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An image of Mercury is seen as taken by NASA's MESSENGER spacecraft at a distance of approximately 17,000 miles (27,359 km) after the spacecraft's closest approach to the planet January 14, 2008. The image shows features as small as 6 miles (10km) in size. Similar to previously mapped portions of Mercury, this hemisphere appears heavily cratered. It also reveals some unique and distinctive features. On the upper right is the giant Caloris basin, including its western portions never before seen by spacecraft. Formed by the impact of a large asteroid or comet, Caloris is one of the largest, and perhaps one of the youngest basins in the solar system. Picture taken January 14, 2008. (NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington/Handout./Reuters) Mon Oct 6, 3:05 PM ET