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Uranus

Uranus

Uranus was the first planet to be discovered that had not been known since antiquity. Although Uranus is just bright enough to be seen with the naked eye, and in fact had appeared in some early star charts as an unidentified star, English astronomer William Herschel was the first to recognize it as a planet in 1781.

The planet's benign appearance gives no hint of a history fraught with catastrophe: Sometime in Uranus's past, a huge collision wrenched the young planet. As a result, the rotation pole of Uranus is now tilted more than 90 degrees from the plane of the planet's orbit. Uranus travels in a nearly circular orbit at an average distance of almost 3 billion kilometers (1.9 billion miles) from the Sun (about nineteen times the distance from Earth to the Sun).

A Somewhat Small Gas Planet

The composition of Uranus is similar to that of the other giant planets* and the Sun, consisting predominantly of hydrogen (about 80 percent) and helium (15 percent). The remainder of Uranus's atmosphere is methane (less than 3 percent), hydrocarbons (mixtures of carbon, nitrogen, hydrogen, and oxygen), and other trace elements. Uranus's color is caused by the methane, which preferentially absorbs red light, rendering the remaining reflected light a greenish-blue color.

Like Jupiter and Saturn, Uranus is a gas planet, although a somewhat small one (at its equator, its radius is about 25,559 kilometers [15,847 miles]). We see the outermost layers of clouds, which are probably composed of icy crystals of methane. Below this layer of clouds, the atmosphere gets thicker and warmer. Deep within the center of Uranus, at extremely high pressure, a core of rocky material is hypothesized to exist, with a mass almost five times that of Earth.

One of the more puzzling aspects of Uranus is the lack of excess heat radiating from its interior. In comparison, the other three giant planets radiate significant excess heat. Astronomers believe that this excess heat is left over from the time of the planets' formation and from continuing gravitational contraction . Why then does Uranus have none? Scientists theorize that perhaps the heat is there but is trapped by layers in the atmosphere, or perhaps the event that knocked Uranus over on its side somehow caused much of the heat to be released early in the planet's history.

Magnetic Field

When the Voyager 2 spacecraft flew by Uranus in 1986, it detected a magnetic field about fifty times stronger than that of Earth. In a surprising twist, the magnetic field's source was not only offset from the center of the planet to the outer edge of the rocky core, but it was also tilted nearly 60 degrees from the planet's rotation axis. From variations in the magnetic field strength detected by Voyager 2, scientists determined that the planet's internal rotation period was 17.2 hours. The winds in the visible cloud layers have rotation periods ranging from about 16 to 18 hours depending on latitude, implying that wind speeds reach 300 meters per second (670 miles per hour) for some regions.

The Moons of Uranus

Within six years of the discovery of Uranus, two moons were discovered. They were subsequently named Titania and Oberon. It was more than sixty years before the next two Uranian moons, Ariel and Umbriel, were discovered. Nearly a century elapsed before Miranda was discovered in 1948, bringing the total of Uranus's large moons to five. Little was known about their surface structure or history until the Voyager 2 spacecraft returned detailed images of the surfaces of these moons.

On Miranda, huge geologic features dominate the small moon's landscape, indicating that some kind of intense heating must have occurred in the past. It is not yet clear whether a massive collision disturbed the small moon, which then reassembled into the current jumble, or whether, in the past, tidal interactions with other moons produced heat to melt and modify the surface, as is the case for Jupiter's moon Io. Oberon, the outermost major moon, shows many large craters, some with bright rays . Titania has fewer large craters, indicating that its surface has been "wiped clean" by resurfacing sometime in the moon's past. Ariel has the youngest surface of the major moons, based on cratering rates. Umbriel is much darker and smoother. Its heavily cratered surface is probably the oldest of the satellites.

In 1986, Voyager 2 discovered ten additional moons, with Puck being the largest. Voyager 2 images of Puck showed it to be an irregularly shaped body with a mottled surface. Voyager 2 did not venture close enough to the other small moons to learn much about them. Since 1986, six more tiny moons have been discovered around Uranus, bringing its total to twenty-one. Little is known about these moons other than their sizes and orbits.

