Fe-rich X-Ray Amorphous Material Records Past Climate and Persistence of Water on Mars

Abstract The assemblage of secondary minerals in martian rocks can help constrain the characteristics of past surface and subsurface fluids as well as past climatic conditions. X-ray amorphous material is an important part of martian surface materials, making up 15–73 wt.% of sedimentary rocks and eolian sediments in Gale crater, Mars. This X-ray amorphous material is variably siliceous, Fe-rich, and contains volatiles, and it therefore likely contains incipient weathering products. To better understand the implications of this material for past aqueous and climatic conditions in Gale crater and elsewhere on Mars, we investigated X-ray amorphous material formation and longevity within terrestrial Fe-rich soils of different ages in terrestrial mediterranean, subarctic, and desert climates using bulk and selective dissolution methods, Rietveld refinements of powder XRD patterns, and transmission electron microscopy. Results indicate that in situ aqueous alteration is required to concentrate Fe into the clay-size material and to form abundant Fe-containing X-ray amorphous material. Cooler climates promote the formation and persistence of Fe-rich X-ray amorphous material whereas warmer climates promote the formation of crystalline secondary phases. Fe-rich X-ray amorphous material formation and persistence on Mars are therefore consistent with past cool and relatively wet environments followed by long-term cold and dry conditions.