This image of a "red spot" on Mercury, which is thought to be the result of a volcanic explosion, was sent to Earth by the Messenger probe in 2011. NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington hide caption

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The surface of Mercury is seen in a picture taken by NASA's Messenger spacecraft as it approached the planet during a fly-by on January 14, 2008. The image was taken about 11,000 miles from the planet and shows a region 300 miles across. NASA/Getty Images hide caption

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