Evolution within the Martian Southern Seasonal Cryptic Terrain from CTX and CRISM

The Martian south seasonal "Cryptic Region" is a large area in the retreating cap that develops a low albedo, but retains the cold temperature of CO₂ ice in equilibrium with the atmosphere. This area is associated with venting of material from underneath seasonal slab ice creating debris fans that darken the upper surface of the seasonal cap. In prior work, we observed the seasonal retreat in four Mars Years (MY) using the MARCI. We found the large-scale boundary of this area was similar in all MY, with many interannual small scale variations. Why the cryptic area occurs where it does and not throughout the retreating seasonal cap remains puzzling and not easily explained by elevation, deposition, or topography. Detailed examination of MARCI mosaics at ~2 km/pixel identified several phenomena that were explored at higher spatial resolution using CTX and CRISM color data in MY34. CTX and CRISM acquisitions were requested for Reynolds and Main Craters, the boundary of Promethei Rupes, and regions in Dorsa Argentea over the "anti-cryptic" area that retains seasonal frost very late. Observations of these 6 regions were taken ~ every 5° of Ls from 180 to 280, followed by every 10° to 20° of Ls up to 360 in order to monitor seasonal changes in color, albedo and terrain evolution. An additional complication is the large planet-scale dust event that occurred in MY34. This flattened contrast in CTX images, but most were clear enough to observe the evolution of seasonal frost patterns. As noted at AGU last year, the dust storm did not cause enhanced or early retreat of the seasonal cap. Instead, the MY 34 dust storm appears to have increased CO₂ deposition over Reynolds Crater perhaps due to surface cooling from increased opacity that lead to deposition as frost sublimated from lower latitudes. We also observe that poleward facing steep scarps or slopes retain seasonal frost the longest. Local equatorward slopes initiate venting, and albedo feedback (dark covered material sublimating faster) propagates seasonal frost retreat. As seen in HiRISE and CRISM over other sites, late season brightening is also observed over Reynolds and Promethei Rupes. This may be due to dust sinking into the remaining seasonal ice; however removal of the surface layer by winds seems likely, based on processes observed in the north.