The north polar ice cap of Mars changes its face during the course of the year: a permanent cap composed mainly of water ice is observed in the summer season, as shown in this HRSC image. During the winter season, temperatures fall below -125 degrees Celsius, cold enough for carbon dioxide to precipitate and build a thin (1-2 meters) seasonal polar cap of carbon dioxide ice. At this time of the year, the ice cap is often covered by thick carbon dioxide clouds and thus difficult to observe with cameras from orbit.