase, observations of the Earth showed strong argon emission, providing a good source for calibration and demonstrating the potential of the technique. At the Moon, our initial observations over Mare Crisium show a first unambiguous remote sensing of calcium in the lunar regolith. Data obtained are broadly consistent with current understanding of mare and highland composition. Ground truth is provided by the returned Luna 20 and 24 sample sets.

2. Crustal Thickness of the Moon: New Constraints from Gravity Inversions Using Polyhedral Shape Models

   Hajime Hikida, and Mark A. Wieczorek

   Icarus, 2007.

   abstract url

   A new method is presented for estimating crustal thickness from gravity and topography data on the Moon. By calculating analytically the exterior gravitational field for a set of arbitrarily shaped polyhedra, relief along the crust–mantle interface can be inverted for that satisfies the observational constraints. As this method does not rely upon filtering the Bouguer anomaly, which was required with previous inversions performed in the spherical-harmonic domain, and as the dramatic variations in spatial quality of the lunar gravity field are taken into account, our crustal thickness model more faithfully represents the available data. Using our model results, we investigate various aspects of the prominent nearside impact basins. The crustal thickness in the central portion of the Orientale and Crisium basins is found to be close to zero, suggesting that these basins could have conceivably excavated into the lunar mantle. Furthermore, given our uncertain knowledge of the density of the crust and mantle, it is possible that the Humorum, Humboldtianum, Nectaris, and Smythii basins could have excavated all the way through the crust as well. The crustal structure for most of the young impact basins implies a depth/diameter ratio of about 0.08 for their excavation cavities. As noted in previous studies, however, the crustal structure of Imbrium and Serenitatis is anomalous, which is conceivably a result of enhanced rates of post-impact viscous relaxation caused by the proximity of these basins to the Procellarum KREEP Terrane. Impact basins older than Smythii show little or no evidence for crustal thinning, suggesting that these ancient basins were also affected by high rates of viscous relaxation resulting from higher crustal temperatures early in the Moon’s evolution. The lithosphere beneath many young basins is found to be supporting a downward directed force, even after the load associated with the mare basalts is removed, and this is plausibly attributed to superisostatic uplift of the crust–mantle interface. Those basins that are close to achieving a pre-mare isostatic state are generally found to reside within, or close to, the Procellarum KREEP Terrane.

3. The BepiColombo Laser Altimeter (BELA): Concept and Baseline Design

   N. Thomas, T. Spohn, J.-P. Barriot, and 24 more authors

   Planetary and Space Science, 2007.

   abstract url

   The BepiColombo Laser Altimeter (BELA) has been selected for flight on board the European Space Agency’s BepiColombo Mercury Planetary Orbiter (MPO). The experiment is intended to be Europe’s first planetary laser altimeter system. Although the proposed system has similarities to the Mercury Laser Altimeter (MLA) currently flying on board NASA’s MESSENGER mission to Mercury, the specific orbit and construction of the MPO force the use of novel concepts for BELA. Furthermore, the base-lined range-finding approach is novel. In this paper, we describe the BELA system and show preliminary results from some prototype testing.

4. Minimum-Variance Multitaper Spectral Estimation on the Sphere

   Mark A. Wieczorek, and Frederik J. Simons

   Journal of Fourier Analysis and Applications, 2007.

   abstract url

   We develop a method to estimate the power spectrum of a stochastic process on the sphere from data of limited geographical coverage. Our approach can be interpreted either as estimating the global power spectrum of a stationary process when only a portion of the data are available for analysis, or estimating the power spectrum from local data under the assumption that the data are locally stationary in a specified region. Restricting a global function to a spatial subdomain — whether by necessity or by design — is a windowing operation, and an equation like a convolution in the spectral domain relates the expected value of the windowed power spectrum to the underlying global power spectrum and the known power spectrum of the localization window. The best windows for the purpose of localized spectral analysis have their energy concentrated in the region of interest while possessing the smallest effective bandwidth as possible. Solving an optimization problem in the sense of Slepian (1960) yields a family of orthogonal windows of diminishing spatiospectral localization, the best concentrated of which we propose to use to form a weighted multitaper spectrum estimate in the sense of Thomson (1982). Such an estimate is both more representative of the target region and reduces the estimation variance when compared to estimates formed by any single bandlimited window. We describe how the weights applied to the individual spectral estimates in forming the multitaper estimate can be chosen such that the variance of the estimate is minimized.

