Variability of rock texture and morphology correlated with the clay-bearing units at Mawrth Vallis, Mars

The clay units at Mawrth Vallis have been well-characterized in hyperspectral data; however, a similar study of high spatial resolution High Resolution Imaging Science Experiment (HiRISE) data has not been previously conducted. Here the textures of the clay units are described and related to mineralogy across the central Mawrth Vallis region. The nontronite-bearing rocks appear tan in HiRISE COLOR data and are polygonally fractured with polygons 2-5m across. In some cases, the fractures appear wider and/or have darker fill or the rocks are a darker brown. The montmorillonite-bearing rocks appear blue with regular polygons 0.5-1.5m across; sometimes, there are larger polygons surrounded by regular polygons, a square fracture pattern, or the color appears yellow or mottled blue-yellow. Kaolinite-rich rocks are the brightest outcrops and are nonpolygonally fractured. Regions with spectra consistent with hydrated silica or the ferrous mineral component do not have unique textures. Hydrated silica-bearing rocks appear yellow or mottled with a regular polygonal texture or yellow with hummocky appearance with no polygons. It is also possible that dust/sand on the surface alters the montmorillonite spectrum to appear like that of hydrated silica. The ferrous component may be expressed as mottled coloring or as a bright fracture fill. The nontronite- and montmorillonite-bearing units have remarkably consistent textures in this region, allowing them to be uniquely identified in the Mawrth Vallis region in nonhyperspectral data sets such as CTX and HiRISE. The morphology of the polygons in these two units suggests that their formation is likely dominated by desiccation and controlled by composition.