Insight into gully formation on Mars with CRISM on the Mars Reconnaissance Orbiter

Gullies are widespread on Mars, with most occurrences found in the southern hemisphere. Indicative of recent downslope movement, multiple alternative models have been proposed for their formation, including groundwater release, melting of snow or near-surface ground ice, dry granular flows, or different CO2-lubricated flows. Ongoing morphological changes to gully channels and aprons observed with the High Resolution Imaging Science Experiment (HiRISE) over intervals as short as one Martian year have indicated seasonal activity consistent with models for gully formation driven by CO2 frost sublimation as well as dry granular flow. To determine if compositional information could provide additional insight into the mechanics of gully formation and seasonal activity, we have analyzed over 100 images of gullies and their apron deposits taken with the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) on the Mars Reconnaissance Orbiter (MRO) over multiple Martian years. Newly processed prototype Map-projected Targeted Reduced Data Record (MTRDR) hyperspectral image cubes were used to identify and extract spectral information. Additional Mars Orbital Laser Altimeter (MOLA) and HiRISE DTM data were used to obtain topographical information. Most gullies observed are spectrally indistinct from their surroundings, most likely due to dust coatings. Where spectral contrast was observed, gullies predominantly exposed and transported underlying mafic material downslope. Rarely were hydrated minerals or alteration products observed in place within these gullies or within their apron deposits, indicating very limited chemical reaction with liquid water. Where detected, hydrated minerals include phyllosilicates and sulfates, and usually occur in a pre-existing layer that is exposed and subsequently transported downslope. Gullies do not show evidence for in situ precipitation or alteration as a result of long lived water-rock interactions. Finally, spectral evidence for seasonal CO2 ice is observed in mid- to high latitude gully channels and apron deposits. This observation is consistent with CO2-driven processes playing a greater role in the formation and modification of these features at higher latitudes. Taken all together, our observations suggest a limited role for liquid water in the formation of gullies and their deposits.