5. Gravity and Topography of the Terrestrial Planets

   M. A. Wieczorek

   In Treatise on Geophysics, 2007.

   abstract url

   This chapter reviews our current knowledge of the gravity and topography of the terrestrial planets and describes the methods that are used to analyze these data. First, a general review of the mathematical formalism that is used in describing the properties of gravitational fields and topography is given. Next, the basic properties of the gravity and topography of the Earth, Venus, Mars, and the Moon are characterized. Following this, the relationship between gravity and topography is quantified, and techniques by which geophysical parameters can be constrained are detailed. Such analysis methods include crustal thickness modeling, the analysis of geoid/topography ratios, and modeling of the spectral admittance and correlation functions. Finally, the major results that have been obtained by modeling the gravity and topography of the Earth, Venus, Mars, and the Moon are summarized.

2006

1. Lateral Variations of Lunar Crustal Thickness from the Apollo Seismic Data Set

   H Chenet, P Lognonne, M Wieczorek, and 1 more author

   Earth and Planetary Science Letters, 2006.

   abstract url

   Seismic waves generated by meteoroid impacts have been detected by the Apollo lunar seismic network. These waves are sensitive to the crustal structure beneath both the seismic stations and the impact sites. We use a Markov chain Monte-Carlo algorithm in order to invert for lateral variations of crustal thickness on the Moon. The inversion uses travel times of seismic waves originating from multiple meteoroid impact locations. Previous seismic investigations constrained the crustal thickness solely for Apollo stations 12, 14 and 16, whereas our approach enables to estimate the crustal thickness at locations far from the Apollo network, and to build a first crustal thickness map based on seismic data. Here we compare our crustal thickness estimates based on seismic travel times to those based on inversions of the topography and gravity field. Both methods turn out to be coherent in the sense that highland sites possess a thicker crust than mare sites.

2. Understanding the Lunar Surface and Space-Moon Interactions

   P. Lucey, R. L. Korotev, J. J. Gillis, and 15 more authors

   Reviews in Mineralogy and Geochemistry, 2006.

   url
3. Thermal and Magmatic Evolution of the Moon

   C. K. Shearer, P. C. Hess, M. A. Wieczorek, and 13 more authors

   Reviews in Mineralogy and Geochemistry, 2006.

   url
4. Spatiospectral Concentration on a Sphere

   Frederik J. Simons, F. A. Dahlen, and Mark A. Wieczorek

   SIAM Review, 2006.

   abstract url

   We pose and solve the analogue of Slepian’s time-frequency concentration problem on the surface of the unit sphere to determine an orthogonal family of strictly bandlimited functions that are optimally concentrated within a closed region of the sphere or, alternatively, of strictly spacelimited functions that are optimally concentrated in the spherical harmonic domain. Such a basis of simultaneously spatially and spectrally concentrated functions should be a useful data analysis and representation tool in a variety of geophysical and planetary applications, as well as in medical imaging, computer science, cosmology, and numerical analysis. The spherical Slepian functions can be found by solving either an algebraic eigenvalue problem in the spectral domain or a Fredholm integral equation in the spatial domain. The associated eigenvalues are a measure of the spatiospectral concentration. When the concentration region is an axisymmetric polar cap, the spatiospectral projection operator commutes with a Sturm–Liouville operator; this enables the eigenfunctions to be computed extremely accurately and efficiently, even when their area-bandwidth product, or Shannon number, is large. In the asymptotic limit of a small spatial region and a large spherical harmonic bandwidth, the spherical concentration problem reduces to its planar equivalent, which exhibits self-similarity when the Shannon number is kept invariant.

