Martian gullies as seen by the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM)

Over 100 high resolution targeted images of martian gullies and debris fans have been obtained by the CRISM instrument currently orbiting Mars aboard the Mars Reconnaissance Orbiter. A survey of these images provides some new constraints on the origin and evolution of these features. We have found no evidence for the recent formation of hydrated minerals in these features, even the most recent light-toned ones. Ice clouds are sometimes visible in the vicinity of the gullies, but these are seasonal. A wide range of mineralogies is observed and includes phyllosilicates, mafics and sulphates. The distribution of the minerals and their relationship to the underlying rocks suggest that the gully-forming process exposes underlying rocks and moves them downslope. When more than one gully is present on a given slope, both vertical and horizontal stratigraphy within the source region is revealed. The wispy nature of the deposits, especially when specific spectral indicators are examined, suggests that the gullies are the result of many small events. The lack of dust coverage on some of the spectrally distinct gullies could indicate fairly recent activity, although eolian erosion could also be a factor. Many other gullies that are indistinct spectrally from their surroundings could be old and inactive, or could indicate very active dust deposition. The processes dominating the modification of the gullies can be established by combining CRISM observations with other nearby geomorphic indicators.