North and south subice gas flow and venting of the seasonal caps of Mars: A major geomorphological agent

Dark polygons associated with fans and spots appear during the spring on the southern seasonal cap. The basal sublimation of the translucent cap and the venting of the CO2 gas are responsible for their formation, as previously proposed for the spots and fans. Dark polygons appear when dark material emerges from elongated vents, whereas spots and fans form from point sources. A class of erosive features (etched polygons) is associated with depressions a few meters to tens of meters in diameters connected to a network of radiating troughs ("spiders''). Spiders are shaped by the scouring action of the confined gas converging toward point sources, whereas the etched polygons result from the forced migration of the CO2 gas over longer distances. The minimum age of the spiders is 10 4 years. They result from one of the most efficient erosive processes on Mars, displacing 2 orders of magnitude more dust per year than a typical dust storm or than all the dust devils during the same time period. In the north, parts of the seasonal cap are translucent between Ls = 355 degrees and Ls = 60 degrees and are associated with spots, fans, dark polygons, and possibly spiders, suggesting that the basal sublimation and venting of the cap triggers a subice gas and dust flow that is modifying the morphology of the surface layer. However, perennial features are extremely uncommon on the north regolith, indicating that the conditions for their formation or conservation are not met. The reduced basal energy budget of the north cap compared to the south and the shorter seasonal life time of the north translucent ice may explain the relative scarcity of features in the north. The polar layered deposits contain the stratigraphic record of climatic changes and catastrophic events. Both polar deposits may have been locally disrupted by the seasonal subice gas flow and the stratigraphic record may have been partially lost.