5. The Constitution and Structure of the Lunar Interior

   M. A. Wieczorek, B. J. Jolliff, A. Khan, and 13 more authors

   Reviews in Mineralogy and Geochemistry, 2006.

   url

2005

1. Constraints on the Martian Lithosphere from Gravity and Topography Data

   V. Belleguic, P. Lognonné, and M. Wieczorek

   Journal of Geophysical Research Planets, 2005.

   abstract url

   Localized spectral admittances of the large Martian volcanoes are modeled by assuming that surface and subsurface loads are elastically supported by the lithosphere. In order to model the case where the load density differs from that of the crust, a new method for calculating gravity anomalies and lithospheric deflections is developed. The modeled gravity anomalies depend upon the elastic thickness, crustal thickness, load density, and crustal density, and these parameters were exhaustively sampled in order to determine their effect on the misfit between the observed and modeled admittance function. We find that the densities of the Martian volcanoes are generally well constrained with values of 3200 \textpm 100 kg m 3, which is considerably greater than those reported previously. These higher densities are consistent with those of the Martian basaltic meteorites, which are believed to originate from the Tharsis and Elysium volcanic provinces. The crustal density is constrained only beneath the Elysium rise to be 3270 \textpm 150 kg m 3. If this value is representative of the northern lowlands, then Pratt compensation is likely responsible for the approximately 6-km elevation difference between the northern and southern hemispheres. The elastic thicknesses of the major Martian volcanoes (when subsurface loads are ignored) are found to be the following: Elysium rise (56 \textpm 20 km), Olympus Mons (93 \textpm 40 km), Alba Patera (66 \textpm 20 km), and Ascraeus Mons (105 \textpm 40 km). We have also investigated the effects of subsurface loads, allowing the bottom load to be located either in the crust as dense intrusive material or in the mantle as less dense material. We found that all volcanoes except Pavonis are better modeled with the presence of less dense material in the upper mantle, which is indicative of either a mantle plume or a depleted mantle composition. An active plume beneath the major volcanoes is consistent with recent analyses of cratering statistics on Olympus Mons and the Elysium rise, which indicate that some lava flows are as young as 10–30 Myr, as well as with the crystallization ages of the Shergottites, of which some are as young as 180 Myr.

2. Localized Spectral Analysis on the Sphere

   Mark A. Wieczorek, and Frederik J. Simons

   Geophysical Journal International, 2005.

   abstract url

   It is often advantageous to investigate the relationship between two geophysical data sets in the spectral domain by calculating admittance and coherence functions. While there exist powerful Cartesian windowing techniques to estimate spatially localized (cross-)spectral properties, the inherent sphericity of planetary bodies sometimes necessitates an approach based in spherical coordinates. Direct localized spectral estimates on the sphere can be obtained by tapering, or multiplying the data by a suitable windowing function, and expanding the resultant field in spherical harmonics. The localization of a window in space and its spectral bandlimitation jointly determine the quality of the spatiospectral estimation. Two kinds of axisymmetric windows are here constructed that are ideally suited to this purpose: bandlimited functions that maximize their spatial energy within a cap of angular radius \texttheta 0, and spacelimited functions that maximize their spectral power within a spherical harmonic bandwidth L. Both concentration criteria yield an eigenvalue problem that is solved by an orthogonal family of data tapers, and the properties of these windows depend almost entirely upon the space–bandwidth product N 0 = (L + 1) \texttheta 0/\pi . The first N 0 - 1 windows are near perfectly concentrated, and the bestconcentrated window approaches a lower bound imposed by a spherical uncertainty principle. In order to make robust localized estimates of the admittance and coherence spectra between two fields on the sphere, we propose a method analogous to Cartesian multitaper spectral analysis that uses our optimally concentrated data tapers. We show that the expectation of localized (cross-)power spectra calculated using our data tapers is nearly unbiased for stochastic processes when the input spectrum is white and when averages are made over all possible realizations of the random variables. In physical situations, only one realization of such a process will be available, but in this case, a weighted average of the spectra obtained using multiple data tapers well approximates the expected spectrum. While developed primarily to solve problems in planetary science, our method has applications in all areas of science that investigate spatiospectral relationships between data fields defined on a sphere.

