Satellites and Rings of Uranus

By Dave Hawksett

Uranus RingsOn 10 March 1977, the planet Uranus passed directly in front of a faint star. By watching how long the star was hidden (occulted) by Uranus, astronomers hoped to measure the diameter of the planet with more accuracy than ever before. High above the Indian Ocean, astronomers on board the Kuiper Airborne Observatory (a modified jet aircraft) saw the star 'blink' 5 times, 35 minutes before passing behind Uranus, and again after the occultation was over. The reason for this was that Uranus, like Saturn had a system of rings around it.

There are ten main rings in total, all of which are very narrow, in contrast to Saturn's broad rings. They are also very dark: as black as coal dust. In fact they would be invisible to someone who was actually there. First imaged close up by the Voyager 2 spacecraft in 1986, these rings are probably composed of countless icy boulders a metre or so across. 

Voyager 2 also imaged Uranus's large family of moons, five of which had been known about before the encounter. The two largest, Titania at 1,578 kilometres across (980 miles) and Oberon at 1,523 kilometres across (950 miles) were discovered in 1787 by William Herschel, six years after he discovered Uranus itself. Closer to the planet, Ariel and Umbriel are moderately smaller and were first spotted in 1851. Miranda, the innermost was discovered in 1948. Ten more icy moons were first seen by Voyager 2, More small moons have since been discovered, bringing the grand total to 21.

The five major moons all have densities around one-and-a-half times that of water, and so are mostly made of ice. Oberon and Umbriel are characterised by impact craters, but Ariel and Titania also show signs of ancient geological activity. Huge canyons extend across their surfaces, as if they have undergone some kind of tectonic activity in the past.

The star of the show was Miranda. This world is only some 472 kilometres across (293 miles) across, but its surface is twisted and contorted, and shows a remarkable variety of features. The patchwork of terrains looks like the tiny moon had been ripped apart and randomly put back together again. In fact, many astronomers believe this is exactly what happened. It is possible that, perhaps a billion or so years ago, a large comet smashed Miranda to pieces. The pieces then gradually came back together under their own gravity. The contorted geology may be a result of the rock and ice trying to separate into layers after Miranda reformed.