Polar ice cap

A polar ice cap or polar ice sheet is a high-latitude region of a planet or moon that is covered in ice. There are no requirements with respect to size or composition for a body of ice to be termed a polar ice cap, nor any geological requirement for it to be over land; only that it must be a body of solid phase matter in the polar region. This causes the term 'polar ice cap' to be somewhat of a misnomer, as the term ice cap itself is applied with greater scrutiny as such bodies must be found over land, and possess a surface area of less than 50,000 km²: larger bodies are referred to as ice sheets.

The composition of the ice will vary. For example Earth's polar ice caps are mainly water ice, while Mars's polar ice caps are a mixture of solid phase carbon dioxide and water ice.

Polar ice caps form because high-latitude regions receive less energy in the form of solar radiation from the sun than equatorial regions, resulting in lower surface temperatures. Seasonal variations of the ice caps will take place due to varied solar energy absorption as the planet or moon revolves around the sun. Additionally, in geologic time scale, the ice caps may grow or shrink due to climate variation. See ice age, polar climate.

The Arctic ice cap is currently shrinking, whereas the Antarctic ice cap appears to be thickening, possibly as a result of anthropogenic global warming as described in references in Antarctic ice sheet.

Earth

File:Antarctica satellite orthographic.jpg

A satellite composite image of Antarctica

Earth's north pole is covered by floating pack ice (sea ice) over the Arctic Ocean, the Arctic ice pack. Portions of the ice that don't melt seasonally can get very thick, up to 3–4 meters thick over large areas, with ridges up to 20 meters thick. One-year ice is usually about a meter thick. The area covered by sea ice ranges between 9 and 12 million km². In addition, the Greenland ice sheet covers about 1.71 million km² and contains about 2.6 million km³ of ice.

The land mass of the Earth's south pole, in Antarctica, is covered by the Antarctic ice sheet. It covers an area of almost 14 million km² and contains 25-30 million km³ of ice. Around 70% of the fresh water on the Earth is held in this ice sheet. In addition, the West Antarctic Ice Sheet covers 3.2 million km² and the Ross Ice Shelf covers 0.5 million km². See Climate of Antarctica.

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  Extent of the Arctic ice-pack in September, 1978-2002
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  Extent of the Arctic ice-pack in February, 1978-2002
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  Earth as seen from Apollo 17, with southern polar ice cap visible (Courtesy NASA)

Mars

Main articles: Planum Australe and Planum Boreum

Mars's north polar region with ice cap, composite of Viking 1 orbiter images (Courtesy NASA/JPL-Caltech)

The planet Mars also has polar ice caps, but they consist of frozen carbon dioxide as well as water. The ice caps change with the Martian seasons-the carbon dioxide ice sublimes in summer, uncovering a surface of layered rocks, and then reforms in winter.