Uranus

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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).

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.

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.

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 modifythe 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.

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.

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

Exploration Programs (Volume 2);; Herschel Family (Volume 2);; Nasa (Volume 3);; Neptune (Volume 2);; Robotic Exploration of Space (Volume 2).

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.

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