Spectral constraints on the formation mechanism of recurring slope lineae

Recurring slope lineae (RSL) exhibit multiple lines of evidence for a wet origin. In the southern midlatitudes, they form on steep, equator-facing slopes that are warm during southern summer. The formation temperature, seasonality, and other geomorphic characteristics are suggestive of water-related formation, perhaps dense brines. We examined Compact Reconnaissance Imaging Spectrometer for Mars images of all confirmed RSL sites from the southern midlatitudes and the equatorial region to understand the composition of RSL and/or RSL-associated deposits. We did not detect any spectral signature attributable to water; however, a distinct and consistent spectral signature is observed at most sites, indicating enhanced abundances or distinct grain sizes of both ferric and ferrous minerals in RSL-related materials compared to adjacent non-RSL slopes. Like the RSL themselves, the strength of these signatures varies as a function of season. The observed spectral changes may indicate removal of a fine-grained surface component during RSL flow, precipitation of ferric oxides, and/or wetting of the substrate. Key Points CRISM analysis of RSL Consistent and diagnostic spectral characteristics associated with RSL Spectral behavior consistent with wetting of the substrate