March 7, 2008 -- Of the many surprises unearthed at Saturn during the ongoing survey by the Cassini science probe, few rival the delightful find that Rhea, one of the ringed planet's larger moons, has an entourage in orbit all its own.
Saturn is not the only planet to have rings. All the large, gaseous worlds of the outer solar system have them, though none are as bold and intricate as Saturn's bands. But for the first time, scientists have evidence that rings of debris, dust and other small particles can gather in orbit around a moon, not just a planet.
The Cassini science team took two years to publicize their finding, not sure they'd be taken seriously if they didn't first eliminate all other possible explanations for the unusual data.
The spacecraft's fields and particles instruments were busy tallying up electrons during a 2005 pass by Rhea when a most unexpected thing happened: the data dropped out, not just once, but three times. And then came the clincher: three more matching dropouts on the other side of the moon.
"When you see something symmetric, it's like 'OK, this is something we need to take seriously,'" said Candice Hansen-Koharcheck, one of dozens of Cassini scientists tapped to come up with alternative explanations for the data.
"They took a long time analyzing this," she added. "They know if they said there are rings around Rhea people would say, 'Come on...that's too far-fetched.'"
The ring particles apparently hid the electrons from Cassini's view, like a game of celestial peekaboo. Other theories, such as a previously undetected lunar atmosphere, were dismissed.
"After ruling out many other possibilities, we said these are most likely rings. No one was expecting rings around a moon," said Cassini scientist Geraint Jones, the lead author on a paper about the discovery appearing in this week's issue of Science.
The reason the ring didn't show up in Cassini's photographs is because the particles are too big to reflect like mist when backlit by the sun and too small for direct imagery.
"We think it is mostly rubble, about 1 centimeter in size," Hansen-Kohascheck said. "If you were standing on Rhea, you probably could see it."
The disk measures several thousand miles from end to end. An additional dust cloud may extend up to 3,000 miles, or 5,900 kilometers, from the center of the moon.
Scientists believe the ring may be remnants of an asteroid or comet that crashed into the moon eons ago. The smash-up would have left a cloud of gas and solid particles around Rhea. When the gas dissipated, the particles remained.
Scientists ran dozens of computer simulations to verify that Rhea, Saturn's second-largest moon after Titan, could hang on to a ring, considering the physics of its size and location, without the particles migrating away.
The moon is about 950 miles (1,500 km) in diameter.
The discovery has scientists wondering if other moons in the solar system might have rings too. "It's worth taking a look," said Hansen-Kohascheck. "We haven't seen hints of rings around the others, but we weren't looking for them either."
Related Links: