In this image, we can see that the north pole is surrounded by a vast “sea” of basaltic sand dunes. In northern winter a seasonal polar cap composed of carbon dioxide ice (dry ice) forms and the surrounding dunes become covered with frost. In the spring, the ice sublimates (evaporates directly from ice to gas) loosening and moving tiny dust particles.

The bright portions of the dunes in this image are areas still covered with seasonal frost while dark spots are areas where the frost is gone or dark dust has cascaded down the sides of the dune.

The dunes imaged here are similar to barchan dunes that are commonly found in desert regions on Earth. Barchan dunes are generally crescent-shaped with a steep slip face bordered by horns oriented in the downwind direction. Barchan dunes form by winds blowing mainly in one direction and thus are good indicators of the dominant wind direction when the dunes formed.

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.