NASA/JPL/Malin Space Science Systems
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This Mars Global Surveyor (MGS) image, released late last month, was taken during southern spring as a seasonal carbon dioxide frost cap sublimated away. Sublimation is a process whereby ice turns directly to vapor, skipping the liquid phase.

Frost remaining in shallow cracks and depressions create the polygonal pattern, astronomers said. Similar polygons occur in the frozen polar regions on Earth. But on our planet the shapes are caused by the freeze and thaw of water ice that does not generally sublimate.

Mars' south polar cap is covered in carbon dioxide ice, commonly known as dry ice. Underneath it, scientist now know, is water ice, which in some spots is exposed when the dry ice sublimates (see Image of the Day, 24 April 2003).

The picture covers an area about 1.9 miles wide (3 kilometers) near 71.9S, 11.1W. Sunlight illuminates the scene from the left.

-- Robert Roy Britt

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