Mars Polar Gully Modification and Possible Formation from Condensed Volatiles

The apparently young age of Martian gullies, a morphology indicative of groundwater seepage and surface runoff, and conditions of the near-surface that preclude long-term storage and flow of liquid water has presented a challenge to geologists and theoreticians trying to explain the origin of these enigmatic features. Various hypotheses have been presented and published, including those that invoke obliquity changes and enhanced volatile concentration in the past. Here we present results from MOC image analysis, together with theoretical arguments, showing that water ice is preferentially concentrated in polar gullies by progressive cold trapping in the presence of CO2 frost. Thermal buffering by the CO2 maintains the high concentrations of water within pole-facing gullies until near the summer equinox, when midnight sun incident on steep slopes causes sublimation of the CO2. Water ice may melt under ideal conditions in this scenario or, at the very least, play a role with the CO2 in lubricating surface materials. This process is possible in the present day and is consistent with other observations from surface and orbital Mars platforms showing enhanced volatile concentration over and above atmospheric column abundances. This process likely plays a role in the modification and possibly in the formation of gullies, especially in polar regions of Mars where sufficient volatiles can be concentrated.