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Mercury, named for the fast-footed Roman messenger of the gods, is the closest planet to the Sun. Doing justice to its name, Mercury circles the Sun in a mere 88 days, compared to 365 days for Earth, and travels through space at nearly 50 kilometers (31 miles) per second, faster than any other planet in the solar system. Mercury's surface temperatures range from a scorching 467 degrees Celsius (872 degrees Fahrenheit) to a bone-chilling -183 degrees Celsius (-300 degrees Fahrenheit).
Like our Moon, the planet is heavily cratered, showing impact scars from countless meteor bombardments. Old lava flows and quake faults also mark its crust. Although it has very little atmosphere, scientists have found water ice inside deep craters at the north and south poles of this hot little globe.
Mercury has been visited by only one spacecraft, Mariner 10, which flew by the planet three times in 1974 and 1975. Mariner 10 mapped about half of the planet's surface, during which time a thin atmosphere and a magnetic field were discovered. Mercury's thin atmosphere consists of atoms blasted off its surface by the solar wind, a constant stream of particles coming from the outer layer of the Sun. Because Mercury is so hot, these atoms quickly escape into space. In contrast to the stable atmospheres of Earth and Venus, Mercury's atmosphere is constantly being replenished. Amazingly, radar observations of Mercury's north pole (a region not mapped by Mariner 10) show evidence of water ice in the protected shadows of some craters.
For more information, visit NASA's Solar System Exploration site.
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