The Possible Role of Condensed Volatiles in Martian Gully Formation

The geographic confinement of martian gullies to middle to polar latitudes and preference for poleward-facing slopes implies an origin that is linked to volatile phase changes affected by solar insolation. What this volatile is, where it resides, and how it carves the gullies is a matter of considerable uncertainty and debate. Early springtime dark "spots" confined to some gully channels in the southern hemisphere have recently been identified, the first reported documented seasonal changes. The spots are absent later in the spring after complete defrosting, revealing a dark substrate, indicating that the spots were associated with defrosting of the seasonal south polar CO2 cap. Explanations consistent with liquid H2O and solid CO2 thermophysical stability will be discussed.