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Hansen Press Release (02/02/2011)

Three images of the same location taken at different times on Mars show seasonal activity causing sand avalanches and ripple changes on a Martian dune. The High Resolution Imaging Science Experiment (HiRISE) camera on NASA’s Mars Reconnaissance Orbiter took these images, centered at 84 degrees north latitude and 233.2 degrees east longitude.  Dune fields at high latitudes are covered every year by a seasonal polar cap of condensed CO2 (dry ice). 

The sequential images, which each show an area 285 meters by 140 meters, depict the before and after morphology of the dune in one Mars year, with new alcoves and extension of the debris apron on the slipface, or steeply sloping leeward surface, of the dune caused by the grainfall, and new wind ripples on the debris apron.

The top image was taken first, in the Martian summer when the dunes were free of seasonal dry ice.  The middle image was acquired in the spring when the region was covered by a layer of seasonal ice.   Spring evaporation of the seasonal layer of ice is manifested as dark streaks of fine particles carried to the top of the ice layer by escaping gas.  Gas flow under the ice as the ice sublimates – changes from solid to gas – from the bottom destabilizes the sand on the dune, and causes the sand to avalanche down the dune slipface.

 The third image shows the resulting changes revealed the following summer after the frozen layer of ice was gone.  Comparison of the middle and lower images shows the correlation of seasonal activity with locations of change of dune morphology. 


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