19. Sand Dunes in Nili Patera
This high-resolution image was taken by the Mars Orbiter Camera on the Mars Global Surveyor spacecraft on March 11, 1999, on the first day of the regular mapping phase of the mission. The view shows a series of large sand dunes superposed on a cratered and highly textured substrate. The dunes are on the caldera floor of the Nili Patera volcano, in the Syrtis Major region of Mars. The crescent shape of the dunes (called barchans) is a result of a locally limited sand supply and indicates that the sand-driving winds blew across this region from the upper right to the lower left. The dunes are darker than their surroundings, suggesting that at least here the sand particles consist of dark materials. Some of the individual barchan dunes in this view are more than 70 meters (231 feet) high. The image is slightly over 3 kilometers (2 miles) wide.
Portion of Mars Orbiter Camera image FHA-00451, NASA/JPL/Malin Space Science Systems.