Formation of Clays, Ferrihydrite, and Possible Salts in Hydrae Chasma, Mars

We used multiple orbital data sets to analyze the morphology, composition, and stratigraphy of clays and other hydrated deposits within Hydrae Chasma. Fe/Mg-smectites are concentrated along the lower floor of the chasma, adjacent to pit craters. There are at least two distinct smectites that vary in their morphology and spectral properties, with a more Mg-rich smectite associated with a thin brighter upper layered deposit and a more Fe-rich smectite in a lower darker massive deposit. We also identified a younger hydrated unit with spectral features similar to ferrihydrite. The spectra of this unit lack an absorption between 2.28 and 2.31 µm, but display a drop in reflectance near 2.3 µm. The ferrihydrite-type detections correspond to cleaner surfaces of light-toned deposits further north and higher in elevation than the smectites. A few small outcrops in the southern chasma floor appear brighter than the clays and display an additional narrow absorption at 2.4 µm in combination with the smectite features, which may indicate a smectite plus perchlorate mixture. There are no valleys along the plateau that intersect Hydrae Chasma and, consequently, any water that created the Fe/Mg-smectites and ferrihydrite must have been from groundwater in the subsurface or surface water sourced from within the chasma. A terraced fan and associated valley to the north of the clays provide support for melting snow/ice within the chasma and this same water may have also altered materials lower in elevation where the ferrihydrite-bearing unit formed under colder conditions than the smectites. The change in stratigraphy from smectites to ferrihydrite is consistent with a decrease in global temperature during the Hesperian.