North Pole (MOC2-334)

Global View (MOC2-402)

South Pole (MOC2-384)

Mars' dry ice polar caps...

The Martian polar regions can become cold enough for the atmosphere itself (carbon dioxide) to condense in the polar regions during winter. The combination of this and the eccentricity of Mars' orbit (as well as other factors) causes an annual cycle in surface pressure or atmospheric mass. The surface pressure can vary by upto 1/3 of its mean value of 6 millibars because of this effect.

The amplitude of this pressure cycle is dependent on factors such as the amount of dust in the atmosphere, or the thermal properties of the dry ice that is deposited on the ground. For instance, 20 years ago the Viking spacecraft observed that the southern dry ice seasonal cap did not completely disappear during southern summer. This was attributed to dust in the atmosphere lowering the amount of sunlight that could reach the cap, thus stopping it from completely subliming away.

The condensation of carbon dioxide causes a north-south mass flow from the subliming (spring) to the condensing (autumn) cap, which can be quite significant in strength. In fact winds associated with this mass flow may be one of the main causes of the dust storms that periodically sweep across the planet.