This image shows defrosting sand dunes near the north polar region of Mars.
Around Mars’ North Pole is a vast region or “sea” of sand dunes that become covered with carbon dioxide frost or ice in the northern hemispheres winter. The light areas indicate that parts of the dunes are still covered in frost or ice.
As Mars’ Northern hemisphere enters into spring and begins to warm, the carbon dioxide sublimates (turns directly from a solid to a gas). The carbon dioxide sublimates in surprising ways, with trapped gas bursting through the ice in jets that leave dark streaks when the wind is blowing
During the summer, all the frost will have sublimed leaving dark sand dunes. The unfrosted dunes are dark because the sand is derived from dark volcanic rocks.Written by: Alfred McEwen (10 May 2008)
More info and image formats at http://hirise.lpl.arizona.edu/PSP_007801_2610
Image: NASA/JPL/University of Arizona