Three images of the same location taken at different times show seasonal activity causing sand avalanches and ripple changes on a Martian dune. Every year, dune fields at high latitudes are covered by a seasonal polar cap of condensed carbon dioxide (dry ice). Sand sliding down the dune carves out new alcoves at the top and adds to the debris apron on the bottom. The top image was taken in the Martian summer when the dunes were free of seasonal dry ice. Spring found the region covered by a layer of seasonal ice (middle). Evaporation of this seasonal ice layer shows up as dark streaks of fine particles carried to the top of the ice layer by escaping gas. As the ice changes from solid to gas, gas flowing underneath destabilizes the sand and causes it to avalanche down the dune. The bottom image shows the resulting changes, revealed during the following summer after the ice was gone. New wind ripples can be seen on the debris apron. The High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter took these images, each showing an area of 285 x 140 meters, centered at 84 degrees North latitude and 233.2 degrees East longitude. (Credit: NASA/JPL/The University of Arizona)Northern Mars Landscape Actively Changing -- Science Daily
ScienceDaily (Feb. 4, 2011) — Sand dunes in a vast area of northern Mars long thought to be frozen in time are changing with both sudden and gradual motions, according to research using images from a NASA orbiter.
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My Comment: I guess Mars is more windy than what it is given credit for.
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