Wednesday, May 12, 2010

Icy Northern Dunes

Like Earth, Mars has seasonal polar caps that grow in the winter and retreat in the spring, but on Mars the seasonal caps are composed primarily of carbon dioxide (dry ice). Carbon dioxide is the major component of the Martian atmosphere, and a significant fraction of the mass of the atmosphere is cycled through the seasonal caps every year.

This image shows sand dunes that are mostly covered by seasonal frost/ice in the northern spring. When the springtime sun shines on the ice, some of it penetrates to the base of the ice and warms the dark sand dune surface below. The warm sand evaporates the carbon dioxide ice from below, building gas pressure that apparently breaks the ice and carries sand to the surface as the pressure is released. The sand then cascades down the surface of the ice, forming the streaks seen in this image.

Photo image: NASA/JPL/University of Arizona

Notes: These sand dunes are located in the Vastitas Borealis, just south of the Planum Boreum. For the "streaks" link above, I chose to link to "Avalanches" as opposed to dark slope streaks, the reason being that, while the streaks of sand upon the ice are somewhat reminiscent of slope streaks, they are probably more similar to a very small scale avalanche of sand cascading down the ice. Dark slope streaks are formed when a small avalanche of bright dust and sand reveals a darker-colored substrate underneath; however, that is not the case here. Here, the opposite is happening, where dark sand is covering up the lighter-colored ice.

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