Dunes of sand-sized materials have been trapped on the floors of many Martian craters. This is one example, from a crater in Noachis Terra, west of the giant Hellas impact basin.

The High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter captured this view on Dec. 28, 2009.

The dunes here are linear, thought to be due to shifting wind directions. In places, each dune is remarkably similar to adjacent dunes, including a reddish (or dust colored) band on northeast-facing slopes. Large angular boulders litter the floor between dunes.

The most extensive linear dune fields know in the solar system are on Saturn's large moon Titan. Titan has a very different environment and composition, so at meter-scale resolution they probably are very different from Martian dunes.

This image covers a swath of ground about 1.2 kilometers (three-fourths of a mile) wide, centered at 42.7 degrees south latitude, 38.0 degrees east longitude. It is one product from HiRISE observation ESP_016036_1370. The season on Mars is southern-hemisphere autumn. Other image products from this observation are available at http://hirise.lpl.arizona.edu/ESP_016036_1370.

The University of Arizona, Tucson, operates the HiRISE camera, which was built by Ball Aerospace & Technologies Corp., Boulder, Colo. NASA's Jet Propulsion Laboratory, a division of the California Institute of Technology, Pasadena, manages the Mars Reconnaissance Orbiter for the NASA Science Mission Directorate, Washington. Lockheed Martin Space Systems, Denver, is the prime contractor for the project and built the spacecraft.