The rings of Uranus are intermediate in complexity between the more extensive set around Saturn and the simpler systems around Jupiter and Neptune. The rings of Uranus were discovered on March 10, 1977, by James L. Elliot, Edward W. Dunham, and Jessica Mink. William Herschel had also reported observing rings in 1789; modern astronomers are divided on whether he could have seen them, as they are very dark and faint.By 1977, nine distinct rings were identified. Two additional rings were discovered in 1986 in images taken by the Voyager 2 spacecraft, and two outer rings were found in 2003–2005 in Hubble Space Telescope photos. In the order of increasing distance from the planet the 13 known rings are designated 1986U2R/ζ, 6, 5, 4, α, β, η, γ, δ, λ, ε, ν and μ. Their radii range from about 38,000 km for the 1986U2R/ζ ring to about 98,000 km for the μ ring. Additional faint dust bands and incomplete arcs may exist between the main rings. The rings are extremely dark—the Bond albedo of the rings' particles does not exceed 2%. They are probably composed of water ice with the addition of some dark radiation-processed organics.
The majority of Uranus's rings are opaque and only a few kilometres wide. The ring system contains little dust overall; it consists mostly of large bodies 20 cm to 20 m in diameter. Some rings are optically thin: the broad and faint 1986U2R/ζ, μ and ν rings are made of small dust particles, while the narrow and faint λ ring also contains larger bodies. The relative lack of dust in the ring system may be due to aerodynamic drag from the extended Uranian exosphere.
The rings of Uranus are thought to be relatively young, and not more than 600 million years old. The Uranian ring system probably originated from the collisional fragmentation of several moons that once existed around the planet. After colliding, the moons probably broke up into many particles, which survived as narrow and optically dense rings only in strictly confined zones of maximum stability.
The mechanism that confines the narrow rings is not well understood. Initially it was assumed that every narrow ring had a pair of nearby shepherd moons corralling it into shape. In 1986 'Voyager 2' discovered only one such shepherd pair (Cordelia and Ophelia) around the brightest ring (ε), though the faint ν would later be discovered shepherded between Portia and Rosalind.
1977Mar, 10
Astronomers discover the rings of Uranus.
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Events on 1977
- 23Mar
Watergate scandal
The first of The Nixon Interviews (12 will be recorded over four weeks) are videotaped with British journalist David Frost interviewing former United States President Richard Nixon about the Watergate scandal and the Nixon tapes. - 5Jul
Zulfikar Ali Bhutto
Military coup in Pakistan: Zulfikar Ali Bhutto, the first elected Prime Minister of Pakistan, is overthrown. - 4Aug
Jimmy Carter
U.S. President Jimmy Carter signs legislation creating the United States Department of Energy. - 15Aug
Search for extraterrestrial intelligence
The Big Ear, a radio telescope operated by Ohio State University as part of the SETI project, receives a radio signal from deep space; the event is named the "Wow! signal" from the notation made by a volunteer on the project. - 21Nov
God Save the Queen
Minister of Internal Affairs Allan Highet announces that the national anthems of New Zealand shall be the traditional anthem "God Save the Queen" and "God Defend New Zealand".