CHANGES IN GEOCHEMICAL ENVIRONMENT DETECTED AT MAWRTH VALLIS, MARS, THROUGH CHANGES IN THE MINERALOGIC RECORD OBSERVED IN ORBITAL IMAGERY

Mineralogical evidence for multiple aqueous alteration settings at Mawrth Vallis, Mars, indicates changes in the geochemical environment. We investigated VNIR spectra from multiple CRISM images to identify phyllosilicate, iron oxide/hydroxide, and sulfate minerals and overlayed these detections on HRSC and HiRISE imagery to view changes in composition in the stratigraphy. A thick layered horizon of Fe/Mg-smectite is covered by a unit containing Al-phyllosilicates, opal, and allophane. Smaller outcrops of sulfates are observed in many locations as spectral “doublet” features in between the phyllosilicate units and are likely due to mixtures of sulfates and phyllosilicates. These may represent evaporite or hydrothermal settings. CRISM spectral features were compared with laboratory spectra of mineral mixtures and field samples in order to constrain the composition of the small jarosite and alunite outcrops. The Mawrth Vallis region of Mars exhibits some of the thickest outcrops of layered clay-rich material that includes stratigraphic changes in clay chemistry, redox reversals, and isolated sulfates. Changes from Fe-smectite to jarosite represent pH shifts from neutral/alkaline to acidic environments. Similarly, montmorillonite deposits transforming to kaolinite/alunite also indicate decreasing pH. Assemblages containing gypsum and opal are characteristic of evaporitic salars, while siliceous sinter, jarosite and/or alunite can represent hydrothermal settings including acidic groundwater or fumerole vents. Redox changes in any of these settings represent a potential energy source for microbes. Coordinated analysis of analog field sites with orbital remote sensing can enable documentation of changing environments on Mars and a better understanding of potential habitable environments or ecosystems there.