2004

1. Correction to “Localized Gravity/Topography Admittance and Correlation Spectra on Mars: Implications for Regional and Global Evolution”

   Patrick J. McGovern, Sean C. Solomon, David E. Smith, and 7 more authors

   Journal of Geophysical Research: Planets, 2004.

   url
2. Crustal Structure of Mars from Gravity and Topography

   G. A. Neumann, M. T. Zuber, M. A. Wieczorek, and 3 more authors

   Journal of Geophysical Research Planets, 2004.

   abstract url

   Mars Orbiter Laser Altimeter (MOLA) topography and gravity models from 5 years of Mars Global Surveyor (MGS) spacecraft tracking provide a window into the structure of the Martian crust and upper mantle. We apply a finite-amplitude terrain correction assuming uniform crustal density and additional corrections for the anomalous densities of the polar caps, the major volcanos, and the hydrostatic flattening of the core. A nonlinear inversion for Moho relief yields a crustal thickness model that obeys a plausible power law and resolves features as small as 300 km wavelength. On the basis of petrological and geophysical constraints, we invoke a mantle density contrast of 600 kg m 3; with this assumption, the Isidis and Hellas gravity anomalies constrain the global mean crustal thickness to be >45 km. The crust is characterized by a degree 1 structure that is several times larger than any higher degree harmonic component, representing the geophysical manifestation of the planet’s hemispheric dichotomy. It corresponds to a distinction between modal crustal thicknesses of 32 km and 58 km in the northern and southern hemispheres, respectively. The Tharsis rise and Hellas annulus represent the strongest components in the degree 2 crustal thickness structure. A uniform highland crustal thickness suggests a single mechanism for its formation, with subsequent modification by the Hellas impact, erosion, and the volcanic construction of Tharsis. The largest surviving lowland impact, Utopia, postdated formation of the crustal dichotomy. Its crustal structure is preserved, making it unlikely that the northern crust was subsequently thinned by internal processes.

3. Thickness of the Martian Crust: Improved Constraints from Geoid-to-Topography Ratios

   M. A. Wieczorek, and M. T. Zuber

   Journal of Geophysical Research Planets, 2004.

   abstract url

   The average crustal thickness of the southern highlands of Mars was investigated by calculating geoid-to-topography ratios (GTRs) and interpreting these in terms of an Airy compensation model appropriate for a spherical planet. We show that (1) if GTRs were interpreted in terms of a Cartesian model, the recovered crustal thickness would be underestimated by a few tens of kilometers, and (2) the global geoid and topography signals associated with the loading and flexure of the Tharsis province must be removed before undertaking such a spatial analysis. Assuming a conservative range of crustal densities (2700–3100 kg m 3), we constrain the average thickness of the Martian crust to lie between 33 and 81 km (or 57 \textpm 24 km). When combined with complementary estimates based on crustal thickness modeling, gravity/topography admittance modeling, viscous relaxation considerations, and geochemical mass balance modeling, we find that a crustal thickness between 38 and 62 km (or 50 \textpm 12 km) is consistent with all studies. Isotopic investigations based on Hf-W and Sm-Nd systematics suggest that Mars underwent a major silicate differentiation event early in its evolution (within the first 30 Ma) that gave rise to an “enriched” crust that has since remained isotopically isolated from the “depleted” mantle. In comparing estimates of the thickness of this primordial crust with those obtained in this study, we find that at least one third of the Martian crust has an origin dating from the time of accretion and primary differentiation. Subsequent partial melting of the depleted mantle would have given rise to the remaining portion of the crust. While we predict that a large portion of the crust should be composed of ancient “enriched” materials, a representative sample of this primordial crust does not currently exist among the known Martian meteorites.

