Rings around Rhea: Possible first time discovery

William Atkins
Friday, 07 March 2008 18:55

Science - Space

Scientists working with NASA’s Cassini spacecraft have found what appears to be a ring system around Saturn’s second largest moon, Rhea. If this tenuous ring system is verified, it will be the first time that a ring has been found around a planetary moon.         

At least one ring, including a wide debris disk, appears to have been detected by instruments onboard the Cassini probe, such as the Magnetospheric Imaging Instrument, that are designed to detect particles orbiting about Saturn and its moons.

The discovery was announced on March 6, 2008 by lead author in the paper, American planetary scientist Geraint Jones, a scientist at the Mullard Space Science Laboratory, University College, in London, England.

Jones states, “Until now, only planets were known to have rings, but now Rhea seems to have some family ties to its ringed parent Saturn.” [Johns Hopkins University: “Saturn’s Moon Rhea May Also Have Rings”]
The rings were discovered when short but distinct decreases in electron frequency were found on data produced by Cassini’s Magnetospheric Imaging Instrument.

But, the Jones team is not quite sure if the ring is real or not.

Jones states, "We can't say definitely that there are rings surrounding Rhea. But the data we have are a real puzzle, and the only reasonable explanation we've been able to come up with is the debris disk proposal." [National Geographic: “Saturn Moon May Have Rings--A First”]

Further observations from Cassini’s imaging instruments will help to prove the existence of the ring system.

The moon Rhea is about 950 miles (1,530 kilometers) in diameter, composed one-fourth of rocky material and three-fourths of water ice. Its surface is heavily cratered.

The debris disk appears to extend for several thousands of miles. Like most debris disks, this one probably consists of particles sized from small pebbles to large boulders. The Jones team states that a dust cloud might extend from the surface of the moon to out to several thousands of miles.

The Jones team used data taken by Cassini when it made a close flyby of Rhea in November 2005. The Cassini-Huygens mission is a joint project of NASA, the European Space Agency, and the Italian Space Agency.

What formed the rings? Please read on.