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.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006