Martian polar ice caps

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 CO2 ice (dry ice). When the poles are again exposed to sunlight, the frozen CO2 sublimes. These seasonal actions transport large amounts of dust and water vapor, giving rise to Earth-like frost and large cirrus clouds.Layers exposed.Close view of layers exposed.Layers visible along edge of northern ice cap.Layers exposed in northern ice cap, as seen by HiRISE under HiWish programClose view of layers exposed in northern ice cap, as seen by HiRISE under HiWish programChanges in south polar surface from 1999 to 2001, as seen by Mars Global Surveyor.'Swiss cheese' ice formations as seen by MGS.Swiss cheese-like ice formations as seen by MGS, showing layers.Close-up of Swiss cheese terrain, as seen by MGS.HiRISE view of south polar terrain.Star burst channels caused by escaping CO2 gas, as seen by HiRISE. Such channels, also called spiders, may be about 500 m in diameter and 1 m deep.Spider on the second Martian day of spring, as seen by HiRISE.Same spider 14 Martian days later, as seen by HiRISE. Notice increased dark fans caused by outgassing of carbon dioxide carrying dark material.Wide view of plumes, as seen by HiRISE under HiWish program Many of the plumes show spiders when enlarged.Spiders, as seen by HiRISE under HiWish programPlumes and spiders, as seen by HiRISE under HiWish programThis HiRISE image shows layers running roughly up and down, with faint polygonal fracturing (mostly rectangular).South polar layers, as seen by THEMIS.Close-up of layers in wall of McMurdo crater, as seen by HiRISE.Layers exposed in a valley on the north polar ice cap as observed by Mars Odyssey. Click on image to enlarge to see clouds of dust caused by winds coming off the cap.Chasma Boreale streamlined feature, as seen by HiRISE.Chasma Boreale, as seen by HiRISE.Steep scarp, as seen by HiRISE.North polar layers on the side of a valley, as seen by HiRISE. Layers erode differently, depending on what direction they face. On one side they are straight, as if cut by a knife.Chasma Boreale channels, as seen by HiRISE. 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 CO2 ice (dry ice). When the poles are again exposed to sunlight, the frozen CO2 sublimes. 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. 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 (km3) 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. 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. In July 2018, Italian scientists reported the discovery of 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, the first known stable body of water on the planet. Research based on slight changes in the orbits of spacecraft around Mars over 16 years found that when one hemisphere experiences winter, approximately 3 trillion to 4 trillion tons of carbon dioxide freezes out of the atmosphere onto the northern and southern polar caps. This represents 12 to 16 percent of the mass of the entire Martian atmosphere. These observations support predictions from the Mars Global Reference Atmospheric Model—2010. Both polar caps show layered features, called polar-layered deposits, that result from seasonal ablation and accumulation of ice together with dust from Martian dust storms. Information about the past climate of Mars may be eventually revealed in these layers, just as tree ring patterns and ice core data do on Earth. Both polar caps also display grooved features, probably caused by wind flow patterns. The grooves are also influenced by the amount of dust. The more dust, the darker the surface. The darker the surface, the more melting. Dark surfaces absorb more light energy. There are other theories that attempt to explain the large grooves. The bulk of the northern ice cap consists of water ice; it also has a thin seasonal veneer of dry ice, solid carbon dioxide. Each winter the ice cap grows by adding 1.5 to 2 m of dry ice. In summer, the dry ice sublimates (goes directly from a solid to a gas) into the atmosphere. Mars has seasons that are similar to Earth's, because its rotational axis has a tilt close to our own Earth's (25.19° for Mars, 23.44° for Earth). During each year on Mars as much as a third of Mars' thin carbon dioxide (CO2) atmosphere 'freezes out' during the winter in the northern and southern hemispheres. Scientists have even measured tiny changes in the gravity field of Mars due to the movement of carbon dioxide. The ice cap in the north is of a lower altitude (base at -5000 m, top at -2000 m) than the one in the south (base at 1000 m, top at 3500 m). It is also warmer, so all the frozen carbon dioxide disappears each summer.The part of the cap that survives the summer is called the north residual cap and is made of water ice. This water ice is believed to be as much as three kilometers thick. The much thinner seasonal cap starts to form in the late summer to early fall when a variety of clouds form. Called the polar hood, the clouds drop precipitation which thickens the cap. The north polar cap is symmetrical around the pole and covers the surface down to about 60 degrees latitude. High resolution images taken with NASA's Mars Global Surveyor show that the northern polar cap is covered mainly by pits, cracks, small bumps and knobs that give it a cottage cheese look. The pits are spaced close together relative to the very different depressions in the south polar cap.