Uranus - Planetary Rings

Planetary Rings

Uranus has a complicated planetary ring system, which was the second such system to be discovered in the Solar System after Saturn's. The rings are composed of extremely dark particles, which vary in size from micrometers to a fraction of a meter. Thirteen distinct rings are presently known, the brightest being the ε ring. All except two rings of Uranus are extremely narrow—they are usually a few kilometres wide. The rings are probably quite young; the dynamics considerations indicate that they did not form with Uranus. The matter in the rings may once have been part of a moon (or moons) that was shattered by high-speed impacts. From numerous pieces of debris that formed as result of those impacts only few particles survived in a limited number of stable zones corresponding to present rings.

William Herschel described a possible ring around Uranus in 1789. This sighting is generally considered doubtful, as the rings are quite faint, and in the two following centuries none were noted by other observers. Still, Herschel made an accurate description of the epsilon ring's size, its angle relative to the Earth, its red color, and its apparent changes as Uranus traveled around the Sun. The ring system was definitively discovered on March 10, 1977 by James L. Elliot, Edward W. Dunham, and Douglas J. Mink using the Kuiper Airborne Observatory. The discovery was serendipitous; they planned to use the occultation of the star SAO 158687 by Uranus to study the planet's atmosphere. When their observations were analyzed, they found that the star had disappeared briefly from view five times both before and after it disappeared behind the planet. They concluded that there must be a ring system around the planet. Later they detected four additional rings. The rings were directly imaged when Voyager 2 passed Uranus in 1986. Voyager 2 also discovered two additional faint rings bringing the total number to eleven.

In December 2005, the Hubble Space Telescope detected a pair of previously unknown rings. The largest is located at twice the distance from the planet of the previously known rings. These new rings are so far from the planet that they are called the "outer" ring system. Hubble also spotted two small satellites, one of which, Mab, shares its orbit with the outermost newly discovered ring. The new rings bring the total number of Uranian rings to 13. In April 2006, images of the new rings with the Keck Observatory yielded the colours of the outer rings: the outermost is blue and the other red. One hypothesis concerning the outer ring's blue colour is that it is composed of minute particles of water ice from the surface of Mab that are small enough to scatter blue light. In contrast, the planet's inner rings appear grey.