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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.〔(The National Snow and Ice Data Center Glossary )〕 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. ==Earth== File:North pole september ice-pack 1978-2002.png|Extent of the Arctic sea-ice in September 1978–2002 File:North pole february ice-pack 1978-2002.png|Extent of the Arctic sea-ice in February 1978–2002 File:The Earth seen from Apollo 17.jpg|Earth as seen from Apollo 17, with southern polar ice cap visible (courtesy NASA) 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Polar ice cap」の詳細全文を読む スポンサード リンク
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