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Figure 11.5 Voyager 2 image of the rings of Uranus showing slight colour differences. The bright white ring at the bottom is Epsilon. (Photo: NASA)

the rings were not empty but contained fine dust. This may have originated from collisions between the larger particles that form the main rings, or from the surrounding moons.

The outer ring is the most massive, and its particles are kept in orbit by the gravitational influence of two moons, Cordelia and Ophelia. The outermost ring is about 100 km wide but only 10 m to 100 m thick. The ring material is probably composed of chunks of ice, covered by a layer of carbon.

The Uranian rings were the first rings to be discovered after Saturn's. This was an important finding, since we now know that rings are a common feature of planets, not a peculiarity of Saturn alone. Uranus's rings are much darker than those of Saturn and are much harder to see from Earth.

The outermost ring of Uranus, discovered in 2005, has been found to be bright blue. This ring is only the second known blue ring in the solar system - the outer ring of Saturn is also blue. Astronomers suspect both rings owe their colour to forces acting on dust in the rings that allow smaller particles to survive while larger ones are recaptured by nearby moons. Hubble discovered a new moon, now called Mab, orbiting Uranus in the same region as its blue ring. Mab is a dead rocky world about 24 km in diameter. Meteoroid impacts continually blast dust off the surface of Mab and this may contribute to the material in the blue ring of Uranus.

The temperature in the upper atmosphere of Uranus is about -200°C. At this low temperature, methane and water condense to form clouds of ice crystals. Because methane freezes at a lower temperature than water, it

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