Martian polar ice caps

North polar cap in 1999

South polar cap in 2000

The planet Mars has two permanent polar ice caps of water ice and some dry ice (frozen carbon dioxide, CO₂). Above kilometer-thick layers of water ice permafrost, slabs of dry ice are deposited during a pole's winter,^[1][2] lying in continuous darkness, causing 25–30% of the atmosphere being deposited annually at either of the poles. When the poles are again exposed to sunlight, the frozen CO₂ sublimes.^[3] These seasonal actions transport large amounts of dust and water vapor, giving rise to Earth-like frost and large cirrus clouds.

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.^[4] The northern polar cap has a diameter of about 1000 km during the northern Mars summer,^[5] and contains about 1.6 million cubic km of ice, which if spread evenly on the cap would be 2 km thick.^[6] (This compares to a volume of 2.85 million cubic km (km³) for the Greenland ice sheet.) The southern polar cap has a diameter of 350 km and a thickness of 3 km.^[7] 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.^[8] Both polar caps show spiral troughs, which analysis of SHARAD ice penetrating radar has shown are a result of roughly perpendicular katabatic winds that spiral due to the Coriolis Effect.^[9][10]

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 CO₂ builds up under a slab, elevating and ultimately rupturing it. This leads to geyser-like eruptions of CO₂ 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.^{[11][12][13][14]}

In 2018, Italian scientists reported that measurements of radar reflections may show a subglacial lake on Mars, 1.5 km (0.93 mi) below the surface of the southern polar layered deposits (not under the visible permanent ice cap), and about 20 km (12 mi) across; If confirmed, this would be the first known stable body of water on the planet.^[15][16] However, the radar reflections may show solid minerals or saline ice instead of liquid water.^[17][18]