2002

1. Localized Gravity/Topography Admittance and Correlation Spectra on Mars: Implications for Regional and Global Evolution

   Patrick J. McGovern, Sean C. Solomon, David E. Smith, and 7 more authors

   Journal of Geophysical Research: Planets, 2002.

   abstract url

   From gravity and topography data collected by the Mars Global Surveyor spacecraft we calculate gravity/topography admittances and correlations in the spectral domain and compare them to those predicted from models of lithospheric flexure. On the basis of these comparisons we estimate the thickness of the Martian elastic lithosphere (Te) required to support the observed topographic load since the time of loading. We convert Te to estimates of heat flux and thermal gradient in the lithosphere through a consideration of the response of an elastic/plastic shell. In regions of high topography on Mars (e.g., the Tharsis rise and associated shield volcanoes), the mass-sheet (small-amplitude) approximation for the calculation of gravity from topography is inadequate. A correction that accounts for finite-amplitude topography tends to increase the amplitude of the predicted gravity signal at spacecraft altitudes. Proper implementation of this correction requires the use of radii from the center of mass (collectively known as the planetary “shape”) in lieu of “topography” referenced to a gravitational equipotential. Anomalously dense surface layers or buried excess masses are not required to explain the observed admittances for the Tharsis Montes or Olympus Mons volcanoes when this correction is applied. Derived Te values generally decrease with increasing age of the lithospheric load, in a manner consistent with a rapid decline of mantle heat flux during the Noachian and more modest rates of decline during subsequent epochs.

2001

1. The Role of Magma Buoyancy on the Eruption of Lunar Basalts

   Mark A. Wieczorek, Maria T. Zuber, and Roger J. Phillips

   Earth and Planetary Science Letters, 2001.

   abstract url

   It has long been recognized that mare basalts on the Moon are preferentially located both on the Earth-facing hemisphere and within large impact basins. A popular model that accounts for this observation assumes that these magmas were denser than the lunar crust, that they accumulated at the crust^mantle interface, and that eruptions occurred only when this magma chamber became overpressurized. In this paper, we re-evaluate this model and argue that it is not consistent with the available data nor with models of dike propagation. As an alternative hypothesis, we propose that magma buoyancy is the predominant factor that determines whether mare basalts erupt at the surface or form crustal intrusions instead. We have computed the densities of mare basaltic magmas and find that some are, in fact, less dense than the Moon’s upper anorthositic crust. Based on the widely accepted view that the lunar crust becomes more mafic with depth, we also show that all mare basaltic magmas should be less dense than the lower portion of the crust. Thus, if the upper anorthositic crust was regionally removed by an impact event, then any mare basaltic magma could have risen to the surface there based on buoyancy considerations alone. In support of this model, we note that mare basalts are indeed found wherever geophysical crustal thickness models predict the upper crust to be absent. Furthermore, many of the basalts that erupted within the anorthositic highlands are predicted to be less dense than the underlying crust based on remote sensing data. The high titanium flows within Oceanus Procellarum are somewhat problematical to our model in that an anorthositic crust is predicted to be present beneath them. Using results from recent lunar thermal models, we suggest that these magmas may have overcome their negative buoyancy in the crust by possessing superliquidus temperatures. If magma buoyancy does indeed control whether or not a basaltic eruption will occur, then this implies that the quantity of magma produced beneath the South Pole-Aitken basin was about 10 times less than that of the nearside. ß 2001 Elsevier Science B.V. All rights reserved.

