<P> The planet Mars has two permanent polar ice caps . During a pole's winter, it lies in continuous darkness, chilling the surface and causing the deposition of 25--30% of the atmosphere into slabs of CO ice (dry ice). When the poles are again exposed to sunlight, the frozen CO sublimes . These seasonal actions transport large amounts of dust and water vapor, giving rise to Earth - like frost and large cirrus clouds . Clouds of water - ice were photographed by the Opportunity rover in 2004 . </P> <P> The caps at both poles consist primarily of water ice . Frozen carbon dioxide accumulates as a comparatively thin layer about one metre thick on the north cap in the northern winter, while the south cap has a permanent dry ice cover about 8 m thick . The northern polar cap has a diameter of about 1000 km during the northern Mars summer, and contains about 1.6 million cubic km of ice, which if spread evenly on the cap would be 2 km thick . (This compares to a volume of 2.85 million cubic km (km) for the Greenland ice sheet .) The southern polar cap has a diameter of 350 km and a thickness of 3 km . The total volume of ice in the south polar cap plus the adjacent layered deposits has also been estimated at 1.6 million cubic km . Both polar caps show spiral troughs, which recent analysis of SHARAD ice penetrating radar has shown are a result of roughly perpendicular katabatic winds that spiral due to the Coriolis Effect . </P> <P> The seasonal frosting of some areas near the southern ice cap results in the formation of transparent 1 m thick slabs of dry ice above the ground . With the arrival of spring, sunlight warms the subsurface and pressure from subliming CO builds up under a slab, elevating and ultimately rupturing it . This leads to geyser - like eruptions of CO gas mixed with dark basaltic sand or dust . This process is rapid, observed happening in the space of a few days, weeks or months, a rate of change rather unusual in geology--especially for Mars . The gas rushing underneath a slab to the site of a geyser carves a spider - like pattern of radial channels under the ice . </P>

The polar caps on mars are most probably made up of