Rings and Seasons

In 1977, astronomers discovered that Uranus has a ring system. Voyager 2 studied the rings in detail when it flew by Uranus in 1986. There are nine well-defined rings, plus a fainter ring and a wider fuzzy ring. Unlike the broad system of Saturnian rings, the main Uranian rings are narrow. The rings are not perfectly circular and also vary in width. Like the rings of Saturn, the Uranian rings are thought to be composed mainly of rocky material (ranging in size from dust particles to house-sized boulders) mixed with small amounts of ice.

The atmosphere of Uranus has often been called bland, and even boring. These epithets are a consequence of fate and unfortunate timing. It was fate that caused the early collision of Uranus with a large body, creating the planet's extreme axial tilt, which in turn created extreme seasons. It was unfortunate timing that the Voyager 2 encounter (which gave us our highest resolution pictures) occurred at peak southern summer, when we had a view of only the southern half of the planet. Historically, this season is when Uranus has appeared blandest in the past.

As Uranus continues its eighty-four-year-long progression around the Sun, its equatorial region is now receiving sunlight again, and parts of its northern hemisphere are being bathed in solar radiation for the first time in decades. Today, images from the Hubble Space Telescope are revealing multiple bright cloud features and stunning banded structures on Uranus. It is fascinating to speculate how Uranus will appear to us by the time it reaches equinox in 2007.

see also Exploration Programs (volume 2); Herschel Family (volume 2); NASA (volume 3); Neptune (volume 2); Robotic Exploration of Space (volume 2).

Heidi B. Hammel

Bibliography

Bergstralh, Jay T., Ellis D. Miner, and Mildred Shapley Matthews, eds. Uranus. Tucson, AZ: University of Arizona Press, 1991.

Miner, Ellis D. Uranus: The Planet, Rings, and Satellites, 2nd ed. Chichester, UK: Praxis Publishing, 1998.

Standage, Tom. The Neptune File: A Story of Astronomical Rivalry and the Pioneers of Planet Hunting. New York: Walker and Company, 2000.

*There are four giant planets in the solar system: Jupiter, Saturn, Uranus, and Neptune

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Uranus

Uranus

Uranus, the seventh planet from the Sun, was probably struck by a large object at some point in its history. The collision knocked the planet sideways, giving it a most unique orbit. Unlike the other planets, whose axes are generally upright on their orbits, Uranus rotates on its side with its axis in the plane of its orbit.

It takes the planet slightly more than 84 Earth years to complete one revolution around the Sun and almost 18 Earth hours to complete one rotation about its axis. Because Uranus's polesand not its equatorface the Sun, each pole is in sunlight for 42 continuous Earth years.

Discovery of the planet

Uranus was discovered in 1781 by German astronomer William Herschel (17381822) during a survey of the stars and planets. At first, Herschel thought he had spotted a comet, but the object's orbit was not as elongated as a comet's would normally be. It was more circular, like that of a planet. Six months later, Herschel became convinced that this body was indeed a planet. The new planet was given two tentative names before astronomers decided to call it Uranus, the mythological father of Saturn.

Uranus is about 1.78 billion miles (2.88 billion kilometers) from the Sun, more than twice as far from the Sun as Saturn, its closest neighbor. Thus, the discovery of Uranus doubled the known size of the solar system.

Uranus is 31,800 miles (51,165 kilometers) in diameter at its equator, making it the third largest planet in the solar system (after Jupiter and Saturn). It is four times the size of Earth. Similar to Jupiter, Saturn, and Neptune, Uranus consists mostly of gas. Its pale blue-green, cloudy atmosphere is made of 83 percent hydrogen, 15 percent helium, and small amounts of methane and hydrocarbons. Uranus gets its color because the atmospheric methane absorbs light at the red end of the visible spectrum and reflects light at the blue end. Deep down into the planet, a slushy mixture of ice, ammonia, and methane surrounds a rocky core.

Voyager 2 mission to Uranus

Most of what is known about Uranus was discovered during the 1986 Voyager 2 mission to the planet. The Voyager 2 space probe left Earth in August 1977. It first visited Jupiter in July 1979, then Saturn in August 1981.

