Polar ice sheet

A polar ice cap or polar cap is a high-latitude region of a planet, dwarf planet, or natural satellite 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 something of a misnomer, as the term ice cap itself is applied more narrowly to bodies that are over land, and cover 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 caps are mainly water ice, whereas Mars's polar ice caps are a mixture of solid 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.

Earth's polar caps have changed dramatically over the last 12,000 years. Seasonal variations of the ice caps takes place due to varied solar energy absorption as the planet or moon revolves around the Sun. Additionally, in geologic time scales, the ice caps may grow or shrink due to climate variation.

Extent of the Arctic sea-ice in September 1978 – 2002

Extent of the Arctic sea-ice in February 1978 – 2002

The Blue Marble, Earth as seen from Apollo 17 with the southern polar ice cap visible (courtesy NASA)

Earth's North Pole is covered by floating pack ice (sea ice) over the Arctic Ocean. Portions of the ice that do not 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 1 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. When the ice breaks off (calves) it forms icebergs scattered around the northern Atlantic.

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