Saturn Moon May Have Rings -- A First

Richard A. Lovett
for National Geographic News
March 6, 2008
Rhea, Saturn's second largest moon, may be the first known moon with its own small system of rings, astronomers announced.

Although no one has actually seen Rhea's rings, scientists say that odd data collected during a 2005 flyby of the Cassini spacecraft provides strong clues.

"We can't say definitely that there are rings surrounding Rhea," said planetary scientist Geraint Jones.

"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."

Jones, of University College London, led a study on the Cassini data to be published in tomorrow's issue of the journal Science.

Missing Electrons

Rhea sweeps through Saturn's magnetic field, which traps charged particles in a bubble zone that includes the tiny moon's orbit.

Jones' team had expected to see a dip in the amount of electrons in Rhea's wake as the surface of the 950-mile-wide (1,529-kilometer-wide) moon intercepted the particles.

The researchers also expected Cassini to find a thin halo of dust kicked up by the micrometeorites that constantly bombard Rhea's surface.

The team did detect these features, but they also found a surprise: The amount of charged particles decreased along a region extending outward on each side to about eight times the moon's diameter.

The electron decrease was far too much to be accounted for by the dust attributed to meteorite impacts.

"About 70 percent of all the electrons had gone before we entered the wake itself," Jones said. "This was a real puzzle."

The idea that the previously unremarkable moon might have very thin rings has many scientists excited but cautious.

"It's an indirect measurement," said John Spencer, a planetary scientist at the Southwest Research Institute in Boulder, Colorado, who was not involved in the study.

"So it still has to be confirmed by actually taking pictures. But it's very interesting."

Study leader Jones agrees that pictures will be important. Attempts have been made to capture an image by looking backward at Rhea when its orbit brings it closer to the sun, he said.

The hope was that the ring halo would be backlighted like dust particles on a dirty windshield, but the cameras were unable to see anything.

That doesn't mean the rings aren't there, Jones stressed.

Rings could still block electrons if the largest particles were each roughly the size of a tennis ball and were scattered a kilometer (0.6 mile) apart. Such a system would be very difficult to see in photographs.

The source of the rings could be particles blasted high into space by meteor impacts on Rhea's surface, Jones added.

Under the right conditions, these particles would remain in orbit, forming rings that computer models show to be stable for millions of years, he said.

(Related news: "Saturn's Rings as Old as Solar System, Study Says" [December 13, 2007].)

Wacky Family

The most exciting part of the find for many astronomers is that it's yet another oddity from Saturn's system of unusual moons.

"No two are alike," said Carolyn Porco of the Space Science Institute in Boulder, Colorado. Porco is leader of the Cassini Imaging Team but was not directly involved in this study.

Each moon "is a world unto itself, with something special to teach us," she wrote in an email. (Read: "Saturn's 'Walnut Moon' Mystery Solved" [July 18, 2007].)

"[W]e might as well be exploring an alien planetary system orbiting a distant star. … The sense of adventure and novelty is no less."

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