Polar caps

Mars possesses ice caps at both poles, which mainly consist of water ice; however, there is frozen carbon dioxide (dry ice) present on their surfaces. Dry ice accumulates in the northern polar region (Planum Boreum) in winter only, subliming completely in summer, while the south polar region additionally has a permanent dry ice cover up to eight metres (25 feet) thick.^[38] This difference is due to the higher elevation of the south pole.

So much of the atmosphere can condense at the winter pole that the atmospheric pressure can vary by up to a third of its mean value. This condensation and evaporation will cause the proportion of the noncondensable gases in the atmosphere to change inversely.^[39] The eccentricity of Mars's orbit affects this cycle, as well as other factors. In the spring and autumn wind due to the carbon dioxide sublimation process is so strong that it can be a cause of the global dust storms mentioned above.^[40]

The northern polar cap has a diameter of approximately 1,000 km during the northern Mars summer,^[41] and contains about 1.6 million cubic kilometres of ice, which if spread evenly on the cap would be 2 km thick.^[42] (This compares to a volume of 2.85 million cubic kilometres for the Greenland ice sheet.) The southern polar cap has a diameter of 350 km and a maximum thickness of 3 km.^[43] Both polar caps show spiral troughs, which are believed to form as a result of differential solar heating, coupled with the sublimation of ice and condensation of water vapor.^[44][45] Both polar caps shrink and regrow following the temperature fluctuation of the Martian seasons; there are also longer-term trends that are not fully understood.

During the southern hemisphere spring, solar heating of south polar dry ice deposits leads in places to accumulation of pressurized CO₂ gas below the surface of the semitransparent ice, warmed by absorption of radiation by the darker substrate. After attaining the pressure needed to burst through, the gas erupts in geyser-like plumes. While the eruptions have not been directly observed, they leave evidence in the form of "dark dune spots" and lighter fans atop the ice, representing sand and dust carried aloft by the eruptions, and a spider-like pattern of grooves created below the ice by the outrushing gas.^[46][47] (see Geysers on Mars.) Eruptions of nitrogen gas observed by Voyager 2 on Triton are thought to occur by a similar mechanism.