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.[1] 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.[2] The northern polar cap has a diameter of about 1000 km during the northern Mars summer,[3] and contains about 1.6 million cubic km of ice, which if spread evenly on the cap would be 2 km thick.[4] (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.[5] 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.[6] 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.[7][8]
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.[9][10][11][12]
In 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.[13][14]
^Hess, S.; Henry, R.; Tillman, J. (1979). "The seasonal variation of atmospheric pressure on Mars as affected by the south polar cap". Journal of Geophysical Research. 84: 2923–2927. Bibcode:1979JGR....84.2923H. doi:10.1029/JB084iB06p02923.
^Darling, David. "Mars, polar caps". Encyclopedia of Astrobiology, Astronomy, and Spaceflight. Retrieved 2007-02-26.
^"MIRA's Field Trips to the Stars Internet Education Program". Mira.or. Retrieved 2007-02-26.
^Carr, Michael H.; Head, James W. (2003). "Oceans on Mars: An assessment of the observational evidence and possible fate". Journal of Geophysical Research. 108 (5042): 24. Bibcode:2003JGRE..108.5042C. doi:10.1029/2002JE001963. S2CID 16367611.
^Phillips, Tony. "Mars is Melting, Science at NASA". Archived from the original on 2007-02-24. Retrieved 2007-02-26.
^Plaut, J. J.; et al. (2007). "Subsurface Radar Sounding of the South Polar Layered Deposits of Mars". Science. 316 (5821): 92–5. Bibcode:2007Sci...316...92P. doi:10.1126/science.1139672. PMID 17363628. S2CID 23336149.
^Smith, Isaac B.; Holt, J. W. (2010). "Onset and migration of spiral troughs on Mars revealed by orbital radar". Nature. 465 (4): 450–453. Bibcode:2010Natur.465..450S. doi:10.1038/nature09049. PMID 20505722. S2CID 4416144.
^"Mystery Spirals on Mars Finally Explained". Space.com. 26 May 2010. Retrieved 2010-05-26.
^"NASA Findings Suggest Jets Bursting From Martian Ice Cap". Jet Propulsion Laboratory. NASA. August 16, 2006. Retrieved 2009-08-11.
^Kieffer, H. H. (2000). "Annual Punctuated CO2 Slab-ice and Jets on Mars". Mars Polar Science 2000(PDF). Retrieved 2009-09-06.
^G. Portyankina, ed. (2006). "Simulations of Geyser-type Eruptions in Cryptic Region of Martian South". Fourth Mars Polar Science Conference(PDF). Retrieved 2009-08-11.
^Kieffer, Hugh H.; Christensen, Philip R.; Titus, Timothy N. (May 30, 2006). "CO2 jets formed by sublimation beneath translucent slab ice in Mars' seasonal south polar ice cap". Nature. 442 (7104): 793–796. Bibcode:2006Natur.442..793K. doi:10.1038/nature04945. PMID 16915284. S2CID 4418194.
^Halton, Mary (July 25, 2018). "Liquid water 'lake' revealed on Mars". BBC News. Retrieved July 26, 2018.
^Cite error: The named reference Orosei2018 was invoked but never defined (see the help page).
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