Uranus

Planet Profile

Mass (kg)............................................8.68 x 10^25
Diameter (km)........................................51118
Mean density (kg/m^3) ...............................1290
Escape velocity (m/sec)..............................21300
Average distance from Sun (AU).......................19.18
Rotation period (length of day in Earth hours).......17.9
Revolution period (length of year in Earth years)....84
Obliquity (tilt of axis in degrees)..................97.9
Orbit inclination (degrees)..........................0.77
Orbit eccentricity (deviation from circular).........0.047
Mean temperature (K).................................59
Visual geometric albedo (reflectivity)...............0.56
Atmospheric components...............................83% hydrogen, 15% helium, 2% methane (at depth) Rings................................................Uranus has a system of narrow, faint rings. Ring particles are dark, and could consist of rocky or carbonaceous material.

Uranus Image Gallery

I INTRODUCTION

Uranus (planet), major planet in the solar system, seventh planet from the Sun. Uranus revolves outside the orbit of Saturn and inside the orbit of Neptune (see Solar System). The average distance from Uranus to the Sun is 2.87 billion km (1.78 billion mi). Uranus has an inner rocky core that is surrounded by a vast ocean of water mixed with rocky material. From the core, this ocean extends upward until it meets an atmosphere of hydrogen, helium, and methane. Uranus has 10 known rings and 18 moons.

The mass of Uranus is 14.5 times greater than the mass of Earth, and its volume is 67 times greater than that of Earth. The force of gravity at the surface of Uranus is 1.17 times the force of gravity on Earth. Because of its size and mass, scientists classify Uranus as one of the giant or Jovian (like Jupiter) planets—along with Jupiter, Saturn, and Neptune.
 

Uranus was the first planet that people discovered by using a telescope. Sir William Herschel, a German-born British musician and astronomer, discovered the planet in 1781. Herschel accidentally discovered it while measuring shifts in the positions of stars in the constellation Gemini. He observed that Uranus is a moving object, so he first reported his discovery to the British Royal Society as a comet. However, people had observed and plotted Uranus on star charts dating back to 1690 (believing it was a star).

Astronomers used these earlier observations to identify the object as a planet and to establish its orbit. Herschel originally named the planet Georgium Sidus (Star of George) in honor of King George III of Great Britain. Later, astronomers named the planet after Uranus, a figure who embodied the heavens and was the father of Saturn and the grandfather of Jupiter in Greek and Roman mythology.
 

II OBSERVATION FROM EARTH AND SPACE

Uranus orbits the Sun, varying from 2.74 x 109 km (1.70 x 109 mi) to 3.00 x 109 km (1.86 x 109 mi) in distance from the Sun. The orbit of Uranus traces out a flat region of space called the planet's orbital plane. The orbital plane of Uranus lies close to Earth's orbital plane. As a result, Uranus always crosses the same region of Earth's sky. Uranus, which appears to be a star to the naked eye, is so faint that people did not consider it important enough to include among the stars outlining the familiar constellation.

Through a large telescope, the planet appears as a blue-green disk with a diameter of about 3.5 arc seconds. Arc seconds describe the size of objects in the night sky by giving the size of the angle that the objects block out in the sky (a quarter held at arm's length is approximately 7000 arc seconds).

Because Uranus is so far from Earth (2.84 x 109 km/1.76 x 109 mi), only one spacecraft has visited the planet. During a rare alignment of the four giant planets, the spacecraft Voyager 2, which was launched on August 20, 1977, was able to pass by Jupiter (in 1979), Saturn (in 1981), Uranus (in 1986), and Neptune (in 1989). Scientists launched Voyager 2 with just enough energy to pass Jupiter. However, the strong gravitational pull of Jupiter accelerated the spacecraft as it passed by the planet so that Voyager 2 had enough energy to reach Saturn. As Voyager 2 successively passed each of the four giant planets, the gravitational pull of each planet accelerated the spacecraft enough to help it reach the next planet.

As Voyager 2 passed by Uranus, the spacecraft recorded and transmitted images of the planet, its rings, and some of its moons. Astronomers studying these images discovered five previously undetected rings and ten previously undiscovered moons. In addition to discovering these inner moons, Voyager 2 passed close to Miranda, the 11th satellite from Uranus, and mapped the moon's surface in detail. Surface features of Miranda include craters, canyons, and geologically young systems of ridges and grooves. Because the other large satellites were more distant from the spacecraft's path, Voyager 2 was unable to make detailed images of their surfaces.

III MOTION OF URANUS

Uranus takes 84 years to complete a single revolution around the Sun, so a year on Uranus is 84 times longer than a year on Earth. Uranus spins in place around its axis (an imaginary line that runs down the middle of the planet) once every 17.25 hours, just as Earth spins once every 24 hours. The ends of the axis mark the north and south poles of Uranus, just as Earth's axis marks the North Pole and the South Pole on Earth. Uranus rotates about an axis (the way a plastic globe spins on a rod) that tilts 98° into its orbital plane(the plane created by Uranus's orbit around the Sun). Because of this tilt, one pole of Uranus points almost directly toward the Sun during half of Uranus's 84-year orbit, and the other pole points toward the Sun during the second half. This pattern creates 42-year-long seasons of lightness and darkness, alternately, on each end of Uranus. Despite these long seasons, the difference in temperature between the two poles is not great (the planet's average temperature in its upper atmosphere is about -212° C/-350° F). This uniform temperature indicates that heat is conducted efficiently, or travels easily, throughout the planet.

