Uranus is the seventh planet from the Sun and is the third in the series of four gas giants. Uranus has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. Uranus revolves outside the orbit of Saturn and inside the orbit of Neptune. British astronomer Sir William Herschel unintentionally discovered Uranus in 1781. It had, in fact, been seen many times earlier, but was ignored as simply another star. It is visible to the naked eye like the other five classical planets, but was never recognized as a planet by ancient spectators because of its dimness and slow orbit. It was also the first planet discovered with a telescope.
Atmosphere and Temperatures of Uranus
Planet Uranus is composed mainly of rock and a variety of ices, having only about 15% hydrogen and a little helium. Uranus’s rocky core is enveloped in a mantle of gases and ices. Planet’s atmosphere contains methane, which gives the planet its characteristic color. Uranus looks blue because red light is absorbed by methane in the upper atmosphere. There may be colored bands similar to Jupiter’s, however they are out of sight because of the overlaying methane layer.
Uranus is located in the cold outer reaches of the Solar System and its cloud tops have very low temperature of about -210° C (-346°F). It is the planet with the coldest planetary atmosphere in the Solar System. Uranus has a complex, layered cloud structure, with water considered to make up the lowest clouds and methane makes up the uppermost layer of clouds.
On the contrary the interior of Uranus is mostly composed of ices and rock. Observations show that Uranus does not have a rocky core like Jupiter and Saturn, however unlike those planets its mass is more or less evenly distributed. Uranus’ atmosphere has about 83% hydrogen, 15% helium and 2% methane.
Orbit and Rotation of Uranus
It takes Uranus 84 Earth years to complete one rotation around the Sun. The planet needs 17hr 15min to rotate around its axis, which is inclined 98° to the plane of the planet’s orbit around the Sun. The rotational period of the interior of Uranus is 17 hours, 14 minutes. On the other hand, as on all giant planets, its upper atmosphere experiences extremely strong winds in the direction of rotation. At some latitudes, for example about two-thirds of the way from the equator to the south pole, atmosphere can move much faster, making a full rotation in as little as 14 hours.
Magnetic Field
Voyager’s observations discovered that the magnetic field of Uranus is quiet peculiar. The reason behind this is that it does not originate from the planet’s geometric center and it is tilted at 59° from the axis of rotation. Actually the magnetic dipole is moved from the center of the planet towards the south rotational pole by almost one third of the planetary radius. This strange geometry results in a extremely asymmetric magnetosphere, in which the magnetic field strength on the surface in the southern hemisphere can be as low as 0.1 gauss (10 µT), while in the northern hemisphere it can be as high as 1.1 gauss (110 µT) and the usual field at the surface is 0.23 gauss (23 µT).
Rings
Similarly to other gas giants, Uranus has rings, a magnetosphere, and numerous moons. In 1977 American astronomer James L. Elliot discovered the presence of five rings around the equator of Uranus. Starting from the innermost ring, these rings were named as Alpha, Beta, Gamma, Delta, and Epsilon. These form a 9400 km wide belt extending to 51,300 km from the planet’s centre. In January 1986, four more rings were discovered during the exploratory flight of Voyager 2.
Like Jupiter’s rings, they are very dark, however similarly to Saturn’s rings they are composed of rather large particles up to 10 meters in diameter. All of Uranus’ rings are very faint – the brightest is known as the Epsilon ring.
Moons of Uranus
Uranus has 27 moons. Unlike other moons in the solar system, which have their names mainly from the classical mythology, Uranus’ moons got their names from the writings of Shakespeare and Pope. Most of Uranus’ moons have almost circular orbits, however the outer 4 are much more elliptical. At least 21 of Uranus moons orbit its equator in the east-west rotation. Oberon and Titania are the largest moons and were discovered by Herschel in 1787. The next two, Umbriel and Ariel, were discovered in 1851 by the British astronomer William Lassell. Cordelia is the closest of Uranus’ ten ‘inner moons’.
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[Images: www.nasa.org]
