Nanoscale Analyses of X‑ray Amorphous Material from Terrestrial Ultramafic Soils Record Signatures of Environmental Conditions Useful for Interpreting Past Martian Conditions

The secondary mineral assemblage of martian rocks and sediments is critical to the interpretation of past conditions on Mars. X-ray amorphous material that is variably Mg/Fe/Si-rich and Al-poor, and that likely contains secondary alteration products, makes up between 15 and 73 wt % of Gale crater materials measured by the CheMin instrument and is also prevalent on the martian surface in other locations. Despite its prevalence, the structure and origin of this material and its implications for past martian environments remains unknown. Here, we employ transmission electron microscopy and synchrotron microprobe analyses of Fe/Si/Mg-rich and Al-poor ultramafic soils from the mediterranean climate Klamath Mountains of California and subarctic climate Tablelands of Newfoundland, Canada to help interpret environmental conditions during the formation of chemically similar X-ray amorphous material in Gale crater, Mars. X-ray amorphous material includes globular amorphous silica and chemically heterogeneous nanospherical amorphous material and nanocrystalline phases. Amorphous silica is present only in soils that undergo extensive periods of cyclic freezing conditions. X-ray amorphous material from the subarctic Tablelands is significantly richer in Mg and Si than material from the warmer Klamath Mountains. Nanocrystallites in both examined soils are richer in Fe than the truly amorphous material. However, Fe-rich nanocrystallites contain significantly more Mg and Si in the subarctic Tablelands, whereas Klamath Mountains soils contain highly Fe-enriched nanocrystallites incorporating little Mg and Si and potentially contain nanocrystalline smectites. These characteristics provide helpful identifiers to interpret past environmental conditions during the formation of X-ray amorphous material both in situ and in returned samples from Mars.