2. The Composition and Origin of the Lunar Crust: Constraints from Central Peaks and Crustal Thickness Modeling

   Mark A. Wieczorek, and Maria T. Zuber

   Geophysical Research Letters, 2001.

   abstract url

   Spectral-reflectance data of lunar central peaks have revealed that the Moon’s crust varies both laterally and vertically in composition. We correlate the depths of origin of materials that make up central peaks with a geophysically derived dual-layered crustal thickness model and find that the peak compositions are consistent with this stratified model. Specifically, peaks composed exclusively of rocks containing more than 85% plagioclase (by volume) come from this model’s upper crust, whereas peaks that contain some norite or gabbro-norite come from the model’s lower crust. Extrapolating these data we find that the Moon’s upper crust is composed of 88\textpm4% plagioclase, corresponding to 29 to 32 wt.% Al2O3. The most-mafic lower portion of the crust is composed of 65\textpm8% plagioclase, having an Al2O3 content that lies between 18 and 25 wt.%. We show that the lower portion of the crust is consistent with having formed by cumulate flotation in a lunar magma ocean.

3. A Serenitatis Origin for the Imbrian Grooves and South Pole-Aitken Thorium Anomaly

   Mark A. Wieczorek, and Maria T. Zuber

   Journal of Geophysical Research: Planets, 2001.

   abstract url

   The northwest corner of the Moon’s South Pole-Aitken (SPA) basin contains a high abundance of thorium and a unique Imbrian aged geomorphologic unit that consists of “grooves and mounds” (referred to here as the Imbrian grooves). Because the location of these features are almost antipodal to the Imbrium basin, where high-thorium ejecta and seismic energy are expected to have converged, an Imbrium origin for these units has long seemed certain. By modeling the deposition of impact ejecta on the Moon, we have investigated whether the convergence of Imbrium’s ejecta at its antipode could be the origin of both the Imbrian grooves and SPA thorium anomaly. As a result of the Moon’s rotation, our results show that ejecta from this basin should converge more than 12^∘ west of its antipode. Both the Imbrian grooves and thorium anomaly within SPA, however, reside slightly to the east of Imbrium’s antipode. In an attempt to reconcile this disparity, the effects of a putative oblique Imbrium impact have been qualitatively investigated. While this model can distribute ejecta in the general vicinity of the Imbrian grooves, the planform of our modeled antipodal ejecta is distinctly different from that which is observed. As an alternative explanation for the origin of these features, we find that the modeled distribution of ejecta from an oblique Serenitatis impact is surprisingly similar to the planform of the Imbrian grooves, with the exception that it is offset directly to the east. This eastward offset is likely to be an artifact of our not being able to properly include the effects of the Moon’s rotation in our oblique impact models. We conclude that the Imbrium grooves and SPA thorium anomaly are most consistent with having an origin from the convergence of ejecta antipodal to the Serenitatis basin. If this conclusion can be substantiated once quantitative ejecta scaling relations for oblique impacts are determined, then this implies that (1) the Serenitatis target contained a high abundance of thorium and (2) the convergence of seismic energy at the antipodes of either the Imbrium or Serenitatis basin was not sufficient to cause substantial surface modification. Extrapolating this result to Mercury suggests that the “hilly and lineated” terrain antipodal to the Caloris basin was formed by the convergence of ejecta, and not seismic waves.

2000

1. Major Lunar Crustal Terranes: Surface Expressions and Crust-Mantle Origins

   B. J. Jolliff, J. J. Gillis, L. A. Haskin, and 2 more authors

   Journal of Geophysical Research Planets, 2000.

