And trapped mud and sediment caught in turbidity currents on the ocean floor cut deep channels into the continental shelves, Musselwhite noted. Gullies in polar pit walls. The planet's atmosphere is mostly composed of CO2. Under some atmospheric pressure, CO2 condenses from the atmosphere and into Mars' surface. Mars has been pummeled by impacts, so its surface is typically porous, spongy gravel.
Gas seeps through the surface and condenses in the pores of rock.
In wintertime the cliff surface gets so cold that its temperature falls below the freezing point of CO2, which at low pressure goes directly to solid. As the cold wave moves from the surface, the pore space is completely filled in. When spring comes, dry ice warms up and expands. Since all the rock pore space is filled, pressure builds until the ice turns to liquid. Liquid CO2 takes up more volume than dry ice, so pressure continues to build.