Introduction to Uranus
Uranus, the seventh planet from the sun. Uranus was the first planet to be discovered with a telescope, Mercury, Venus, Mars, Jupiter, and Saturn being readily visible to the unaided eye. Uranus was the first planet to be discovered since ancient times. When Sir William Herschel sighted Uranus in 1781, he at first mistook it for a comet. Under good viewing conditions, it is just visible to a person of keen eyesight. German astronomer Johann E. Bode named Uranus after a sky god in Greek mythology. Uranus and Neptune are similar in size and appearance. In the late 1970s, astronomers discovered that Uranus was encircled by a series of nine narrow rings. Unlike the rings of Saturn, these rings are dark and are probably composed of particles of rock.
Uranus at a glanceDistance from the sun: Shortest--1,703,360,000 mi (2,741,300,000 km); Greatest--1,866,360,000 mi (3,003,620,000 km); Average--1,784,860,000 mi (2,872,460,000 km).Distance from Earth: Shortest--1,604,300,000 mi (2,581,900,000 km); Greatest--1,961,900,000 mi (3,157,300,000 km).Diameter at equator: 31,763 mi (51,118 km).Length of year: 84 Earth years.Rotation period: 17 hours 14 minutes.Temperature: 355 °F (215 °C).Atmosphere: Hydrogen, helium, methane.Number of satellites: At least 27.Orbit and Rotation
Uranus moves around the sun in an elliptical (oval-shaped) orbit. It takes Uranus around 30,685 Earth days, or 84 Earth years to complete one revolution. While orbiting the sun, Uranus also rotates on its axis. Axis is defined as a hypothetical line through the planets center. The planet's interior (ocean and core) takes about 17 hours 14 minutes to spin around once on its axis. However, the atmosphere rotates faster than that. The fastest winds on Uranus blow at about 450 miles per hour (720 km/h). These winds are measured about two-thirds of the way from the equator to the South Pole. Therefore, this area toward the South Pole makes one complete rotation every 14 hours.
Scientists measure the tilt of a planet relative to a line at a right angle to the orbital plane, an imaginary surface touching all points of the orbit. Most planets' axes tilt less than 30. But Uranus's axis tilts 98, so that the axis lies almost in the orbital plane. Astronomers believe that this tilt was caused by a collision with an Earth-sized planet.
Physical Characteristics
Uranus is a huge ball of gas and liquid. At the equator its diameter is about 31,763 miles (51,118 km), which is four times that of Earth. The planet has a virtually uniform appearance; images of the planet show only a few clouds visible through the atmosphere. Data from spacecraft instruments established that Uranus has a magnetic field. The surface of Uranus is made up of blue-green clouds consisting of tiny methane crystals which have frozen out of the planet's atmosphere. Far below these clouds are thicker cloud layers made up of crystals of ammonia ice and liquid water. Further downabout 4,700 miles (7,500 km) below the visible cloud topsthere probably exists an ocean of liquid water containing dissolved ammonia. The center of the planet may be rocky and about the size of Earth. Some astronomers believe Uranus may have a molten core some 4,000 miles (6,400 km) in diameter surrounded by a layer of superheated water some 5,000 miles (9,000 km) deep. Scientists strongly doubt Uranus has any form of life.
Uranus has a mass (quantity of matter) 14 1/2 times larger than that of Earth. However, the mass of Uranus is only about 1/20 as large as that of Jupiter.
The average density of Uranus is 1.27 grams per cubic centimeter, or about 1 1/4 times the density of water. The density of Uranus is similar to that of Jupiter and 1/4 of Earth. An object weighing 100 pounds on Earth would weigh around 90 pounds on Uranus. This is because the force of gravity at the surface of Uranus is around 90 per cent of that at the surface of Earth.
The atmosphere of Uranus contains about 83 per cent hydrogen, 15 per cent helium, 2 per cent methane, and small amounts of ethane and other gases. The presence of methane gives the planet a greenish appearance. The atmospheric pressure beneath the methane cloud layer is about 19 pounds per square inch (130 kilopascals), or about 1.3 times the atmospheric pressure at the surface of Earth.
The visible clouds of Uranus are of pale blue-green all over the surface of the planet. Images taken by Voyager 2 show very faint bands within the clouds parallel to the equator. These images have been processed for high contrast by computers. These bands consist of different concentrations of smog produced as sunlight breaks down methane gas. There are a few small spots on the planet's surface, which are probably violently swirling masses of gas resembling a hurricane.
