A Mastcam Multispectral Investigation of Rock Variability in Gale Crater, Mars: Implications for Alteration in the Clay-Sulfate…

The Mars Science Laboratory Curiosity rover has explored over 650 m of strata since landing in 2012. After leaving Vera Rubin Ridge (VRR), the rover traversed though the phyllosilicate-rich region, Glen Torridon, and the Mg-sulfate-bearing strata that lies above it, with excursions onto the Greenheugh Pediment and Amapari Marker Band. These unique stratigraphic units were investigated using Curiosity's instrument suite which includes the Mast Camera (Mastcam) pair of multispectral imagers. Mastcam's narrowband filters are sensitive to iron-bearing phases and some hydrated minerals. We used Mastcam spectra, in combination with chemical data from Curiosity's CheMin, APXS and ChemCam instruments, to assess the variability of rock spectra and interpret the mineralogy, depositional origin, and diagenesis in the clay-sulfate transition and surrounding regions. To constrain the Mg-sulfate detection capabilities of Mastcam and aid in the analyses of multispectral data, we conducted a laboratory investigation with different mixtures of phyllosilicates, hydrated Mg-sulfate, and basalt. We identify four new classes of rock spectra since leaving VRR. The localities and cross-instrument comparisons with these Mastcam spectra suggest that: (1) two of the new classes are inherent to dusty and pyroxene-rich surfaces on the Amapari Marker Band; (2) one new class is associated with the relatively young, basaltic, Greenheugh Pediment; and (3) one new class indicates areas subjected to intense aqueous alteration with an amorphous Fe-sulfate component, primarily in the clay-sulfate transition region. Our laboratory investigation shows the ease at which hydrated Mg-sulfates can be masked by other materials, requiring ≥ 90 wt. % of hydrated Mg-sulfate to exhibit a hydration signature in Mastcam spectra. Thus, we conclude that it would be challenging for Mastcam to detect Mg-sulfate in an intimate mixture. However, we can use these results to constrain the abundance of hydrated Mg-sulfates along Curiosity's future traverse. Together these results imply significant compositional changes along the traverse since leaving VRR, and they support the hypothesis of wet-dry cycles in the clay-sulfate transition. The new set of Mastcam spectral classes will provide a baseline for Curiosity's further exploration of Mount Sharp, where hydrated sulfates, minor phyllosilicates, and other hydrous mineral signatures have been identified from orbit.