   abstract url

   In light of global remotely sensed data, the igneous crust of the Moon can no longer be viewed as a simple, globally stratified cumulus structure, composed of a flotation upper crust of anorthosite underlain by progressively more mafic rocks and a residual-melt (KREEP) sandwich horizon near the base of the lower crust. Instead, global geochemical information derived from Clementine multispectral data and Lunar Prospector gamma-ray data reveals at least three distinct provinces whose geochemistry and petrologic history make them geologically unique: (1) the Procellarum KREEP Terrane (PKT), (2) the Feldspathic High-lands Terrane (FHT), and (3) the South Pole-Aitken Terrane (SPAT). The PKT is a mafic province, coincident with the largely resurfaced area in the Procellarum-Imbrium region whose petrogenesis relates to the early differentiation of the Moon. Here, some 40% of the Th in the Moon’s crust is concentrated into a region that constitutes only about 10% of the crustal volume. This concentration of Th (average ∼5 ppm), and by implication the other heat producing elements, U and K, led to a fundamentally different thermal and igneous evolution within this region compared to other parts of the lunar crust. Lower-crustal materials within the PKT likely interacted with underlying mantle materials to produce hybrid magmatism, leading to the magnesian suite of lunar rocks and possibly KREEP basalt. Although rare in the Apollo sample collection, widespread mare volcanic rocks having substantial Th enrichment are indicated by the remote data and may reflect further interaction between enriched crustal residues and mantle sources. The FHT is characterized by a central anorthositic region that constitutes the remnant of an anorthositic craton resulting from early lunar differentiation. Basin impacts into this region do not excavate significantly more mafic material, suggesting a thickness of tens of kilometers of anorthositic crust. The feldspathic lunar meteorites may represent samples from the anorthositic central region of the FHT. Ejecta from deep-penetrating basin impacts outside of the central anorthositic region, however, indicate an increasingly mafic composition with depth. The SPAT, a mafic anomaly of great magnitude, may include material of the upper mantle as well as lower crust; thus it is designated a separate terrane. Whether the SPA basin impact simply uncovered lower crust such as we infer for the FHT remains to be determined.

2. The “Procellarum KREEP Terrane”: Implications for Mare Volcanism and Lunar Evolution

   M. A. Wieczorek, and R. J. Phillips

   Journal of Geophysical Research Planets, 2000.

   abstract url

   Geophysical, remote-sensing, and sample data demonstrate that the Procellarum and Imbrium regions of the Moon make up a unique geochemical crustal province (here dubbed the Procellarum KREEP Terrane). Geochemical studies of Imbrium’s ejecta and the crustal structure of the Imbrium and Serenitatis basins both suggest that a large portion of the lunar crust in this locale is composed of a material similar in composition to Apollo 15 KREEP basalt. KREEP basalt has about 300 times more uranium and thorium than chondrites, so this implies that a large portion of Moon’s heat-producing elements is located within this single crustal province. The spatial distribution of mare volcanism closely parallels the confines of the Procellarum KREEP Terrane and this suggests a causal relationship between the two phenomena. We have modeled the Moon’s thermal evolution using a simple thermal conduction model and show that as a result of the high abundance of heat-producing elements that are found in the Procellarum KREEP Terrane, partial melting of the underlying mantle is an inevitable outcome. Specifically, by placing a 10-km KREEP basalt layer at the base of the crust there, our model predicts that mare volcanism should span most of the Moon’s history and that the depth of melting should increase with time to a maximum depth of about 600 km. We suggest that the 500-km seismic discontinuity that is observed in the Apollo seismic data may represent this maximum depth of melting. Our model also predicts that the KREEP basalt layer should remain partially molten for a few billion years. Thus the Imbrium impact event most likely excavated into a partially molten KREEP basalt magma chamber. We postulate that the KREEP basalt composition is a by-product of mixing urKREEP with shallow partial melts of the underlying mantle. Since Mg-suite rocks are likely derived from crystallizing KREEP basalt, the provenance of these plutonic rocks is likely to be unique to this region of the Moon.

1999

1. Lunar Multiring Basins and the Cratering Process

   Mark A. Wieczorek, and Roger J. Phillips

   Icarus, 1999.