In 2006, the Hubble Space Telescope revealed a large, dark spot made of whirling gases in Uranus's northern hemisphere. It measured 1,100 miles (1,700 km) by 1,900 miles (3,000 km). Scientists believe the spot formed because spring was coming to Uranus's northern hemisphere.
Temperature
The temperature of Uranuss atmosphere is about 355 F. (215 C.). In the interior, the temperature rises rapidly, reaching perhaps 12,600 F. (7000 C.) in the rocky core and 4200 F. (2300 C.) in the ocean. As Uranus is tilted 98 on its axis, its poles receive more sunlight during a Uranian year than does its equator. Therefore, Uranus radiates as much heat into space as it gets from the sun. However, this extra heat seems to be dispensed fairly evenly over the planet, due to its weather system.
Satellites
Astronomers have discovered about 27 satellites of Uranus. The 5 largest satellites were discovered between 1787 and 1948. But, there are some more small moons that are yet to be discovered. Most of our information available today about Uranus comes from the flight of the United States spacecraft Voyager 2. In early 1986, the spacecraft Voyager 2 passed within about 50,000 miles (80,000 km) of Uranus. Images sent to Earth by the spacecraft revealed 10 small, previously unknown satellites and two previously unknown rings. Later, more satellites were discovered by the Earth-based telescopes.
Miranda, the smallest of the five large satellites, has some unique surface features. It has three oddly shaped regions called ovoids. Each ovoid is 120 to 190 miles (200 to 300 km) across. The outer areas of an ovoid resemble racetracks, with parallel ridges and canyons wrapped about the center. However, ridges and canyons crisscross one another arbitrarily in the center.
There are a number of rings around Uranus. The exact composition of the rings is unknown, but they probably consist of small chunks and dust-sized grains of ice coated in a layer of material that contains carbon. Out of the many rings of Uranus ten are dark and narrow. Their width ranges from less than 3 miles (5 km) to 60 miles (100 km) and they are around 33 feet (10 m) thick. Outside the narrow rings lie two faint, dusty rings. There is also a broad, less-distinct ring closer to the planet than the others.
Satellites of UranusNameMean distance from UranusDiameterDiscoveredCordelia 30,930 mi (49,770 km)25 mi (40 km)1986Ophelia 33,420 mi (53,790 km)27 mi (43 km)1986Bianca 36,770 mi (59,170 km)32 mi (51 km)1986Cressida 38,390 mi (61,780 km)50 mi (80 km)1986Desdemona 38,950 mi (62,680 km)40 mi (64 km)1986Juliet 39,990 mi (64,350 km)58 mi (94 km)1986Portia 41,070 mi (66,090 km)84 mi (135 km)1986Rosalind 43,460 mi (69,940 km)45 mi (72 km)1986Cupid 46,480 mi (74,800 km)15 mi (24 km)2003Belinda 46,760 mi (75,260 km)50 mi (81 km)1986Perdita 47,490 mi (76,420 km)50 mi (80 km)1986Puck 53,440 mi (86,010 km)101 mi (162 km)1985Mab 60,730 mi (97,730 km)20 mi (32 km)2003Miranda 80,720 mi (129,900 km)293 mi (472 km)1948Ariel 118,690 mi (191,020 km)720 mi (1,158 km)1851Umbriel 165,470 mi (266,300 km)726 mi (1,169 km)1851Titania 271,100 mi (436,300 km)981 mi (1,578 km)1787Oberon 362,580 mi (583,520 km)946 mi (1,523 km)1787Francisco 2,657,000 mi (4,276,000 km)7 mi (12 km)2001Caliban 4,493,000 mi (7,231,000 km)61 mi (98 km)1997Stephano 4,973,000 mi (8,004,000 km)12 mi (20 km)1999Trinculo 5,284,000 mi (8,504,000 km)12 mi (20 km)2001Sycorax 7,568,000 mi (12,180,000 km)118 mi (190 km)1997Margaret 8,917,000 mi (14,350,000 km)7 mi (11 km)2003Prospero 10,100,000 mi (16,260,000 km)19 mi (30 km)1999Setebos 10,820,000 mi (17,420,000 km)19 mi (30 km)1999Ferdinand 12,990,000 mi (20,900,000 km)7 mi (12 km)2001Magnetic Field
Uranus has a powerful magnetic field. The axis of the field is tilted 59 from the planet's axis of rotation. The magnetic field encloses high-energy, electrically charged particlesmostly electrons and protonsin radiation belts around the planet. These particles emit radio waves as they travel back and forth between the magnetic poles. Voyager 2 detected these waves, but they are too weak to be detected on Earth.