Two pairs of side-by-side, before and after images from the High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter illustrate changes in the shape of edges of dark sand dunes in the Nili Patera region of Mars.

The two images on the left are excerpts from a June 30, 2007, observation (late autumn at the site). The two on the right are of the same ground observed 15 weeks later, on Oct. 13, 2007 (winter at the site). Dark dunes overlaid with smaller ripples fill the bottom of the upper pair and cross the middle of the lower pair. In each image, an inset box in the upper left holds a diagram of the ripple crests (yellow lines) and dune edges (green lines) within the inscribed box to the right of the diagram. Changes both in the dune edges and in the ripple crests occurred during the interval between the June observation and the October observation.

White scale bars in each image are 10 meters (33 feet) long. North is toward the top.

A locator map at PIA12857 shows the context for the areas shown in these images. The inset box there labeled 4a-a' indicates the location of the top pair here. The box 4b-b' indicates the location of the bottom pair here. The site is field of dark sand dunes at 9 degrees north latitude, 67 degrees east longitude.

This comparison is part of a study of whether wind-shaped bedforms on Mars -- dunes and ripples -- are actively migrating in present-day atmospheric conditions. It is from a presentation by S. Silvestro, L.K. Fenton and D.A. Vaz at the 41st Lunar and Planetary Sciences Conference, March 2010, reporting that the bedforms at this Nili Patera site are actively migrating. The changes suggest that these dunes are not heavily cemented or crusted.

Other products from the June 30, 2007, HiRISE observation of this dune field are available at http://hirise.lpl.arizona.edu/PSP_004339_1890. Other products from Oct. 13, 2007, observation are available at http://hirise.lpl.arizona.edu/PSP_005684_1890.

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 spacecraft development and integration contractor for the project and built the spacecraft.