MERCURY
Mercury orbits closer to the sun than any other planet, making it dry, hot, and virtually airless. Although the planet's cratered surface resembles that of the moon, it is believed that the interior is actually similar to the earth's, consisting primarily of iron and other heavy elements. This photograph was taken in 1974 by Mariner 10, the first probe to study Mercury in detail.
The planet closest to the sun. Its mean distance from the sun is approximately 58 million km (about 36 million mi); its diameter is 4875 km (3030 mi); its volume and mass are about : that of Earth; and its mean density is approximately equal to that of Earth. Mercury revolves about the sun in a period of 88 days. Radar observations of the planet show that its period of rotation is 58.7 days, or two-thirds of its period of revolution. The planet, therefore, rotates one and a half times during each revolution. Because its surface consists of rough, porous, dark-colored rock, Mercury is a poor reflector of sunlight.

Mariner 10 also detected a magnetic field about 1 percent that of Earth, making Mercury the only other planet with density and composition close to that of Earth to have a magnetic field. Because it has a magnetic field, the outer core of the planet must be a liquid iron compound, which produces a magnetic field as it moves. Scientists believe Mercury's crust acts as an effective insulator to keep the planet's outer core liquid despite the very cold temperatures on the dark side of the planet.

Spectroscopic studies indicate that only an extremely thin atmosphere, containing sodium and potassium, exists on Mercury, its atoms apparently diffusing from the crust of the planet. Collisions with other protoplanets early in the history of the solar system may have stripped away lighter materials, thereby accounting for Mercury's great density. The force of gravity on the planet's surface is about one-third of that on Earth's surface.

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. Unlike that of Earth's moon, the surface of Mercury is criss-crossed by long escarpments, or cliffs, dating perhaps from the period of contraction the planet experienced as it cooled some time early in its history.

In 1991 powerful radio telescopes on Earth revealed unmistakable signals of vast sheets of ice in Mercury's polar regions, areas that had not been covered by Mariner 10.

The point in Mercury's orbit at which the planet is closest to the sun (the perihelion) moves a tiny amount every orbit, but too much to be accounted for by the force of gravity from other planets. A full explanation of the changes in Mercury's perihelion was one of the first confirmations of the theory of relativity.

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