Uranus

Uranus is the seventh planet of the Solar System. It has a diameter of about 52,400 km; a mass 14.6 times that of the Earth and orbits the Sun every 84 years. Its distance from the Sun varies between 18.3 and 20.1 Astronomical Units. (One Astronomical Unit is the mean distance of the Earth from the Sun.) From the Earth Uranus can be seen with a small telescope and is just visible to the naked eye. However, even with large telescopes it can only be seen as a small greenish disk with no surface details visible.

The Discovery of Uranus

A montage of Voyager 2 pictures of UranusA montage of Voyager 2 pictures of Uranus (the blue planet in the centre) and its largest moons. © Voyager 2 Uranus was the first planet to be discovered that was not known to the ancients. William Herschel announced its discovery in 1781 although initially he thought that it was a comet. After he had realised that it was a planet he wanted to call it Georgium Sidus, after his patron King George III. The name Uranus was originally suggested by Bode but it was not until 1850 when John Couch Adams suggested a change of name that it was universally adopted.

Herschel was not the first astronomer to record Uranus but he was the first to recognise that it was not a star. The earliest records of Uranus are by Flamsteed in 1690 (he called it a star, 34 Tauri), 1712 and four times in 1715. There are at least 15 other known sightings by three other astronomers before Herschel's discovery.

Results from Voyager 2

Uranus' atmospheric bands of cloudUranus' atmospheric bands of cloud. Uranus appears to have a featureless blue atmosphere (left). Image enhancement (right) shows that there are, in fact, faint atmospheric bands of cloud. ©Voyager 2 Our knowledge of Uranus has been revolutionised by the amazing results from the close fly-bys of each by Voyager 2 in January 1986.

The most striking observation was that the planet Uranus showed almost no features! It appears as a large green-blue ball in pictures from both Earth and Voyager. Astronomers already knew that Uranus rotated about an axis that is tilted at 98° to the pole of its orbital plane. This means that Uranus is `lying on its side' and rotating in the opposite sense to that of the other planets. The effect on the surface is that the seasons on Uranus are linked to its orbital period about the Sun, so near the poles winter will last 42 earth-years!

The Voyager data favour a model for the internal structure of the planet with a fairly small, rocky core surrounded by a deep superdense atmosphere of gases and ices of water, ammonia and methane. Above this there is an atmosphere of hydrogen and helium with clouds of methane, ammonia and water-ice. The temperature at the top of the clouds is -214°C.

Uranus has a magnetic field from which the rotation rate of the unseen `body' of the planet can be deduced as just over 17 hours. This is smaller than the rate determined from the few cloud structures seen, indicating that there are strong jet streams present on Uranus rather like those on the Earth.

Satellites

Before the Voyager 2 fly-by 5 satellites were known. Two, Titania and Oberon, were found by Herschel in 1787. Ariel and Umbriel were discovered in 1851 and Miranda in 1948. Voyager added another 10 objects of which two are `shepherds' in the rings.

All the satellites have impact craters on them. Ariel and Umbriel are each about 1200 kilometres in diameter but have very different surfaces. Ariel shows signs of geological activity on a large scale with evidence for liquid flows. The temperature is far too low for the liquid to have been water and a mixture of liquid ammonia and water ice has been suggested.

Umbriel is covered in craters and shows almost no sign of geological activity. Its surface is probably similar to what it was shortly after the formation of the solar system.

Titania and Oberon are each about 1600 kilometers in diameter and are both heavily cratered. Both show signs of geological activity.

Miranda is only 500 kilometres in diameter and yet shows incredibly complex jumbled geological structures with faults that appear to be global in size with surface features kilometres deep. These must have been formed in a violent stage in Miranda's life, either due to tidal heating in the past or a violent collision.

Rings

An image of Uranus taken by the Hubble Space TelescopeAn image of Uranus in October 1998, taken by the Hubble Space Telescope. This near-infrared image shows the rings and clouds in Uranus' atmosphere. The cloud systems circle the planet at more than 300 mph (500 km/h). On the right are large bright clouds that were not present in 1989 when Voyager 2 flew past the planet. © Kenneth Seidelmann, U.S. Naval Observatory, and NASA The ring system around Uranus was discovered, almost by accident, when the occultation of a bright star by Uranus was being studied. Small dips in the brightness of the star were seen while the star was some distance away from the disc of the planet and it was deduced that this was due to material in a ring about Uranus obscuring the starlight. The presence of the rings was confirmed by the Voyager spacecraft which added a tenth ring to the nine discovered from the Earth.

The sizes of the particles making up the rings can be estimated from how they scatter sunlight and how they affected the spacecraft's radio signals. There are far less small particles (micron-sized) than in Saturn's rings and many of the particles must measure several centimetres across. The rings are grey and reflect light poorly and are thus likely to be made of dust. They must be younger than 100 million years old and are likely to have been formed from the break-up of a small moon due to collision with a meteoroid or comet.

Due to the angle of the planet’s axis, the poles are subjected to 42 years of savage winter followed by a summer of the same length where they face the Sun. It now appears that this causes dramatic seasonal changes across the planet, as the atmosphere appears to become more dynamic as the northern hemisphere rotates out of winter into summer. By 2007, the Sun will be directly over Uranus’ equator (the equivalent of spring in the northern hemisphere). This has caused a change in the appearance of Uranus from the 1986 fly-by of Voyager 2 when images of the planet showed an almost completely smooth and featureless exterior. HST images have been used to produce a time-lapse movie  of this change. The movie also shows a new feature of the planet’s ring system: they wobble. The slightly flattened shape of Uranus and gravitational forces from the many moons could cause this effect.