Mercury, in astronomy, nearest planet to the sun, at a mean distance of 36 million mi (58 million km); its period of revolution is 88 days. Mercury passes through phases similar to those of the moon as it completes each revolution about the sun, although the visible disk varies in size with respect to its distance from the earth. Because its greatest elongation is 28°, it is seen only for a short time after sunset or before sunrise. Since observation of Mercury is particularly unfavorable when it is near the horizon, the planet has often been studied in full daylight, with the sun's light blocked off.

Mercury has the most elliptic orbit of the planets in the solar system. Its great eccentricity of orbit and its great orbital speed provided one of the important tests of Einstein's general theory of relativity. Mercury's perihelion (its closest point to the sun) is observed to advance by 43″ each century more than can be explained from planetary perturbations using Newton's theory of gravitation, yet in nearly exact agreement with the prediction of the general theory.

Mercury is the smallest planet in the solar system, having a diameter of 3,031 mi (4,878 km); both Jupiter's moon Ganymede and Saturn's moon Titan are larger. Its mean density relatively high, a little less than that of the earth; its core is believed to occupy about 85% of its radius and to consist of a probably solid iron core, surrounded by a liquid iron layer, which is surrounded by a solid iron-sulfide layer. There is 10 times as much sulfur in its crust than is found on the surface of the earth. The planet has a dark surface that reflects relatively little light; it is believed that the surface has been darkened by the deposition of carbon-rich micrometeorites that originate from comets passing close to the sun. Mercury's small mass and proximity to the sun prevent it from having an appreciable atmosphere, although a slight amount of carbon dioxide has been detected.

The surface of Mercury is much like that of the moon, as was shown Mariner 10 in flybys in 1974–75 and Messenger in flybys in 2008–9 and in orbit in 2011–15. Most of its craters were formed during a period of heavy bombardment by small asteroids early in the solar system's history. Messenger, which became the first space probe to orbit Mercury, found solid evidence of ancient volcanism as well and corroborated that there is ice near the planet's north pole in craters where areas are in permanent shadow; measurements by earth-based radar in the 1990s had suggested that there was ice near the poles. Images from Messenger were used to produce (2016) a complete topographic map of Mercury.

It was long thought that Mercury's period of rotation on its axis was identical to its period of revolution, so that the same side of the planet always faced the sun. However, radar studies in 1965 showed a period of rotation of 58.6 days. This results in periods of daylight and night of 88 earth days each, with the daylight temperatures reaching as high as 800°C (450°C). Night temperatures are believed to drop as low as −300°C (−184°C).

The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.

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