As Uranus spins about its axis, material near the planet's equator must travel farther to make one rotation than material near the poles must travel. This equatorial material must then move faster than material at the poles. All material has inertia (the tendency of a moving mass to continue moving in a straight line), and this property makes the fast-moving material near the equator want to fly off from the planet in a straight line. The rest of the planet's mass gravitationally attracts the material and keeps it glued to the planet, but the material's inertia makes the planet bulge out at the equator. The bulge around the equator of Uranus is about 2 percent of the radius, or about 500 km (about 300 mi).

IV COMPOSITION AND STRUCTURE

Uranus contains mostly rock and water, with hydrogen and helium (and trace amounts of methane) in its dense atmosphere. Astronomers believe that Uranus formed from the same material—principally frozen water and rock—that composes most of the planet's moons. As the planet grew, pressures and temperatures 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 about 6650° 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 and extends upward to the planet's atmosphere.

The vast body of liquid on Uranus accounts for most of the planet's volume. Scientists think this ocean consists mostly of water molecules, which are mixed with silicate, magnesium, nitrogen-bearing molecules, and hydrocarbons (molecules composed of carbon and hydrogen). Uranus's ocean is extremely hot (about 6650° C/about 12,000° F). Water at the surface of Earth evaporates, or boils, 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 times stronger than the atmospheric pressure on Earth at sea level. Higher pressure holds molecules in liquids close together and prevents them from spreading out to form vapor.

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. Astronomers believe this atmosphere is relatively calm and inactive, with few storms or clouds. 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 low latitudes. These winds layer Uranus's clouds into bands. Light reflected from Uranus's deep atmosphere is blue-green, because the atmospheric methane absorbs red and 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 mostly hydrogen and helium, Uranus captured a much smaller amount of these elements as the solar system formed. Instead, Uranus captured mostly water. Because water is more 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 a radius 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 but much less dense than Jupiter.

V SPACE AROUND URANUS

Astronomers have identified ten rings of debris encircling Uranus's equator. These extremely dark, narrow rings orbit the planet in the plane of its equator at distances from 3.8 x 104 km (2.4 x 104 mi) to 5.1 x 104 km (3.2 x 104 mi). Many of these rings are made of ice and rock boulders about the size of large beach balls. American astronomer James L. Elliot detected five of the ten rings in 1977. Starting from the innermost ring, these five rings are called Alpha, Beta, Gamma, Delta, and Epsilon. In 1986 images taken by the Voyager 2 spacecraft helped scientists discover five more rings encircling Uranus.

Eighteen moons orbit Uranus. Sixteen of these moons revolve about the planet's equator, moving with the planet in an east-to-west direction. The other two moons orbit Uranus from west to east at a large angle to the planet's equator.

All of Uranus's moons are named for characters in the works of English playwright William Shakespeare and English poet Alexander Pope. The two largest and brightest moons, Oberon and Titania, were discovered by Sir William Herschel in 1787. British astronomer William Lassell detected the two next largest moons, Umbriel and Ariel. American astronomer Gerard Peter Kuiper discovered Miranda in 1948. Voyager 2 helped scientists discover Uranus's 11 innermost moons, each with a diameter of less than 100 km (60 mi). The ninth moon from Uranus was discovered in 1999 from photos that Voyager 2 took in 1986. This moon does not yet have an official name, but is known as S/1986 U 10. In order of their distance from Uranus, these moons are Cordelia (which is closest), Ophelia, Bianca, Cressida, Desdemona, Juliet, Portia, Rosalind, S/1986 U 10, Belinda, and Puck. The last two moons were discovered in 1997 by Canadian astronomer Brett Gladman and collaborators using the 200-inch telescope and a special camera at the Palomar Observatory on Mount Palomar in California. These two distant moons were subsequently named Caliban and Sycorax. Unlike the planet's 16 other moons, these two moons do not orbit in the plane of Uranus's equator. Astronomers believe that Caliban and Sycorax are captured asteroids instead of satellites that formed from the same cloud of planetary nebula (dust and gases that condense into planets) that formed Uranus. The surfaces of Uranus's four largest moons—Ariel, Umbriel Titania, and Oberon—are old, heavily cratered, and geologically inactive. Astronomers believe that these four moons consist of half ice and half rock (see Hale Observatories).

VI ROTATIONAL AND MAGNETIC AXIS

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 oceans contain electrically charged particles called ions. Ocean currents on Uranus make these charged particles move through the ocean, which in turn creates a magnetic field. Scientists believe that ocean currents in the other Jovian planets—Neptune, Saturn, and Jupiter—are created by heat released from the planets' cores. The core of Uranus releases less heat than the other three Jovian planets, however, and astronomers are unsure about what causes ocean currents in Uranus's fluid interior. Uranus's magnetic field is 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's strongly tilted rotational axis, although the magnetic field is offset from the planet's center. The influence of Uranus's magnetic field extends for several hundred thousand kilometers above the planet.

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