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Unlike the rolling volcanic plains of the north, the southern half of Mars is dominated by older, heavily cratered highlands. The distinctly lunar09/14/2009like terrain of Terra Sirenum belies the fact that, unlike the Earth's Moon, Mars has an active atmosphere. In this rendering the perspective is from an altitude of about 30 miles and about 30 degrees from Mars' south pole. The large crater with a central peak at the 2 o'clock position near the horizon is Liu Hsin, with a diameter of 80 miles. Just beyond Liu Hsin is the southern rim of a much larger crater named Copernicus, with a diameter of 180 miles. To the west, high09/14/2009flying cirrus clouds of water ice crystals form complex patterns in the tenuous carbon dioxide atmosphere.