Voyager 2 collected information on Uranus during the first two months of 1986. At its closest approach, on January 24, it came within 50,600 miles (81,415 kilometers) of the planet. Among its most important findings were ten previously undiscovered moons (bringing the total to fifteen) and two new rings (bringing the total to eleven). Voyager also made the first accurate determination of Uranus's rate of rotation and found a large and unusual magnetic field. Finally, it discovered that despite greatly varying exposure to sunlight, the planet is about the same temperature all over: about 346°F (210°C).

Uranus's moons

Before Voyager 2 's visit, scientists believed Uranus had just five moons: Titania, Oberon, Umbriel, Ariel, and Miranda. After a rash of discoveries in the late 1990s, it is now known that Uranus has a complex system of twenty-one natural satellites, each with distinctive features (many of the moons are named for characters in plays by English dramatist William Shakespeare) The five previously discovered moons of Uranus range in diameter from about 980 miles (1,580 kilometers) to about 290 miles (470 kilometers). The largest of the newly discovered moons is 99 miles (160 kilometers) in diameter, just larger than an asteroid. The smallest is a mere 12.5 miles (20 kilometers) wide. The moons

fall into three distinct classes: the original five large ones; the eleven small, very dark inner ones uncovered by Voyager 2; and the five newly discovered much more distant ones.

Voyager 2 determined that the five largest moons are made mostly of ice and rock. While some are heavily cratered and others have steep cliffs and canyons, a few are much flatter. This discovery suggests varying amounts of geologic activity on each moon, such as lava flows and the shifting of regions of lunar crust.

Uranus's rings

The original nine rings of Uranus were discovered only nine years before Voyager 2 's visit. It is now known that Uranus has eleven rings plus ring fragments consisting of dust, rocky particles, and ice. The eleven rings lie between 23,500 and 31,700 miles (38,000 and 51,000 kilometers) from the planet's center. The extremely dark rings range in size from less than 1 mile to 60 miles (0.5 to 95 kilometers) wide.

[See also Solar system ]

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Uranus (in astronomy)

Uranus (yŏŏrā´nəs, yŏŏr´ə–), in astronomy, 7th planet from the sun, at a mean distance of 1.78 billion mi (2.87 billion km), with an orbit lying between those of Saturn and Neptune; its period of revolution is slightly more than 84 years. The first planet discovered in modern times with the aid of a telescope, Uranus was detected in 1781 by Sir William Herschel, who originally thought it to be a comet. Because the calculated orbit of Uranus did not compare accurately with the observed orbit, astronomers concluded that a disturbing influence was present. A study of this irregularity led to the discovery of Neptune in 1846. Uranus has a diameter of c.31,760 mi (46,700 km), roughly 4 times that of the earth, and a mass of about 15 times that of the earth. Like the giant planets Jupiter and Saturn, Uranus has a thick atmosphere of hydrogen, helium, and methane; a relatively low density; and a rapid period of rotation of about 17.9 hr, which causes a polar flattening of over 6%. However, its axis of rotation is tilted 98° to the plane of its orbit. The Voyager 2space probe found that Uranus has the most inclined magnetic field in the solar system, and some astronomers interpret this as evidence that the magnetic field is reversing its polarity. Viewed through a telescope, Uranus appears as a greenish disk, slightly elliptical because of its rapid rotation. Its temperature is estimated to be about -330°F (-200°C), and at this temperature ammonia, the main constituent of the visible cloud cover, would exist in the form of ice crystals. Uranus has 27 known natural satellites with diameters ranging in size from 7 mi (11 km) to 986 mi (1,578 km).

Prior to 1986, only five of Uranus's natural satellites were known: Titania, the largest, and Oberon were discovered by Herschel in 1787; Ariel and Umbriel, by William Lassell in 1851; and Miranda, by Gerard Kuiper in 1948. When Voyager 2 flew by Uranus in 1986, it discovered 10 more natural satellites—Cordelia, Ophelia, Bianca, Cressida, Desdemona, Juliet, Portia, Rosalind, Belinda, and Puck—and confirmed the existence of 11 rings. Two additional satellites, Caliban and Sycorax, were discovered in 1997, and three more, Prospero, Setebos, and Stephano, were found in 1999. Trinculo, a small irregular satellite, was discovered in 2002; eight other small satellites are also irregular, that is, their motion around Uranus is retrograde (motion opposite to that of the planet's rotation). The moons of Uranus are named after characters found in the works of William Shakespeare and Alexander Pope.

