^1,2E. K. Leask,¹B. L. Ehlmann,³M. M. Dundar
Journal of Geophysical Research (Planets) (in Press) Open Access Link to Article [https://doi.org/10.1029/2023JE008259]
¹Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA
²Johns Hopkins University Applied Physics Laboratory, Laurel, MD, USA
³Indiana University-Purdue University, Indianapolis, IN, USA
Published by arrangement with John Wiley & Sons

Terra Sirenum, a region of Noachian highlands southwest of the Tharsis volcanic complex, is unique in the number, proximity, and diversity of orbital detections of secondary minerals, as the sole region found to date hosting large-scale deposits of all of Mars’ major salts (chlorides, sulfates, carbonates) as well as diverse hydrated silicates. We combine mineralogical information, high-resolution imagery, and elevation models to investigate the geologic context of these secondary minerals to understand the sources of water and ions for each type of deposit and their spatial/temporal relationships. Carbonates, where present, are part of Noachian basement rocks exposed through cratering and do not appear associated with evaporative sequences. Numerous small detections of the acid sulfate minerals alunite and jarosite mirror the dominant clay cation in the localities they are found—Al phyllosilicates and Fe phyllosilicates, respectively—suggesting in situ formation. We interpret a previously discovered kaolinite-rich unit overlying Fe/Mg clays across northeast Terra Sirenum as remnants of a widespread ash unit rather than a pedogenic weathering sequence. Sulfate and chloride detections are decoupled, with sulfates in topographic lows likely precipitated from volcanism-associated groundwaters, while chloride detections are consistent with surface water runoff, in some instances clearly post-dating volcanic units capping sulfate detections. Volcanic resurfacing of craters in the region is progressively younger from west to east, and crater statistics-based ages indicate localized sulfate- and chloride-forming processes continue to occur from ∼3.5 to ∼1.4 Ga. We hypothesize that their decoupling points to disconnected, episodic surface and groundwater reservoirs, perhaps separated by a permafrost layer.

¹B. Sutter,¹P. D. Archer,²P. B. Niles,²D. W. Ming,³D. Hamara,³W. V. Boynton
Journalof Geophysical Research (Planets) (in Press) Open Access Link to Article [https://doi.org/10.1029/2024JE008335]
¹Jacobs/JETSII, NASA Johnson Space Center, Houston, TX, USA
²NASA Johnson Space Center, Houston, TX, USA
³Lunar Planetary Laboratory, University of Arizona, Tucson, AZ, USA
Published by arrangement with John Wiley & Sons

The Thermal Evolved Gas Analyzer (TEGA) analysis of surface and icy subsurface Phoenix landing site soils consisted of low (300–700°C) and high (>700°C) temperature CO2 evolutions that were attributed to organic carbon (83–1,484 μgC/g) and Ca-rich carbonate (1.1–2.6 wt.%). Total carbon abundances ranged from 1,143 to 4,905 µgC/g, which is the highest soil carbon concentration so far detected on Mars. Low temperature CO2 was attributed to oxidized organic C (e.g., oxalates, acetates), while hydrocarbon combustion was indicated in two soils by the detection of coevolved CO2 and O2 (perchlorate). Combustion reactions may have prevented the detection of hydrocarbon masses in the Phoenix landing site soils. Organic C was likely derived from meteoritic and igneous/hydrothermal sources, but microbiological sources cannot be excluded. CO2 evolved at high temperatures was consistent with Ca-rich carbonate along with possible minor contributions from macromolecular organic carbon and mineral/glass vesicle CO2. Carbon detected in the Phoenix landing site soil and other landing site soils and sands (e.g., Gale/Jezero craters) would be consistent with global organic C and carbonate in soils and sand across Mars. However, oxidizing water thin films derived from the near-surface ice in the Phoenix soils favor Ca-carbonate over Fe-carbonate, which is likely more stable in the ice-free regions of Mars (e.g., Gale/Jezero craters). The global carbon budget on Mars inferred from these results emphasizes that Mars Sample Return should yield carbon bearing soil/rock that would allow the identification of the origin of carbon and any possible connections to ancient martian microbiology.