Uranus is the third largest of the planets, and its surface, like those of Jupiter and Saturn, is obscured by clouds. Even though it is quite large, the ancients knew nothing of it. In fact, it was not until William Herschel had built a powerful telescope in 1781, was he able to discover it.
Uranus presents a homogeneous cloud, layered with water vapour composing the innermost clouds, and methane the uppermost. Although it is slightly larger in diameter than Neptune, its mass is a little less than its fellow ice giant. Like Neptune, the interior of Uranus is solid ice and rock.
Uranus has a ring system like the other 3 gas giants, as well as a magnetosphere, and a large number of moons, some of which, like Saturn's, are interspersed among the rings.
Uranus's axis of rotation is not 'up-down', as for all the other planets, but tilted so that it is close to parallel to the orbital plane (its pole is pointing towards the Sun). What might have happened to cause this?
Uranus has many natural satellites, and 27 have been found (so far).
Oberon is the second largest moon of Uranus, and the furthest out. It is large enough to have been observed by William Hershel in 1787, just 6 years after he discovered the planet. Everything we know about Oberon comes from the data and images sent to us by Voyager 1 in 1986.
The other large moons of Uranus are Ariel, Titania, and Miranda, but Oberon has the most heavily cratered surface, similar to Umbriel. Oberon also boasts a 6000m mountain.
Neptune is the last of the planets, and its orbit crosses that of the dwarf planet Pluto once every two centuries.
Whereas Saturn is all cloud, Neptune has a solid icey rock centre beneath its clouds.
This is a photograph of Neptune taken in 1989 by Voyager 2, the only probe to get up close to the planet. From 7 million km, this beautiful image reveals wraithlike clouds, the Great Dark Spot, a nearby smudge, and 'Scooter', a rapidly moving bright feature. Scientists believe these features, though transient, are recurring.
Voyager 2 also passed by Triton, the largest moon of Neptune. Triton is four-fifths the size of Earth's Moon, but has an atmosphere. Only three members of the solar system have nitrogen-dominated atmospheres: Earth, Titan (moon of Saturn), and Triton.
At -225°C, Triton has the coldest surface in the solar system. At this extreme temperature, the nitrogen can condense to form a nitrogen ice surface. The poles also contain methane ice. Methane reacts with solar radiation to form red-brown compunds, seen also on Charon, Pluto's moon. Evidence for geyser plumes has also been found.
There is a bumpy surface (known prosaically as the 'cantaloupe region'), and cryovolcanic terrain, which may have been formed by super-cold liquids erupting from Triton's interior to freeze on the surface.
Images from NASA/JPL/USGS.
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