This image was acquired by the Mars Express High-Resolution Stereo Camera on 17 May 2010 and shows a part of the northern polar region of Mars at the northern hemisphere summer solstice.
The polar ice deposits follow the seasonal cycles. Studies made by Mars Express’s OMEGA instrument shows that the cap is covered by frozen water and carbon dioxide ice in winter and spring but by this point in the martian year all of the carbon dioxide ice has warmed and evaporated into the planet’s atmosphere.
Only water ice is left behind, which shows up as bright white areas in this picture. From these layers, large bursts of water vapour are occasionally released into the atmosphere.
In winter, part of the atmosphere recondenses as frost and snow on the northern cap. These seasonal deposits can extend as far south as 45°N latitude and be up to a metre thick.
Another phenomenon occurs on the curved scarps of the northern polar cap, such as the Rupes Tenuis slope (on the left of this image). During spring, the seasonal carbon dioxide layer is covered by water frost. At certain times, winds remove the the millimetre-thick top layer of frozen water, revealing the carbon dioxide ice below.
These processes bear witness to a dynamic water cycle on Mars and may lead to the varying accumulation of water ice over the polar cap.