Windblown sand can be used to tell us the wind direction on Mars. Small-scale features, such as ripples and wind tails, indicate the most recent wind directions.

Wind tails may be the remnants of a formerly widespread mantle of sediment that has been removed. Alternatively, they may have formed when aeolian sediment is deposited in the wind-shadow zone behind obstacles such as the 1.5 meter diameter boulders on the crater rim. Their orientation points in the downwind direction. In the figure, two wind tails extend from some boulders indicating winds from at least two directions.

Ripples occur on the surface of all dunes imaged at HiRISE resolution on Mars. The alignment of ripples often results from the influence of more than one wind direction. In this figure, the ripples are superimposed on a low dome dune. On Earth, ripples on the surface of sand dunes may re-orientate in a matter of hours. The time required to re-orientate ripples on Mars is unknown.

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.