¹R. J. Hopkins,¹A. D. Rogers,¹L. Ehm
Journal of Geophysical Research: Planets 131, e2025JE009299 Open Access Link to Article [https://doi.org/10.1029/2025JE009299]
¹Stony Brook University, Stony Brook, NY, USA
Published by arrangement with John Wiley & Sons

Throughout Gale crater on Mars, the Curiosity rover has found high abundances (15–73 wt%) of X-ray amorphous materials in the rocks and sediments. The composition of this amorphous fraction is primarily calculated by subtracting crystalline abundances measured by the Chemistry & Mineralogy X-Ray Diffractometer (CheMin) from bulk elemental abundances from the Alpha Particle X-Ray Spectrometer (APXS). If any crystalline phase was underrepresented in the CheMin data, the elemental components of that phase would be overestimated in the amorphous fraction. This study examines the possibility of underrepresented crystalline phases in X-ray diffraction data using mixtures of amorphous ferric sulfate (AFS) and crystalline Ca-sulfate. Two compositionally equivalent mixtures were made with different morphologies: first, a simple mixture of AFS and Ca-sulfate grains and second, Ca-sulfate grains with AFS coatings. Each mixture was characterized with Raman spectroscopy, and two X-ray diffractometers (XRDs): a Bragg-Brentano instrument with CuKα radiation and a Debye-Scherrer instrument with CoKα radiation. Raman peaks from Ca-sulfate dominate both mixtures, but are dampened when AFS coats the Ca-sulfate grains. In XRD data, AFS coatings cause an overestimation of the amorphous percentage, with a difference between the known and refined amorphous abundances of 29–34 wt%, compared to 2–2.8 wt% for the uncoated mixtures. The effects of the coatings were slightly amplified with the Debye-Scherrer XRD, primarily due to the increased scattering and absorption of the CoKα radiation. This has implications for interpreting the XRD data of mixed amorphous-crystalline samples on Mars, as any Fe-rich amorphous coating may cause an overestimation of the amorphous abundance.

^1,2Jitse Alsemgeest,¹Fraukje M. Brouwer,³Luis F. Auqué,¹Kirsten van Zuilen,⁴Natalia Hauser,⁴Wolf Uwe Reimold
Journal of Geophysical Research: Planets 131, e2025JE008966 Open Access Link to Article [https://doi.org/10.1029/2025JE008966]
¹Geology and Geochemistry Cluster, Department of Earth Sciences, VU Amsterdam, Amsterdam, The Netherlands
²Nowat GAIA, Faculty of Geo‐Information Science and Earth Observation, University of Twente, Enschede, The Netherlands
³Department of Geosciences, University of Zaragoza, Zaragoza, Spain
⁴Institute of Geosciences, University of Brasília,CEP, Brasília, Brazil
Published by arrangement with John Wiley & Sons

As hydrous minerals have been observed in impact craters on Mars, impact-generated hydrothermal systems (IGHSs) have been considered as potential habitats for life on that planet. The Vargeão Dome, a 12 km wide impact structure in southern Brazil, was formed in basalts with at least two hydrothermal alteration stages. This structure is a rare terrestrial analog for IGHS evolution on Mars. However, the thermochemical evolution of these stages and their relationship to the impact remain unresolved. Two vein-forming alteration stages were identified by multidisciplinary sample analysis of Vargeão Dome. Fractured and deformed white vein fragments from the first stage occur within red veins from the second. This suggests reactivation of the white vein set during the crater excavation and modification stages. Rare Earth Element and Rb-Sr isotope data indicate different fluid sources for the white and red veins and an evolving fluid system. Thermodynamic modeling indicates a cooling sequence from 100–250 to <25°C for the white vein set, whereas goethite and hematite within the red veins indicate that most IGHS activity occurred at temperatures below 40°C. Considering this and accounting for gravity differences, life-supporting IGHSs on Mars may preferentially form impact structures over 28 km in diameter. Furthermore, impact-reactivation suggests elevated habitat potential in structures with pre-existing fault, fracture, and vein systems. These occur in volcanic areas or terrain older than 3.5 Ga, when Mars was wetter and geologically active. Therefore, the search for evidence of IGHS-supported life should be focused on these kinds of terrains.