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, 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. 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 meter 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 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 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. However, the radar reflections may show solid minerals or saline ice instead of liquid water.
Shared features
Freezing of atmosphere Research based on slight changes in the orbits of spacecraft around Mars over 16 years found that each winter, approximately 3 trillion to 4 trillion tons of carbon dioxide freezes out of the atmosphere onto the winter hemisphere polar cap. 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.
Layers
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. China's Zhurong rover that has studied the Utopia Planitia region of Mars has found dunes that lie in different directions. The bright barchans and dark longitudinal dunes is evidence that the predominant wind field underwent a roughly 70° change. The researchers believe the dunes were formed when the tilt changed and caused a shift in the winds. At about the same time, there are changes in the layers in the Martian northern ice caps.
Deuterium enrichment Deuterium is a heavier isotope of hydrogen compared to the element's most common isotope, protium. This makes any celestial body's deuterium statistically much less prone to being carried into space by stellar wind compared to its protium. Evidence that Mars once had enough water to create a global ocean at least 137 m deep has been obtained from measurement of the HDO to H2O ratio over the north polar cap. In March 2015, a team of scientists published results showing that the polar cap ice is about eight times as enriched with deuterium as water in Earth's oceans. This means that Mars has lost a volume of water 6.5 times as large as that stored in today's polar caps. The water for a time may have formed an ocean in the low-lying Vastitas Borealis and adjacent lowlands (Acidalia, Arcadia and Utopia planitiae). Had the water ever all been liquid and on the surface, it would have covered 20% of the planet and in places would have been almost a mile deep. This international team used ESO's Very Large Telescope, along with instruments at the W. M. Keck Observatory and the NASA Infrared Telescope Facility, to map out different isotopic forms of water in Mars's atmosphere over a six-year period.
North polar cap
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