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Uranus
Uranus (diameter 51,100km) is the seventh planet
of the Solar
System. It is
about four times
the diameter and
15 times the mass
of the Earth, and orbits the Sun
every 84 years at a distance of 2.9 billion km. It spins on its axis in just over 17 hours.
Uranus has an impenetrable atmosphere of 83% hydrogen, 15% helium, and 2% methane, which is over 8,000km thick. Beneath it is an ocean of hot water over 10,000km deep. The solid part of the planet is made of rock and
is about the same size as the Earth. At the center is a core of molten rock. A system of 11 faint rings
surrounds the planet. Uranus has five major
moons and ten smaller ones. (See also: Ariel; Miranda; Voyager.)
Uranus is unusual in that its axis is in its plane of orbit, that is,
it “spins on its back.” This may have been caused by an ancient collision with another
planet. (See also: Herschel, (Frederik) William.)
Gas
Rock
Liquid
Vega
Triton
The largest moon of Neptune, some 2,706km across (see: Moons). It has an icy surface, but there are huge geyserlike eruptions, possibly of liquid nitrogen.
U
Umbra
The dark central region of an eclipse.
Universe
Everything that exists. The most popular theory of the origin of the Universe is the Big Bang theory. The known Universe is thought to have an age of about 20 billion years. (See also: Cosmos and Expanding Universe.)
 Valles marineris—The largest rift feature in the Solar System.
The 26km high volcano Ascraeus Mons pokes through the cloud in the Valles Marineris region of Mars
Lower left is the Valles Marineris rift valley, which is the same width as the United States.
V
Valles Marineris
A spectacular canyon on Mars, equal in length to the width of the United States and the largest rift feature in the Solar System.
Van Allen belts
Two radiation belts surrounding the Earth. They were discovered by James Van Allen in 1945. (See also: Explorer.)
Vega
The common name for the star Alpha Lyrae, the fifth brightest in the sky (see: Brightest stars).
 Uranus—The structure of Uranus is similar to Neptune, with helium-hydrogen atmosphere surrounding a liquid helium and hydrogen “mantle” over a small rocky core.
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