MERCURY

MERCURY

MERCURY, in the solar system, 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 1/18 that of the earth;and its mean density is approximately equal to that of the 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.

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. Mariner 10 also detected a magnetic field 1 percent that of the earth. Unlike that of the earth’s moon, the surface of Mercury is crisscrossed by long escarpments, 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 perihelion (the point closest to the sun) in the orbit of Mercury advances at a slow rate. A full explanation of the motion of its orbit according to relativistic concepts was one of the first confirmations of the theory of relativity.