Global documentation of gullies with the Mars Reconnaissance Orbiter Context Camera and implications for their formation

Hypotheses ranging from fluvial processes and debris flows to CO2 frost-lubricated or entirely dry flows have been proposed for the formation of martian gullies. In order to constrain these potential formation mechanisms, we mapped the global distribution of gullies on Mars using >54,000 images from the Mars Reconnaissance Orbiter (MRO) Context Camera (CTX) covering ∼85% of the martian surface at a resolution of ∼6 m/pixel. The results of this mapping effort confirm the results of studies using lower resolution and/or less areally extensive datasets that gullies are confined to the martian mid- to high-latitudes (∼30-80° in both hemispheres). We also find a clear transition in gully orientation with increasing latitude, going from poleward-facing to equator-facing preference. In general, gullies are more developed on poleward-facing walls, and mid-latitude gullies are more developed than those at higher latitudes. Gullies are also found to be strongly correlated with regions of distinct thermophysical properties of sand- to pebble-sized grains, low albedo, and higher thermal inertia. These observations all point to climate, insolation, and thermal properties of the substrate playing key factors in gully formation on Mars, supporting either a melting ground ice or snowpack hypothesis as the source for water involved in gully formation.