^1,2A. L. Knight,^1,2S. J. VanBommel,³R. Gellert,⁴J. A. Berger,^1,2J. G. Catalano,^5,6J. Gross,^1,2J. R. Christian
Journal of Geophysical Research (Planets) (in Press) Link to Article [https://doi.org/10.1029/2024JE008569]
¹Department of Earth, Environmental, and Planetary Sciences, Washington University in St. Louis, St. Louis, MO, USA
²McDonnell Center for the Space Sciences, Washington University in St. Louis, St. Louis, MO, USA
³Department of Physics, University of Guelph, Guelph, ON, Canada
⁴Jacobs JETSII at NASA Johnson Space Center, Houston, TX, USA
⁵NASA Johnson Space Center, Houston, TX, USA
⁶Department of Earth and Planetary Sciences, Rutgers University, Piscataway, NJ, USA
Published by arrangement with John Wiley & Sons

The Mars Exploration Rovers (MER) Spirit and Opportunity, sent to Gusev crater and Meridiani Planum, respectively, determined the chemical composition of martian materials with their Alpha Particle X-ray Spectrometers (APXS). The MER APXS was effective at routinely quantifying major, minor, and select (Ni, Zn, Br) trace elements at levels down to ∼50 ppm but often reached detection limits for other trace elements (e.g., Ga and Ge during typical individual analyses of a single sample). To enable precise quantification of additional trace elements, a database of MER APXS target properties (e.g., location, feature, target, formation, target type, sample preparation) was created, enabling the construction of a library of composite (i.e., summed) spectra with improved statistics. Composite spectra generated from individual spectra with shared characteristics have a higher potential for resolving and thus quantifying trace element peaks. Analyses of composite spectra from Meridiani Planum and Gusev crater indicate that the molar Ga to Al ratio is relatively constant throughout both regions and is in line with predicted values for the martian crust and measured values in martian meteorites. Gallium and aluminum likely do not volatilize and instead remain together during volcanism and aqueous alteration. In contrast, Ge is enriched at least an order of magnitude relative to martian meteorites, and the molar Ge to Si ratio is much more variable across Meridiani Planum and Gusev crater. Enrichment of Ge may be a global phenomenon resulting from volcanic outgassing of volatiles and subsequent overprinting by local mobilization and enrichment via hydrothermal fluids.

^1,2K. Hirata,¹T. Usui,^3,4E. Caminiti,⁵J. Wright,⁶S. Besse
Journal of Geophysical Research (Planets)(in Press) Open Access Link to Article [https://doi.org/10.1029/2024JE008788]
¹Institute of Space and Astronautical Science (ISAS), Japan Aerospace Exploration Agency (JAXA), Sagamihara, Japan
²Department of Earth and Planetary Science, The University of Tokyo, Tokyo, Japan
³LIRA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris, Meudon, France
⁴Université Grenoble Alpes, CNRS, Institut de Planétologie et d’Astrophysique de Grenoble (IPAG), Saint-Martin-d’Hères, France
⁵School of Physical Sciences, The Open University Walton Hall, Milton Keynes, UK
⁶European Space Astronomy Centre (ESAC), European Space Agency (ESA), Madrid, Spain
Published by arrangement with John Wiley & Sons

The observed geochemical heterogeneity on the surface of Mercury is key to understanding the planet’s volcanic activity and mantle conditions. The Caloris basin shows a diversity in elemental composition, spectral properties, and geomorphology, both within and around it. However, the relationship among these characteristics has not been well understood due to the mismatch in spatial resolutions of the available observation data. This study investigates the geochemical end-members around the Caloris basin, overcoming the limitation of the low spatial resolution of MESSENGER’s X-Ray Spectrometer (XRS) data. End-member units are defined using spectral and geomorphological units from MESSENGER’s VIS-NIR spectral data and high-resolution images, with the assumption of homogeneous elemental compositions within each unit. A mixing model is constructed to reproduce the XRS data by mixing the end-members, and we solve the inverse problem to calculate the respective end-member compositions. Five end-member compositions were determined, including those corresponding to the post-Caloris volcanic smooth plains interior and exterior to the basin and surrounding pre-Caloris crust. Two smooth plains units, which are geomorphologically indistinguishable but spectrally distinct, showed a compositional variation consistent with magma evolution through fractional crystallization. This suggests that they originated from parent magmas with a common composition. The pre-Caloris crust units showed a large compositional variation, ranging from low- to high-Mg content, implying the potential existence of high-Mg crusts comparable to the HMR. The observed crustal diversity could be explained by relatively minor heterogeneity in source mantle compositions and/or conditions of partial melting within the mantle.