Dark Dunes on MarsDark dunes make waves across the floor of Proctor Crater, a 93-mile-wide (150-kilometer-wide) basin on Mars, as seen in a picture by the HiRSE camera aboard NASA's Mars Reconnaissance Orbiter released April 28.

The dunes are most likely made of fine, basaltic—or volcanic—sand that collected in the bottom of the crater, according to NASA. Patches of frost seem to light up the tops of some dunes, while large boulders dot the smaller ripples along the dune beds.
Image courtesy NASA/JPL/University of Arizona

Space Photos This Week: Black Holes, Sahara Storm, More

Midsize black holes, a dust "wall" pushing across Africa, and recently made streaks on Mars feature among the week's best space pictures.

May 4, 2010