Evidence points to ring around Saturn moon: study

By Michael Kahn

LONDON (Reuters) - Saturn's second-largest moon Rhea may have a small ring around it -- the first time a moon has been found to have a ring, an international team of researchers reported on Thursday.

The findings from the Cassini spacecraft, published in the journal Science, will help scientists better understand how planets form, said Geraint Jones, who worked on the study while at the Max Planck Institute in Germany.

The four largest planets in the solar system -- Jupiter, Neptune, Saturn and Uranus -- all have rings and Earth probably had one as well at some point billions of years ago, before its moon even existed, Jones said in a telephone interview.

"All planets when they were forming probably had rings at various times," he said. "It is fascinating to find one possibly around a small body like Rhea, which is still with us today."

The Cassini spacecraft was launched in 1997 to examine Saturn. Last month scientists said evidence from the joint European-U.S. mission pointed to the presence of water beneath the frozen surface of another of Saturn's moons, Enceladus.

The researchers believe the latest data point to a ring because when Cassini recently flew past Rhea they detected a surprising lack of electrons surrounding the moon.

Rhea lies within a magnetized bubble that surrounds Saturn and contains trapped ions and electrons. Because of this, the researchers expected to see electrons trailing off closer to the surface as the moon absorbed them.

Instead, the electrons disappeared much sooner -- as if something was blocking them -- leaving a ring of debris as the most likely explanation, Jones said.

"There is evidence something is absorbing electrons around this moon," he said. "A debris disk around the moon is the simplest explanation we can come up with that fits with the data we have."

The ring probably formed when a smaller body smashed into Rhea and sent out a stream of debris that began orbiting the moon, said Iannis Dandouras, a planetary scientist at the National Centre for Scientific Research in France who also worked on the study.

The evidence of a ring is also interesting because it shows that the material was not incorporated into Rhea but was ejected into orbit, Dandouras added.

"It is the first time we found a system of planetary rings around a satellite of a planet," Dandouras said. "A planetary ring tells us the story about the historical formation of moons and planets."

(Reporting by Michael Kahn; Editing by Maggie Fox and Tim Pearce)