Today's Image of Mars comes from HiRISE shows defrosting dunes in the north polar region during late spring. The ice that you see is primarily carbon dioxide ice, which builds up over the winter months. During the spring the carbon dioxide ice begins to sublimate, turning directly from a solid into gas.
By the time this image was taken most of the ice had sublimated. As summer approaches more and more of the ice will sublimate, leaving only small patches, if that, but then when winter comes around the ice will begin to accumulate again! Thus is the never ending cycle in the north polar regions of Mars.
Because there is such consistency on Mars scientists study the differences in the dunes each year to determine what processes, if any are still active on Mars. Click on this image to be taken to the original high resolution image. [See the HiRISE caption for this image]