Titania along with Oberon and Umbriel appear geologically to be relatively quiet. Ariel has surface features that indicate past seismic activity. Miranda shows the most dramatic surface of all, with fracture patterns and sudden landscape changes that indicate that the moon fell apart and then reassembled after a collision in its early history. In 1977, during an occultation by Uranus of a star, astronomers detected a system of nine narrow rings of small, dark particles orbiting around the planet; two more rings, many tiny ringlets, and arcs of rings were later found by Voyager 2. Uranus's rings are distinctly different from those of Jupiter and Saturn. For example, Saturn's rings are very bright and easily seen but Uranus's are very dark, with only 5% of the sunlight being reflected back. Uranus's rings also are very narrow and flat. The widest part of Uranus's outermost ring, the epsilon ring, is 60 mi (97 km) across. The others are only 1 to 2 mi (1.5–3.2 km) wide and barely half a mile (0.8 km) deep.

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Uranus

Uranus The seventh planet in the solar system, discovered in 1781 by Sir William Herschel, although he described it as a comet. It was named Uranus by J. E. Bode. Its equatorial radius is 25 559 km and polar radius 24973 km; volume 6833 km3; mass 86.83 × 1024 kg; mean density 1318 kg/m3; visual albedo 0.51; black-body temperature 35.9 K. The inclination of the equator to the plane of the ecliptic is 97.86°, so the planet is lying on its side (a fact discovered in 1846 by J. Galle). At its closest approach, Uranus is 2581.9 × 106 km from Earth and at its furthest 3157.3 × 106 km. Uranus has an atmosphere, with a surface atmospheric pressure well in excess of 100 bar. The atmosphere is composed of molecular hydrogen (89%) and helium (11%), with aerosols of methane, ammonia ice, water ice, ammonia hydrosulphide, and possibly methane ice (similar to that of Neptune). Wind speeds at the surface are 0-200 m/s and the average surface temperature is about 58 K. Two new satellites, so far unnamed, were discovered in 1997 (and designated S/1997U1 and S/1997U2), moving in eccentric orbits at a mean distance of 5.8 × 106 km from the planet (227 Uranian radii), and are estimated to have diameters of 60 km and 80 km, bringing the total number of known satellites to 17 (see URANIAN SATELLITES) and it is likely that more remain to be discovered. Except for Titan, the uranian satellites are denser than those of Jupiter. Oberon and Titania, the two largest, were discovered in 1787 by Sir William Herschel. Umbriel and Ariel were discovered in 1851 by William Lassell. Miranda was discovered in 1948 by Gerard Kuiper. Ariel, Oberon, and Titania are probably made of water ice, other ices, and silicates. They are believed to be too cold to have a molten core, but on some there are signs of geological activity. The remaining 10 satellites were revealed in images transmitted to Earth from Voyager 2.

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Uranus

Uranus Seventh planet from the Sun, discovered (1781) by Sir William Herschel. Uranus is visible to the naked eye under good conditions. Through a telescope it appears as a small, featureless, greenish-blue disc. Like all the giant planets, it possesses a ring system and a retinue of satellites. Like Pluto, Uranus' axis of rotation is steeply inclined, and its poles spend 42 years in sunlight, followed by 42 years in darkness. Highly exaggerated seasonal variations are experienced by the planet and its satellites. The fly-by of the Voyager 2 probe in 1986 provides most of our knowledge of the planet. The upper atmosphere is about 83% molecular hydrogen, 15% helium, and the other 2% mostly methane. The five largest satellites were known before the Voyager encounter, which led to the discovery of 11 more. Nineteen of its 22 moons are regular satellites, orbiting in or close to Uranus' equatorial plane. They are all darkish bodies composed of ice761 and rock. The main components of Uranus' ring system were discovered in 1977, and others were imaged by Voyager.

http://lpl.arizona.edu/nineplanets/nineplanets/uranus.html; http://wr.usgs.gov

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