A 7,000 kilometre wide view of Uranus' rings, showing icy particles filling in the gaps between the rings.
THE RINGS OF URANUS
Uranus has a ring system. The rings, of which eleven are known, are not very bright and quite narrow (less than 10 kilometres each across). They were the first rings to be discovered for nearly 370 years, and important because they demonstrated that rings are not unique to Saturn. In 1977, astronomers were about to observe the occultation of a star by Uranus. Just beforehand, the star blinked five times. After the occultation by the planet, the star blinked again five times; thus indicating that a series of rings were present.
Ring structure
The innermost of Uranus' rings is 1986 U2R, it is broad and faint. Moving outwards beyond the numbered rings 4,5, and 6, the Alpha and Beta rings are encountered. There is a gap between these and the next set, the Eta, Gamma and Delta rings.
Beyond the very faint Lambda ring, is the Epsilon ring. The Epsilon ring is the brightest and most opaque. It is 20 to 100 kilometres across, which also helps to make it more visible. The Voyager 2 images shown here display the layout of Uranus' rings. Each image has been duplicated and labelled to assist identification. In these images, the Lambda ring is too faint to show up.
Voyager 2 image of the bright Epsilon ring and fainter other rings.	Labelled version.
The Alpha, Beta, Gamma, Delta, and ETA rings.	Labelled version.
Name Distance Width (km) Depth (km) Eccentricity Inclination Albedo 1986U2R 38000 2500 0.1 0 0 0.3 6 41840 1-3 0.1 1 63 0.3 5 42230 2-3 0.1 1.9 52 0.3 4 42580 2 0.1 1.1 32 0.3 Alpha 44720 8-11 0.1 0.8 14 0.3 Beta 45670 7-11 0.1 0.4 5 0.3 Eta 47190 2 0.1 0 2 0.3 Gamma 47630 1-4 0.1 0 11 0.3 Delta 48290 3-9 0.1 0 4 0.3 Lambda 50020 1-2 0.1 0 0 0.3 Epsilon 51140 20-100 0.15 7.9 1 0.3 D = distance from the centre of Uranus to the inner edge of the ring.
Epsilon and shepherds
The Epsilon ring is bounded by the orbits of Ophelia and Cordelia, which act as shepherd moons, maintaining the ring, keeping its margins free of debris. The neatness of the other rings is probably due to the presence of moonlets perhaps just several kilometres across, which are too small to be detected.
Voyager 2 image of Cordelia.	Voyager 2 image of Ophelia.
Cordelia and Ophelia acting as shepherds to Uranus' Epsilon ring.	The orbits of Cordelia, Ophelia, and the Epsilon ring.
Close up images of the Epsilon ring show that it is very similar to Saturn's braided F-ring and appears twisted. Dark regions seen amongst the ring are associated with the affect of charged particles upon methane contained in the icy particles.
The Epsilon ring is eccentric which means that it has a slightly elliptical orbit. Some of the other rings too have surprising orbital properties. The rings 4, 5, and 6 are inclined by about 30o, 50o and 60o. It is not known why this should be.
Composition
The fragments which make up the rings range in size from dust particles to boulders which can be as large as 10 metres across. There appears to be less dust, however, than observed in other planetary ring systems. There is a greater abundance of centimetre scale particles. However, inside the orbit of the Lambda ring, dust particles have formed their own ringlets. The rings of Uranus are very dark, reflecting only a few percent of any sunlight they receive. They have neutral colours, similar to Uranus' small moons, suggesting that meteoritic bombardment of the small moons may be the source of the rings. The small moons have a low mass, and therefore a low escape velocity. Ejecta, kicked up from the surface by colliding meteoroids, will escape from the moon and will enter into orbit around the parent planet. Bombardment as a mechanism of ring formation seems also to be operating within Jupiter's ring system.
A 7,000 kilometre wide view of Uranus' rings, showing icy particles filling in the gaps between the rings.