¹B. Bultel,²M. Wieczorek,³Anna Mittelholz,^4,5Catherine L. Johnson,⁶Jérôme Gattacceca,^7,8,9Valentin Fortier,¹⁰Benoit Langlais
Journal of Geophyisical Research (Planets) Open Access Link to Article [https://doi.org/10.1029/2023JE008111]
¹GEOPS, Université Paris-Saclay, CNRS, Orsay, France
²Institut de Physique du Globe de Paris, Université Paris Cité, CNRS, Paris, France
³Department of Earth and Planetary Sciences, ETH Zurich, Zurich, Switzerland
⁴University of British Columbia, Vancouver, BC, Canada
⁵Planetary Science Institute, Tucson, AZ, USA
⁶Aix-Marseille Univ, CNRS, IRD, INRAE, CEREGE, Aix-en-Provence, France
⁷Université Catholique de Louvain-la-Neuve, Earth and Life Institute, Louvain-la-Neuve, Belgium
⁸Laboratoire G-Time, Université Libre de Bruxelles, Bruxelles, Belgium
⁹Géosciences Montpellier, CNRS, Univ. Montpellier, Montpellier, France
¹⁰ de Planétologie et Géosciences UMR 6112, Nantes Université, Univ Angers, Le Mans Université, CNRS, Nantes, France
Published by arrangement with John Wiley & Sons

Strong magnetic fields have been measured from orbit around Mars over parts of the ancient southern highlands crust and on the surface at the InSight landing site. The geological processes that are responsible for generating strong magnetization within the crust remain poorly understood. One possibility is that intense aqueous alteration of crustal materials, through the process of serpentinization, could have produced magnetite that was magnetized in the presence of a global core-generated magnetic field. Here, we test this idea with geophysical and geochemical models. We first determine the magnetizations required to account for the observed magnetic field strengths and then estimate the amount of magnetite necessary to account for these magnetizations. For the strongest orbital magnetic field strengths, about 7 wt% magnetite is required if the magnetic layer is 10 km thick. For the surface field strength observed at the InSight landing site, 0.4–1.1 wt% magnetite is required if the magnetic layer corresponds to one or more of the three crustal layers observed in the InSight seismic data (with thicknesses from 8 to 39 km). We then investigate the minerals that are produced by aqueous alteration for various possible crustal compositions and water-to-rock ratios using a thermodynamic model. Magnetite abundances up to 6 wt% can be generated for dunitic compositions that could account for the strongest magnetic anomalies. For more representative basaltic starting compositions, however, more than 0.4 wt% can only be generated when using high water-to-rock ratios, which could account for the weaker magnetizations beneath the InSight landing site.

¹J. N. Levin,¹A. J. Evans,²J. C. Andrews-Hanna,¹I. J. Daubar
Journal of Geophysical Research (Planets)(in Press) Link to Article [https://doi.org/10.1029/2024JE008418]
¹Department of Earth, Environmental, and Planetary Sciences, Brown University, Providence, RI, USA
²Lunar and Planetary Laboratory, The University of Arizona, 1629 E University Blvd Tucson AZ, Tucson, AZ, USA
Published by arrangement with John Wiley & Sons

The distribution of KREEP—potassium (K), rare earth elements (REE), and phosphorus (P)—in the lunar crust is an important clue to deciphering the geochemical and thermal evolution of the Moon. Surface measurements of thorium abundance taken by the Lunar Prospector Gamma Ray Spectrometer (LP GRS) instrument have shown that KREEP is concentrated on the lunar nearside surface, mirroring the hemispheric asymmetry observed in the distribution of maria, crustal thickness, and topography. However, the overall lateral and vertical distribution of KREEP within the crust is poorly constrained, leaving uncertainty in estimates of bulk crustal thorium abundance and in the history and evolution of KREEP. In this study, we compared the overall lateral and vertical distribution of lunar KREEP in the upper crust by determining the thorium abundance of material excavated by complex impact craters. We find that the distribution of KREEP on the nearside is consistent with a layer of high-Thorium ejecta from the Imbrium impact mixing with underlying low-Th (<1 ppm) crustal material, suggesting the excavation of a sub-crustal KREEP reservoir with thorium abundances as high as 45–120 ppm by the Imbrium-forming impact. Imbrium ejecta alone does not explain the distribution of thorium on the lunar farside, particularly around the South Pole Aitken basin, suggesting other sources for farside thorium enrichments. Furthermore, our results refute the existence of a large-scale Thorium-enriched layer in the upper 16 km of the farside crust.

¹T.J.McCoy et al. (>10)
Nature 637, 1072-1077 Open Access Link to Article [DOI https://doi.org/10.1038/s41586-024-08495-6]
¹Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Washington, DC, USA

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