MERCURY

in astronomy, innermost planet of the solar system. Its average distance from the Sun is about 58 million km (35,960,000 miles). Except for Pluto, it is the smallest of the nine major planets, having a diameter of about 4,870 km (3,050 miles). Mercury's orbit around the Sun is highly elliptical. Its period of revolutiononly 88 Earth daysis the shortest among the planets and its average orbital speed of 48 km per second the highest. The planet is thus aptly named after the god Mercury, Roman counterpart of the Greek Hermes, fleet-footed messenger of the gods. Mercury also has an extremely low rotation velocity. Because it turns about its axis so slowly, one day on Mercury (one complete rotation) is equal to 59 Earth days. A solar day on the planet (the time from one sunrise to the next, for example) is 176 Earth days, however, owing to the revolution of Mercury around the Sun. Mercury is difficult to observe from Earth because of its proximity to the Sun. Relatively little was known about its topographic details until 1974, when the U.S. Mariner 10 space probe transmitted numerous closeup photographs of the planet. Much of the Mercurian surface is heavily cratered like that of the Moon, but it differs from the latter in various ways. Because Mercury's surface gravity is considerably greater than the Moon's, ejecta reach only 65 percent of the distance they would reach on the Moon. Vast areas of the planet known as intercrater plains are peppered with small craters. They are, however, devoid of large ones. Some of these plains appear to be as old as the heavily cratered terrain, while others seem to be much younger. Their origin is still unknown. The most impressive feature imaged by Mariner 10 is the 1,300-kilometre Caloris basin, centred in Mercury's northern hemisphere. This large circular structure, formed by a huge meteorite impact, is bounded by a ring of mountains, which stands about 1 to 2 km high. Smooth plains appeared after the Caloris impact. Different in character from the intercrater plains, they resemble the lunar maria. Their volcanic origin, however, has not yet been confirmed. The only surface features younger than the smooth plains are certain impact craters, some of which have extensive rays. Still another kind of topographic feature detected on Mercury is sinuous scarps that extend for hundreds of kilometres. These long, steep cliffs may have originated as a result of crustal compression. Various investigators have hypothesized that Mercury's core once consisted of molten iron, which shrank by several kilometres upon cooling. The resultant settling of the planetary crust produced large-scale wrinkles in the form of the scarps. That Mercury, like the Earth, has a large iron core has been suggested by the discovery of a magnetic field in its vicinity. The presence of such a core would account for the high mean density of the planet. Although Mercury is considerably smaller than the Earth, its mean density is about 5.44 grams per cubic cm, as compared to 5.5 grams per cubic cm for the Earth. Mercury's atmosphere is negligible, consisting chiefly of a tenuous layer of hydrogen, helium, sodium, potassium, oxygen, and possibly neon. Trace amounts of argon, carbon dioxide, krypton, and xenon are also present. The planet's gravity is so weak that it cannot retain significant amounts of atmospheric gases. Temperatures at the surface of Mercury fluctuate drastically, ranging from a high of approximately 675 K to a low of about 100 K. any of the first series of manned space flights conducted by the United States (196163). The series began with a suborbital flight about three weeks after the Soviet cosmonaut Yury Gagarin became the first human in space (see Vostok). Alan B. Shepard, Jr., rode a Mercury space capsule dubbed Freedom 7 on a 486-kilometre (302-mile) flight of 15-minute duration, attaining a maximum altitude of 186 km (116 miles). The Freedom 7, like its successor on the second suborbital flight, was launched by a Redstone rocket. Subsequent manned flights in the Mercury program were launched by more powerful Atlas rockets. All capsules in the Mercury series weighed about 1,400 kg (3,000 pounds). The first U.S. manned flight in orbit was that of the Friendship 7, commanded by John H. Glenn. Launched on Feb. 20, 1962, it successfully completed three orbits and landed in the Atlantic Ocean near The Bahamas. The last Mercury flight, Faith 7, launched May 15, 1963, carrying L. Gordon Cooper, Jr., was also the longest, achieving 22 orbits before its landing and successful recovery 34 hours and 20 minutes later. planet closest to the Sun, revolving around it at an average distance of 58 million kilometres. Mercury's orbit is inside the orbit of the Earth, and this creates two important astronomical effects. First, Mercury is never more than 2745 of angle away from the Sun and is thus seen as a morning star just before sunrise or an evening star just after sunset. Second, Mercury exhibits phases much like the Moon: when it lies nearly between the Earth and the Sun (inferior conjunction), it appears as a thin crescent; when it is at its greatest separation (or elongation) from the Sun, the apparent disk is half-illuminated; and when it is on the opposite side of the Sun from the Earth (superior conjunction), its fully illuminated surface is visible. Since these changes in phase occur because of the motion of Mercury in its orbit, its apparent size also varies with the phase. It is largest at inferior conjunction (about 10 of arc, or 1/180 the apparent size of the Moon) and smallest at superior conjunction (about 4 1/2 of arc, or 1/380 the apparent size of the Moon). Mercury, designated in astronomy, was known to be a planet in Sumerian times, some 5,000 years ago. In classical Greece it was called Apollo when it appeared as a morning star and Hermes, for the Greek equivalent of the Roman god Mercury, when it appeared as an evening star. Hermes was the swift messenger of the gods, and the planet's name is thus a reference to its rapid motions relative to the other objects in the sky. also called quicksilver chemical element, liquid metal of Group IIb, or the zinc group, of the periodic table. Mercury was known to the ancient Chinese and Hindus and has been found in an Egyptian tomb of about 1500 BC. For a discussion of the properties and applications of mercury, see Chemical Elements: Zinc group elements. For treatment of the mining, recovery, and refining of mercury, see Industries, Extraction and Processing: Mercury. Latin Mercurius, in Roman religion, god of merchandise and merchants, commonly identified with the Greek Hermes (q.v.), fleet-footed messenger of the gods. His worship was introduced early, and his temple on the Aventine Hill in Rome was dedicated in 495 BC. There he was associated with the goddess Maia, who became identified as his mother through her association with the Greek Maia, mother of Hermes. Both Mercury and Maia were honoured in a festival on May 15, the dedication day of Mercury's temple on the Aventine (built about 500 BC). Mercury is sometimes represented as holding a purse, symbolic of his business functions. Usually, however, artists borrow the attributes of Hermes irrespective of their appropriateness and portray him wearing winged sandals or a winged cap and carrying a caduceus (staff). Additional reading Nontechnical discussions may be found in Robert G. Strom, Mercury: The Elusive Planet (1987); and Bruce C. Murray and Eric Burgess, Flight to Mercury (1977). An excellent collection of Mariner 10 photographs is available in Merton E. Davies et al., Atlas of Mercury (1978). Reports on the results of the Mariner 10 mission are found in Journal of Geophysical Research, 80(17):23412514 (June 10, 1975). Faith Vilas, Clark R. Chapman, and Mildred Shapley Matthews (eds.), Mercury (1988), is a comprehensive collection of technical papers covering knowledge of the planet to the time of publication, with an extensive bibliography. Michael C. Malin