URANUS

seventh planet in order of distance from the Sun. Its low density (1.285 grams per cubic centimetre) and large size (radius four times that of the Earth) place it among the four giant planets, all of which are composed primarily of hydrogen, helium, water, and other volatile compounds and which thus are without solid surfaces. Absorption of red light by methane gas gives the planet a blue-green colour. Uranus, designated in astronomy, spins on its side; its rotation axis is tipped at an angle of 98 relative to its orbit axis. In addition, the magnetic field is tipped at an angle of 59 relative to the rotation axis. The planet has 15 known satellites, ranging up to 789 km in radius, and 10 narrow rings. Its mean distance from the Sun is 2,870.99 million km, and its mean distance from the Earth at closest approach is 2,721.39 million km. Rings of Uranus, photograph taken by Voyager 2. Backlit view shows continuous distribution of fine in astronomy, seventh major planet from the Sun. It was discovered in 1781 by the English astronomer William Herschel during a telescopic survey of the sky. Uranus is almost 15 times as massive as the Earth, and its volume exceeds that of the latter by more than 50 times. Stellar occultation measurements conducted by James Elliot and other American investigators in 1977 revealed the existence of nine sharply defined, narrow, dark rings lying from 1.6 to 1.95 RU (where RU is Uranus' equatorial radius of 26,000 km [16,120 miles]). A tenth narrow ring and broad dust bands between the narrow rings were discovered in 1986 by the U.S. Voyager 2 space probe during its flyby of the planet. Three of the rings are circular, while the others have a large range in eccentricity and are variable (by up to a factor of five) in width. All of the rings consist primarily of boulder-sized chunks of dark matter about 1 m (3 feet) in diameter. The composition of the ring particles has not yet been determined, but investigators speculate that they are made up of rock fragments and water ice intermixed with a black, amorphous polymer, which might account for the coal-black colour of the particles. The gaps between the rings are filled with numerous bright dust particles. In addition to its system of rings, Uranus has at least 15 satellites. The five major moonsMiranda, Umbriel, Ariel, Oberon, and Titaniahave diameters ranging from 472 to 1,578 km (293 to 980 miles). The other moons are substantially smaller and lie inside the orbit of Miranda, the innermost of the major Uranian satellites. Two of the moonlets seem to act as shepherds of the ring dubbed Epsilon, gravitationally confining its constituent particles. Data transmitted by Voyager 2 revealed that Uranus is surrounded by a magnetic field comparable in strength to those around the Earth and Saturn. Trapped within this field is a radiation belt more intense than the Earth's. The axis of the Uranian field is tilted 58.6 from the planet's axis of rotation. This is an exceedingly large inclination when compared with that of the magnetic fields of other planets (e.g., the Earth's field is tilted a mere 11). The magnetic field of Uranus is swept back into a long tail by the solar wind (outflow of charged particles from the Sun's corona). Because the field is inclined so sharply, the tail is twisted much like a corkscrew. An analysis of radio emissions from Uranus' magnetic field has enabled investigators to estimate the rotation period of the planet to be about 17.24 hours. Uranus' axis of rotation is unusual in that it lies in the plane of its orbit, and so Uranus appears to spin on its side. The axes of all other planets in the solar system are roughly perpendicular to the plane of their orbits. The interior of Uranus is thought to consist of a mixture of ice and gas. A rocky core appears to be small or nonexistent. The atmosphere of Uranus appears to extend to an altitude of thousands of kilometres in successive layers. The innermost layer most likely consists of clouds of methane ice, while the upper reaches are composed principally of hydrogen and helium. The atmospheric hydrogen and helium occur in proportions consistent with solar values. The average heat radiated by Uranus is equivalent to that of a blackbodyi.e., an object that emits all radiation incident upon itof the same cross-section radiating at 59.1 K. This is the so-called effective temperature of the planet and is equal to the actual temperature of the atmospheric gases at a pressure of about 0.4 bar (one bar is roughly equal to the Earth's sea-level pressure). Temperature decreases with altitude from this point, reaching a minimum of 52 K at the 70-millibar level, and then increases with height up to a maximum of 750 K at the very top of the atmosphere. Like Jupiter and Saturn, Uranus has bands of clouds oriented parallel to circles of constant latitude. Zonal winds travel in a retrograde direction at low latitudes (near the equator) and in a prograde direction at higher latitudes; the wind velocities are several times greater than those of the Earth and appear to increase with latitude. in Greek mythology, the personification of heaven. According to Hesiod's Theogony, Gaea (Earth), emerging from primeval Chaos, produced Uranus, the Mountains, and the Sea. From Gaea's subsequent union with Uranus were born the Titans, the Cyclopes, and the Hecatoncheires. Uranus hated his offspring and hid them in Gaea's body. She appealed to them for vengeance, but Cronus (a Titan) alone responded. With the harpe (a scimitar) he removed Uranus' testicles as he approached Gaea. From the drops of Uranus' blood that fell on her were born the Furies, the Giants, and the Meliai (ash-tree nymphs). The severed genitals floated on the sea, producing a white foam, from which sprang the goddess of love, Aphrodite. Cronus by his action had separated Heaven and Earth. Uranus also had other consorts: Hestia, Nyx, Hemera, and Clymene. There was no cult of Uranus in classical Greece. This circumstance, together with other elements in the story, suggests pre-Greek origins. The use of the harpe points to an Oriental source, and the story bears a close resemblance to the Hittite myth of Kumarbi. Additional reading A.P. Ingersoll, Uranus, Scientific American, 256(1):3037 (January 1987), provides an introductory review article with illustrations and diagrams. Garry Hunt and Patrick Moore, Atlas of Uranus (1989), offers an in-depth introduction. The first reports of the Voyager 2 encounter written by the Voyager scientists are included in a set of 12 articles in Science, 233(4759):39109 (July 4, 1986), which contains most of the best images of the planet, its satellites, and its rings. Jay T. Bergstralh, Ellis D. Miner, and Mildred Shapley Matthews (eds.), Uranus (1991), with chapters written by specialists in the field, is the definitive reference work on the subject. Ellis D. Miner, Uranus: The Planet, Rings, and Satellites (1990), reviews current knowledge of the planet, with much background information on the Voyager mission. Garry Hunt (ed.), Uranus and the Outer Planets (1982), is a pre-Voyager summary dealing almost exclusively with Uranus, with some interesting historical chapters. Eric Burgess, Uranus and Neptune: The Distant Giants (1988), also focuses primarily on Uranus. Mark Littmann, Planets Beyond: Discovering the Outer Solar System, updated and rev. ed. (1990), chronicles the history of the discovery of Uranus, Neptune, and Pluto. Andrew P. Ingersoll