URANUS (MYTHOLOGY) In Greek mythology, Uranus represented the sky or heaven. He was both the son and husband of GAEA, the earth, by whom he fathered first the hundred-handed giants and the CYCLOPES, whom he banished to Tartarus, and later the TITANS. Gaea, angry because her children were imprisoned, set the Titans against Uranus. CRONUS, their leader, castrated Uranus and succeeded him as ruler of the universe. According to Hesiod's Theogony APHRODITE was born of the foam of Uranus's discarded genitals as they fell in the sea.
Uranus Green with jealousy because the plankton sea of Earth endlessly paints miracles on canvasses Uranus cannot see for the sun hides microscopically the portraits of teeming life in cosmic mystery. carlyle miller
Uranus
Uranus
Uranus: Oberon
Uranus: Umbriel
URANUS (PLANET) Uranus is the seventh PLANET from the Sun and was the first to be discovered since ancient times. Sir William HERSCHEL first observed the planet on Mar. 13, 1781, seeing a featureless bluish green disk that he nevertheless recognized as a highly unusual object. Although Herschel wished to call the newly discovered planet Georgium Sidus (Georgian Star) for King George III of England, and although many French astronomers referred to it as Herschel, Johann Bode's proposal of the name Uranus--the mythological father of Saturn--was over the years accepted more and more widely and finally became universal in the mid-19th century. The only spacecraft to encounter Uranus thus far, VOYAGER 2, collected data on the planet and its rings and satellites over a four-month period, between Nov. 4, 1985, and Feb. 25, 1986. Voyager 2 passed within 107,000 km (66,500 mi) of the center of Uranus--about 81,450 km (50,625 mi) above its cloud tops--on Jan. 14, 1986. Appearance Although it was discovered with a telescope, Uranus reaches a maximum brightness of magnitude 5.5 and can be seen by the naked eye as a faint point of light in a clear, moonless sky. In Voyager 2 photos, Uranus appears as a generally featureless disk, except for considerable darkening toward the edges. Faint banding is seen in contrast-enhanced images, as are transitory cloud plumes. Orbit and Rotation Uranus's average distance from the Sun is 2.875 billion km (1.786 billion mi), and the planet takes 84.013 Earth-years to make one complete revolution about the Sun. The orbit has an eccentricity of 0.0472 and an inclination of 0 deg 46 min. Its period of rotation was determined by Voyager 2 to be 17.24 hours. One startling aspect of Uranus is that its rotation axis is inclined 97 deg. 54 min. from its orbital plane. This means that the poles of Uranus lie nearly in the plane of its orbit around the Sun. Physical Characteristics Uranus has an equatorial diameter of 51,100 km (31,750 mi), almost precisely four times that of the Earth. The dark absorption bands discovered in 1869 in the red part of the spectrum of Uranus were identified in 1932 as being caused by methane gas in Uranus's atmosphere. This red absorption is the major cause of the blue green color of the planet; the methane absorbs much of the red light from the impinging white sunlight, leaving a bluish green color in the reflected sunlight. Measurements by Voyager 2 verified that hydrogen is the main constituent of the visible atmosphere. Helium is the other major constituent, about 15% in terms of molecular weight, which is a larger percentage than in the atmospheres of either Jupiter or Saturn. Together, hydrogen and helium make up more than 99% of Uranus's atmosphere, which extends to a depth of about 8,000 km (5,000 mi). A layer of methane ice clouds was detected by Voyager 2 near a pressure level of about one atmosphere. Deep within Uranus exists a superheated water ocean, perhaps 10,000 km (6,000 mi) deep, which contains large concentrations of ionized chemicals. It is in this ocean that Uranus's magnetic field may originate. Beneath this ionic ocean, an Earth-sized core of molten rocky materials is believed to exist. Infrared measurements from Voyager 2 indicate that Uranus has an effective temperature of -214 deg. C (-353 deg. F). Unlike the other giant planets, there is no evidence of any significant internal heat source. A very extended corona of atomic and molecular hydrogen encompasses all the known rings of the planet, perhaps heated by low-energy electrons. Sunlight falling on this corona causes it to radiate ultraviolet light, a phenomenon that scientists have termed ELECTROGLOW. Rings The discovery on Mar. 10, 1977, that Uranus has rings was one of the most unexpected and exciting events of modern planetary astronomy. Subsequent observations have indicated the presence of 10 narrow rings of dark particles and one broad, diffuse ring, in addition to 100 or more possibly transient ringlets of dust-sized particles seen only in Voyager 2 images of the backlighted rings. The outermost ring (called the epsilon ring), in marked contrast to the rings of Saturn, has almost no particles smaller than about 