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Mercury (planet), one of the planets in the solar system. Mercury orbits closest to the Sun of all the planets, at an average distance of approximately 58 million km (about 36 million mi). The planet’s diameter is 4,879 km (3,032 mi), and its volume and mass are about one-eighteenth that of Earth. Mercury’s mean density is approximately equal to that of Earth and is higher than that of any of the other planets. The force of gravity on the planet's surface is about one-third of that on Earth's surface or about twice the surface gravity on the Moon.

Mercury revolves once about the Sun every 88 days. Radar observations of the planet show that it rotates only once every 58.7 days, two-thirds of its period of revolution. Only three of the planet’s days, therefore, occur during every two of its years. The side facing the Sun gets very hot, while the side facing away quickly cools to frigid temperatures. The point in Mercury's orbit at which the planet is closest to the Sun (called the planet’s perihelion) moves a tiny amount every orbit, too much to be accounted for by the gravitational influence of other planets. The observation of these changes in Mercury's perihelion was one of the first confirmations of Einstein’s theory of relativity, which predicted their existence.

The Mariner 10 spacecraft passed Mercury twice in 1974 and once in 1975. It sent back pictures of a moonlike, crater-pocked surface and reported temperatures to be about 430°C (about 810°F) on the sunlit side and about -180°C (about -290°F) on the dark side. Although Mercury’s surface appears very similar to the surface of the Moon, there are some significant differences. The smooth, lava-like plains on Mercury, for example, are not as dark as the smooth plains (maria) of the Moon. Also unlike the surface of the Moon, the surface of Mercury is crisscrossed by long escarpments, or cliffs, indicating a period of contraction as the planet cooled early in its history.

Mercury is a poor reflector of sunlight because its surface consists of rough, porous, dark-colored rock. The planet’s albedo, or the amount of sunlight it reflects, is only about 12 percent. Earth, in contrast, reflects about 37 percent of the sunlight that strikes it, while Venus, the most reflective planet in the solar system, reflects 65 percent.

Mercury’s high density indicates that the relatively dense and abundant element iron accounts for a large proportion of the planet’s composition. The surface of Mercury, however, contains little iron, suggesting that most of Mercury’s iron is now concentrated in a large iron core. Collisions with other protoplanets early in the history of the solar system may have stripped away much of Mercury’s low-density crust, leaving behind a dense, iron-rich core.

Characteristics of Mercury

In 1991 powerful radio telescopes on Earth revealed signs of vast deposits of ice in the polar regions of Mercury. These ice deposits occur in areas where sunlight never falls, such as crater bottoms near both of the planet’s poles. Similar ice deposits were found during the 1990s near the poles of the Moon by the Clementine and Lunar Prospector spacecrafts.

Scientists use a technique called spectroscopy to conduct studies of the light that Mercury reflects. These studies indicate that planet has only an extremely thin atmosphere, containing sodium and potassium. Apparently these elements slowly escape as gases from the crust of the planet.

Mercury is the only rocky planet other than Earth to have a global magnetic field, which is about 1 percent as strong as Earth's. The presence of the field and its global extent together suggest that the core of the planet is largely liquid iron, which produces a magnetic field as it moves. Scientists believe Mercury's crust insulates the planet's outer core, keeping it liquid despite the very cold temperatures on the dark side of the planet.