   abstract url

   Numerous studies of the lunar gravity field have concluded that the lunar Moho is substantially uplifted beneath the young multiring basins. This uplift is presumably due to the excavation of large quantities of crustal material during the cratering process and subsequent rebound of the impact basin floor. Using a new dual-layered crustal thickness model of the Moon, the excavation cavities of some nearside multiring basins (Grimaldi and larger, and younger than Tranquillitatis) were reconstructed by restoring the uplifted Moho to its preimpact location. The farside South PoleAitken (SPA) basin was also considered due to its importance in deciphering lunar evolution. Restoring the Moho to its preimpact position beneath these basins resulted in a roughly parabolic depression from which the depth and diameter of the excavation cavity could be determined. Using these reconstructed excavation cavities, the basin-forming process was investigated. Excavation cavity diameters were generally found to be on the small side of most previous estimates (for Orientale the modeled excavation cavity lies within the Inner Rook Ring). Additionally, with the exception of the three largest basins (Serenitatis, Imbrium, and South Pole–Aitken) the depth/diameter ratios of the excavation cavities were found to be 0.115 ß 0.005, a value consistent with theoretical and experimental results for impact craters orders of magnitude smaller in size. The three largest basins, however, appear to have significantly shallower depths of excavation compared to this trend. It is possible that this may reflect a different physical process of crater formation (e.g., nonproportional scaling), special impact conditions, or postimpact modification processes. The crustal thickness model also shows that each basin is surrounded by an annulus of thickened crust. We interpret this thickened crust as representing thick basin ejecta deposits, and we show that the radial variation in the thickness of these deposits is consistent with scaling laws obtained from small-scale experimental studies. If multiring basins ever possessed a terraced main crater rim, this terraced zone may be presently unrecognizable at the surface due to the emplacement of ejecta deposits that exceed a few kilometers in thickness exterior to the excavation cavity rim. We also show that the interiors of many basins were superisostatic before mare volcanism commenced. Those basins that were closest to approaching a premare isostatic state lie close to or within an anomalous geochemical province rich in heat-producing elements.

1998

1. Potential Anomalies on a Sphere: Applications to the Thickness of the Lunar Crust

   Mark A. Wieczorek, and Roger J. Phillips

   Journal of Geophysical Research: Planets, 1998.

   abstract url

   A new technique for calculating potential anomalies on a sphere due to finite amplitude relief has been developed. We show that by raising the topography to the nth power and expanding this field into spherical harmonics, potential anomalies due to topography on spherical density interfaces can be computed to arbitrary precision. Using a filter for downward continuing the Bouguer anomaly, we have computed a variety of crustal thickness maps for the Moon, assuining both a homogeneous as well as a dual-layered crust. The crustal thickness maps for the homogeneous model give plausible results, but this model is not consistent with the seismic data, petrologic evidence, and geoid to topography ratios, all of which suggest some form of crustal stratification. Several dual-layered models were investigated, and it was found that only models with both upper and lower crustal thickness variations could satisfy the gravity and topography data. These models predict that the entire upper crust has been excavated beneath the major nearside multiring basins. Additionally, significant amounts of lower crustal material was excavated from these basins, especially beneath Crisium. This model also predicts that mantle material should not have been excavated during the South-Pole Aitken basin forming event, and at that lower crustal material should be exposed the surface in this basin.

1997

1. The Structure and Compensation of the Lunar Highland Crust

   Mark A. Wieczorek, and Roger J. Phillips

   Journal of Geophysical Research: Planets, 1997.

   abstract url

   A new method of interpreting geoid to topographry ratios (GTRs) on a sphere is presented, in which it is shown that the GTR is equivalent to a sum of spectrally weighted degree-dependent admittance. Using this method combined with newly obtained gravity, topography, and near-global surface iron concentrations from the Clementine mission, the structure and compensation of the lunar highland crust have been investigated. Geoid to topography ratios were tested against single-layer Pratt and Airy compensation models, as well as dual-layered Airy models. Regional lateral variations in crustal density are found to play an insignificant role in crustal compensation, and the single-layer and dual-layered Airy models both strongly suggest that the lunar crust is vertically stratified. The depth of the intracrustal interface obtained from these models is consistent with the existence of a 20-km seismic discontinuity beneath the Apollo 12 and 14 sites. A uniform density crust with compensation occurring at the Moho is a viable interpretation of crustal structure only when the extreme limits of the observed GTR distribution are used.