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The grains of sand that make up sand dunes on Earth and Mars have a hazardous existence because of the way that they travel. Wind-blown sand is lifted above the surface of each planet before crashing onto the ground and bouncing in a sequence of repeated hops, a process called saltation.
Sand grains can also roll along the ground as they are blown by the wind, and they are also jostled by other sand gains that are similarly flying across the surface. All of these repeated impacts tend to wear down the sand grains, smoothing them into a more spherical shape and breaking off small fragments that supply the vast dust deposits of Mars. This process (known as comminution) ultimately destroys sand grains and limits the length of time that the particles exist. The fact that we see active sand dunes on Mars today requires that sand particles must be resupplied to replace the grains that are lost over time. Where are the modern day sources of sand on Mars?
The map is projected here at a scale of 25 centimeters (9.8 inches) per pixel. [The original image scale is 26.7 centimeters (10.5 inches) per pixel (with 1 x 1 binning); objects on the order of 80 centimeters (31.5 inches) across are resolved.] North is up.
The University of Arizona, Tucson, operates HiRISE, which was built by Ball Aerospace & Technologies Corp., Boulder, Colo. NASA's Jet Propulsion Laboratory, a division of Caltech in Pasadena, California, manages the Mars Reconnaissance Orbiter Project for NASA's Science Mission Directorate, Washington.