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 hemisphere’s 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.

For information about NASA and agency programs on the Web, visit: http://www.nasa.gov. NASA's Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, Calif., manages the Mars Reconnaissance Orbiter for NASA's Science Mission Directorate, Washington. Lockheed Martin Space Systems is the prime contractor for the project and built the spacecraft. The HiRISE camera was built by Ball Aerospace and Technology Corporation and is operated by the University of Arizona. The image data were processed using the U.S. Geological Survey’s ISIS3 software.