A polar ice cap or polar ice sheet is a high-latitude region of a planet or moon that is covered in ice. This term is somewhat of a misnomer since an ice cap is less than 50,000 km² and is always over land: a larger area of ice is called an ice sheet. Polar ice caps do not have size, composition or geologic requirements of being over land, but they must be centered in the polar region.

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

Both of the Earth's ice caps are currently shrinking, possibly as a result of anthropogenic global warming.

Earth

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Earth's north pole is covered by floating pack ice (sea ice) over the Arctic Ocean. 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.

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 30 million km³ of ice. Around 90% of the fresh water on the Earth's surface 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.

image:north pole september ice-pack 1978-2002.png|Extent of the Arctic ice-pack in September, 1978-2002 image:north pole february ice-pack 1978-2002.png|Extent of the Arctic ice-pack in February, 1978-2002 Image:The_Earth_seen_from_Apollo_17.jpg|Earth as seen from Apollo 17, with southern polar ice cap visible (Courtesy NASA)

Mars

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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 sublimates in summer, uncovering a surface of layered rocks, and forms again in winter.

Poles | Climate change feedbacks and causes | Calota polar