Mars, polar caps

Two variable white areas at the Martian poles, visible to Earth-based observers but studied in detail by the Mariner and Viking Orbiter spacecraft and more recently by the Mars Global Surveyor and Mars Odyssey orbiters. Each cap grows under a haze of cloud during the winter in its hemisphere, reaching a latitude of 50° N or S at its greatest extent. During summer it retreats to leave a small irregular patch about 500 km across. This residual (permanent) cap probably consists almost entirely of water ice, while the variable cap appears to be a frost of frozen carbon dioxide, which is appears to be deposited out of the thin Martian atmosphere by dust storms in winter. At latitudes greater than 80°, the polar terrain exhibits a crater-free layered structure up to 6 km thick that possibly represents successive deposition of wind-borne dust. The covering of carbon dioxide is about 1 meter thick over the north pole but appears to be thicker over the south pole, where it is estimated to be up to 8 meters in depth. In summer the evaporation of carbon dioxide back into the atmosphere accompanies the retreat of the caps.

单词	mars, polar caps
释义	Mars, polar caps Mars, polar caps Two variable white areas at the Martian poles, visible to Earth-based observers but studied in detail by the Mariner and Viking Orbiter spacecraft and more recently by the Mars Global Surveyor and Mars Odyssey orbiters. Each cap grows under a haze of cloud during the winter in its hemisphere, reaching a latitude of 50° N or S at its greatest extent. During summer it retreats to leave a small irregular patch about 500 km across. This residual (permanent) cap probably consists almost entirely of water ice, while the variable cap appears to be a frost of frozen carbon dioxide, which is appears to be deposited out of the thin Martian atmosphere by dust storms in winter. At latitudes greater than 80°, the polar terrain exhibits a crater-free layered structure up to 6 km thick that possibly represents successive deposition of wind-borne dust. The covering of carbon dioxide is about 1 meter thick over the north pole but appears to be thicker over the south pole, where it is estimated to be up to 8 meters in depth. In summer the evaporation of carbon dioxide back into the atmosphere accompanies the retreat of the caps.
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