20 cm (8 in). The other rings also seem to be deficient in small particles. Possibly the extended hydrogen corona exerts a drag on the orbiting ring particles that preferentially removes the smaller particles, causing them to fall into the planet. Because collisions between larger particles create smaller particles, atmospheric-drag forces could virtually destroy the rings in geologically short time periods. The rings are therefore relatively young or else are replenished by the breakup of small satellites. Satellites Uranus has five major satellites: MIRANDA, discovered by Gerard Kuiper in 1948; Ariel and Umbriel, discovered by William Lassell in 1851; and Titania and Oberon, discovered by William Herschel in 1787. Ten small satellites were also discovered in Voyager 2 photographs, all of them orbiting Uranus well inside the orbit of Miranda. The five major satellites--and probably the ten smaller ones as well--appear to be in synchronous rotation; that is, they keep their same faces toward Uranus as they orbit the planet. Much like the ring particles, the smaller satellites appear to have surfaces as dark as coal. The major satellites have somewhat brighter surfaces, reflecting from 19% (Umbriel) to 40% (Ariel) of the sunlight that falls on them. In addition to being the darkest of the major satellites, Umbriel also has the fewest geological features. The other four satellites display increasingly complex geologies with decreasing distance from Uranus. Oberon and Titania are remarkably similar in size, density color, and reflectivity. Titania, however, has a much larger fraction of small craters and more fractures across its surface, implying that Titania has a geologically younger surface. Oberon has one mountain that rises to an altitude of at least 20 km (12 mi) above the surrounding surface. Although Umbriel and Ariel have similar diameters and densities, they differ dramatically in the appearance of their surfaces. Ariel's surface is literally covered with fractures and fault systems. Parallel fractures in several areas bound valleys that appear to have glacierlike flows along their floors. Water-ammonia ices become fluid at much lower temperatures than pure water ice and might form the observed glaciers. Miranda is the innermost and smallest of the major moons. its surface consists of an old, heavily cratered, rolling terrain with relatively uniform reflectivity, and a younger, complex terrain in three nearly rectangular regions characterized by parallel grooves, an abundance of vertical structures, and large brightness differences. The materials out of which Miranda was formed may not have melded together to form a uniform surface; its present bizarre appearance may be a frozen record of the late stages of the development of this satellite. Bibliography: Burgess, Eric, Uranus and Neptune (1988); Ingersoll, Andrew P., "Uranus," Scientific American, January 1987; Hunt, Garry, and Moore, Patrick, Atlas of Uranus (1989); Johnson, Torrence V., et al., "The Moons of Uranus," Scientific American, April 1987; Laeser, Richard P., et al., "Engineering Voyager 2's Encounter with Uranus," Scientific American, November 1986; O'Meary, S. J., "A Visual History of Uranus," Sky & Telescope, November 1985; Stone, E. C., and Miner, E. D., "The Voyager 2 Encounter with the Uranian System," Science, July 4, 1986. SATELLITES AND RINGS OF URANUS --------------------------------------------------------------- Average Distance from Period of Center of Uranus Revolution Name km mi (days) --------------------------------------------------------------- 1986U2R ring 38,000 24,000 6 ring 41,850 26,000 5 ring 42,240 26,250 4 ring 42,580 26,460 Alpha ring 44,730 27,790 Beta ring 45,670 28,380 Eta ring 47,180 29,320 Gamma ring 47,630 29,600 Delta ring 48,310 30,020 Cordelia 49,700 30,900 0.33 1986U1R ring 50,040 31,090 Epsilon ring 51,160 31,790 Ophelia 53,800 33,400 0.38 Bianca 59,200 36,800 0.43 Cressida 61,800 38,400 0.46 Desdemona 62,700 39,000 0.48 Juliet 64,600 40,100 0.49 Portia 66,100 41,100 0.51 Rosalind 69,900 43,400 0.56 Belinda 75,300 46,800 0.62 Puck 86,000 53,400 0.76 Miranda 129,900 80,700 1.41 Ariel 190,900 118,600 2.52 Umbriel 266,000 165,300 4.15 Titania 436,300 271,100 8.70 Oberon 583,400 362,500 13.46 --------------------------------------------------------------- Diameter Orbital Orbital or Ring Width Inclination Eccentricity km mi (degrees) --------------------------------------------------------------- 2,500 1,500 0? 0? 2 1 0.063 0.0010 3 2 0.052 0.0019 2 1 0.032 0.0011 10 6 0.014 0.0008 9 6 0.005 0.0004 2 1 0.002 0.000 3 2 0.011 0.000 6 4 0.004 0.000 40 25 0? 0? 2 1 0? 0? 58 36 0.001 0.0079 50 30 0? 0? 50 30 0? 0? 60 35 0? 0? 60 35 0? 0? 80 50 0? 0? 80 50 0? 0? 60 35 0? 0? 60 35 0? 0? 170 105 0? 0? 484 301 4.2 0.0027 1,160 720 0.3 0.0034 1,190 740 0.36 0.0050 1,610 1,000 0.14 0.0022 1,550 960 0.10 0.0008 ---------------------------------------------------------------
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