First Ring Found Around a Moon
Saturn's Satellite Rhea Has a Rocky and Dusty Debris Ring
The first lunar ring has been found around a moon of the planet best known for its rings: Saturn.
At one time it was believed that Saturn was the only planet with a ring. After the Voyager missions we learned that all the giant planets have rings, although they are not as large as Saturn's. Now the Cassinin spacecraft has discovered that one of Saturn's moons, Rhea, has a ring around it. This is the first discovery of a ring around a moon.
"Many years ago we thought Saturn was the only planet with rings. Now we may have a moon of Saturn that is a miniature version of its even more elaborately decorated parent," says Candy Hansen, a Cassini scientist from NASA's Jet Propulsion Lab in California.
How Rhea's Ring Was Discovered
When the Cassini spacecraft flew past Rhea, it sampled the dust surrounding the moon directly. Cassini detected a gradual drop in the number of electrons on either side of Rhea. It also detected sharp and brief drops in electrons on each side of the moon. From this it was determined that material in these locations was shielding Cassini from the normal flow of electrons. The gradual drop would be explained by a cloud of dust and debris around the moon and the sharp drop-off corresponds with a rocky ring blocking the electrons. The rings of Uranus were discovered by similar methods.
"Seeing almost the same signatures on either side of Rhea was the clincher," says Geraint Jones, a Cassini scientist at the Mullard Space Science Laboratory in London. "After ruling out many other possibilities, we said these are most likely rings. No one was expecting rings around a moon."
The Composition and Structure of the Lunar Ring
Rhea contains a broad disk of debris and at least one ring around it. The moon is approximately 950 miles in diameter, with the debris disk several thousand miles wide. The rocky particles in the ring and disk are estimated to be in size from pebbles to boulders. There is a possibility that an additional dust cloud extends 3,000 miles from Rhea's center, which would be eight times the radius of the moon.
How the Ring Got There
After the discovery, numerical simulations showed that, indeed, Rhea and Saturn's gravitational fields would allow for any rings that formed around Rhea to remain in place for a long time. A collision is one way that the ring may have been formed. An asteroid of comet could have smashed into the moon in the distant past and the resulting debris is what is now orbiting Rhea. Another of Saturn's moons, Mimas, shows the scar of a very large impact in the past. In fact, the moon Mimas was the pockmarked inspiration for the Death Star in the movie Star Wars.
The Future of Lunar Ring Studies
Since the discovery of Rhea's ring, more observations are being planned. "Like finding planets around other stars, and moons around asteroids, these findings are opening a new field of rings around moons," says Norbert Krupp, a Cassinin scientist from the Max Planck Institute.
Source: Jet Propulsion Laboratory
