This view from the High Resolution Imaging Science Experiment (HiRISE)
camera on NASA's Mars Reconnaissance Orbiter shows two classes of aeolian
bedforms within Proctor Crater. The relatively bright, small ridges are
ripples. From their study on Earth, and close-up examination by the MER
rovers (roving elsewhere on Mars), we know that ripples are composed of
fine sand (less than 200 microns in diameter) or fine sand coated with
coarser sand and granules.
The larger, darker bedforms are dunes composed of sand, most likely of
fine size. Ripples tend to move slower than dunes. Because of this, over
time, ripples get covered with dust, possibly explaining the bright tone
visible here. The dunes are dark probably because they are composed of
basaltic sand (derived from dark, volcanic rock) that is blown by the wind
enough that dust does not sufficiently accumulate to change their color.
This area in Proctor Crater, at 47.8 degrees south latitude and 30.7
degrees east longitude, is being monitored by HiRISE to document any
changes over time.
This image is a portion of the HiRISE observation catalogued as
ESP_011909, taken on Feb. 9, 2009.
NASA's Jet Propulsion Laboratory, a division of the California Institute
of Technology in Pasadena, manages the Mars Reconnaissance Orbiter for
NASA's Science Mission Directorate, Washington. Lockheed Martin Space
Systems, Denver, is the prime contractor for the project and built the
spacecraft. The High Resolution Imaging Science Experiment is operated by
the University of Arizona, Tucson, and the instrument was built by Ball
Aerospace & Technologies Corp., Boulder, Colo.