Uranus (planet) - astronomy.

« V COMPOSITION AND STRUCTURE A Interior of Uranus Uranus contains mostly rock and water, with hydrogen and helium (and trace amounts of methane) in its dense atmosphere.

Astronomers believe that Uranus, likeNeptune, formed from the same material—principally frozen water and rock—that composes most of the planet’s moons.

As the planet grew, pressures andtemperatures in the planet’s interior increased, heating the planet’s frozen water into a hot liquid. Uranus probably has a relatively small rocky core (smaller in size than Earth’s core), with a radius no larger than 2,000 km (1,240 mi) and a temperature of about6650°C (12,000°F).

Uranus’s core may be small because most of the rock composing the planet remains mixed with the body of water that surrounds the core andextends upward to the planet’s atmosphere. The vast body of liquid on Uranus accounts for most of the planet’s volume.

This compressed, slushy liquid is sometimes described as an ocean or as ice.

Scientists thinkthis ocean consists mostly of water molecules, which are mixed with silicate, magnesium, nitrogen-bearing molecules such as ammonia, and hydrocarbons (moleculescomposed of carbon and hydrogen) such as methane.

Uranus’s ocean is extremely hot (about 6650°C/about 12,000°F).

Water at the surface of Earth evaporates, orboils, at 100°C (212°F).

The ocean on Uranus remains liquid at such a high temperature, however, because the pressure deep in Uranus is about five million timesstronger than the atmospheric pressure on Earth at sea level.

Higher pressure holds molecules in liquids close together and prevents them from spreading out to formvapor. B Atmosphere The atmosphere of Uranus, which contains hydrogen, helium, and trace amounts of methane, extends about 5,000 km (about 3,100 mi) above the planet’s ocean.

Atthe time of the Voyager 2 flyby in 1986, the atmosphere was relatively calm and inactive, with few storms or clouds, but Hubble Space Telescope images showed moreactivity in 2001.

Winds blow parallel to the equator of Uranus, moving in the same direction as the planet’s rotation at high latitudes, and opposite to the rotation at lowlatitudes.

These winds layer Uranus’s clouds into bands.

Light reflected from Uranus’s deep atmosphere is blue-green, because the atmospheric methane absorbs redand orange light.

Unlike the other giant planets, Uranus radiates little heat into space from its deep interior. Although Uranus is one of the giant planets, it is smaller and has a different chemical composition than Saturn and Jupiter.

While Saturn and Jupiter are made of mostlyhydrogen and helium, Uranus captured a much smaller amount of these elements as the solar system formed.

Instead, Uranus captured mostly water.

Because water ismore dense than hydrogen and helium, Uranus is more compact than Jupiter or Saturn.

Jupiter, for example, has a radius of 71,355 km (44,338 mi) while Uranus has aradius of 25,548 km (15,875 mi).

If Uranus had the same mass it has now but consisted of the lighter elements hydrogen and helium, the planet would be larger butmuch less dense than Jupiter.

Uranus is also slightly less massive, and thus less dense and less compact, than Neptune, which is otherwise very similar in composition.As a result, the radius of Uranus is slightly larger than the radius of denser Neptune. C Magnetic Field Uranus, like Earth, is surrounded by a magnetic field, a region of space that exerts a small force on electrically charged or magnetic material.

Uranus’s deep oceanscontain electrically charged particles called ions.

Ocean currents on Uranus circulate these charged particles, which in turn creates a magnetic field.

Scientists believethat ocean currents in the other Jovian planets—Neptune, Saturn, and Jupiter—are created by heat released from these planets’ cores.

The core of Uranus releases lessheat than the other three Jovian planets, however, and astronomers are unsure about what causes ocean currents in the planet’s fluid interior.

Uranus’s magnetic fieldis similar in strength to Earth’s magnetic field.

Uranus’s magnetic axis (the line joining the north and south poles of its magnetic field) is aligned with the planet’sstrongly tilted rotational axis, although the magnetic field is offset from the center of the planet.

The influence of Uranus’s magnetic field extends for several hundredthousand kilometers above the planet. VI RINGS AND MOONS Astronomers have identified 13 rings of debris encircling Uranus’s equator.

An inner set of extremely dark, narrow rings orbit the planet in the plane of its equator atdistances from 38,000 km (24,000 mi) from the center of the planet.

Many of these rings are made of ice and rock boulders about the size of large beach balls.

Severalobservatories first detected five of the ten rings in 1977.

Starting from the innermost ring, these five rings were called Alpha, Beta, Gamma, Delta, and Epsilon.

In 1986images taken by the Voyager 2 spacecraft helped scientists discover five more rings encircling Uranus.

In 2005 astronomers using the Hubble Space Telescope reportedthe discovery of two new rings.

These rings are so far from the planet that they make up a second ring system.

The innermost of these more distant rings is about67,000 km (41,632 mi) from the planet’s center and the outmost about 97,700 km (60,708 mi) from the center. Astronomers have found at least 27 moons that orbit Uranus.

Uranus’s moons are named for characters in the works of English playwright William Shakespeare andEnglish poet Alexander Pope.

The two largest and brightest moons, Titania and Oberon, were discovered by Sir William Herschel in 1787.

British astronomer WilliamLassell detected the two next largest moons, Umbriel and Ariel.

The surfaces of these four largest moons are old, heavily cratered, and geologically inactive.Astronomers believe that these four moons consist of half ice and half rock.

American astronomer Gerard Peter Kuiper discovered a smaller fifth moon, Miranda, in1948. Voyager 2 helped scientists discover 11 of Uranus’s inner moons, each with a diameter of less than 100 km (60 mi).

The tenth moon of the group, 1986U10, wasdiscovered in 1999 from photos that Voyager 2 took in 1986.

This moon was later named Perdita.

In order of their distance from Uranus, these inner moons areCordelia , Ophelia, Bianca, Cressida, Desdemona, Juliet, Portia, Rosalind, Belinda, Perdita, and Puck. Two more distant moons were discovered in 1997 by Canadian astronomer Brett Gladman and collaborators using the 200-inch telescope and a special camera at thePalomar Observatory on Mount Palomar in California.

These moons were subsequently named Caliban and Sycorax.

In 1999 the same group reported the discovery ofthree additional small, distant moons: Prospero, Setebos, and Stephano.

Prospero and Setebos are even more distant from Uranus than Sycorax, while Stephano’saverage orbital distance lies between those of Caliban and Sycorax.

Unlike the planet’s other moons, these five outer moons orbit Uranus in the direction opposite thatin which the planet rotates and follow highly eccentric orbits that are inclined to the plane of Uranus’s equator.

Astronomers believe that these oddball satellites arecaptured asteroids rather than satellites that formed from the same planetary nebula (cloud of dust and gases that condenses into planets) that formed Uranus ( see Hale Observatories). The Hubble Space Telescope enabled astronomers in 2005 to detect two more moons, named Mab and Cupid.

Mab has a diameter of about 19 km (12 mi).

Astronomersbelieve meteoroid impacts with Mab continually replenish dust in the newly discovered outer ring system.. »