Uranus Moons Named Caliban and Sycorax

May 01, 1998

ITHACA, N.Y. -- Cornell University astronomer Philip Nicholson and his colleagues have proposed names for the two recently discovered moons of the planet Uranus. They are Caliban and Sycorax, both characters in Shakespeare's play "The Tempest." The names are likely to be approved by the International Astronomical Union.

The astronomers detail their discovery of the two moons in a report in the April 30 issue of the magazine Nature. They confirm that Caliban and Sycorax are the faintest planetary moons yet imaged by ground-based telescopes. The discovery of the two moons was reported on Oct. 31 by Nicholson and colleagues Joseph Burns, professor of engineering and astronomy at Cornell, Brett Gladman of the Canadian Institute for Theoretical Physics at the University of Toronto, and J.J. Kavelaars of McMaster University, Canada.

The team used light-sensitive semiconductors, called charge-coupled devices, attached to the 5-meter Hale telescope on Mount Palomar, Calif., to track the irregular, or non-circular, orbits of the two moons. Regular satellites orbit near a planet's equatorial plane. The two moons are the first irregular satellites discovered around Uranus.

Both Caliban and Sycorax, the astronomers write, are unusually red in color, which suggests a link with the recently discovered populations of comet-like bodies called trans-Neptunian objects, which orbit the sun beyond the orbit of Neptune, and Centaurs, which cross the orbits of the outer planets.

Both trans-Neptunians and Centaurs, say the researchers, have a wide range of reddish colors, perhaps resulting from the bombardment of their organic-rich icy surfaces. Nicholson says this bombardment could be from cosmic rays or from the sun's ultraviolet radiation. The methane on the moons' surfaces, he says, would be "cooked" by the radiation into hydrocarbons, showing up as a dark red through a telescope's filters.

The two moons, say the researchers, are presumed to have been captured by Uranus early in the history of the solar system. "My guess is that the moons were once trans-Neptunians and they became Centaurs and were captured by Uranus and became satellites," says Nicholson. Since the newly discovered moons are likely to have been captured by Uranus soon after its formation, the Nature article notes, "their physical properties may provide clues to conditions in the early solar system."

The process of capture could have taken two forms, Nicholson says. The moons could have been trapped by Uranus gravity as they came close to the planet. Another theory, he says, is that in the early days of the solar system Uranus might have been surrounded by a gaseous nebula that would have caused a drag on the objects' movement as they came close to the planet.

Nicholson estimates that Caliban, the smaller of the two moons, has a diameter of 60 kilometers (37 miles) and is orbiting Uranus at an average distance of about 7.2 million kilometers (4.5 million miles), taking 1.6 years to complete one revolution. Sycorax, he estimates, has a diameter of 120 kilometers (74.5 miles) and takes 3.5 years to complete one orbit of Uranus at a mean distance of about 12.2 million kilometers (7.5 million miles) from the planet. However, he says, Sycorax has a much more elliptical orbit than Caliban, bringing it as close as 6 million kilometers (3.7 million miles) to the planet.

The composition of the two moons, says Nicholson, "is probably a plum-pudding mixture of rocks and ice."

All 15 previously known satellites of Uranus lie on fairly evenly spaced, nearly circular orbits. Most recently Voyager 2, in 1985 and 1986, discovered 10 small, dark inner moons.

Jupiter has eight known irregular satellites, of which the last, Leda, was discovered in 1974. Saturn has one, Phoebe, discovered in 1898, and Neptune has one, Nereid, discovered in 1949.

To see images of the two newly discovered moons of Uranus, go to Gladman's page on the World Wide Web at http://www.cita.utoronto.ca/~gladman/uranus.html.

Cornell University

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