Water ice on Mars (winter). During the Martian winter much of the frozen water (blue) on Mars is obscured by layers of carbon dioxide frost or snow (dry ice, green). This image shows the northern hemisphere of Mars. During a Martian year (687 Earth days), as much as a third of Mars' carbon dioxide atmosphere freezes. The water-rich soil layer beneath the frost is the Martian equivalent of permafrost. This image, showing the composition of elements in the top metre of the surface of Mars, is based on data from the gamma- ray spectrometer on board NASA's Mars Odyssey spacecraft. For an image of water ice on Mars during the Martian summer see R350/207.