Saturn's moon Rhea could be a mini version of its ringed parent, and the first moon known to have rings of its own.
Scientists detected hints of the rings when the Cassini spacecraft flew by the moon, Saturn's second largest, in November 2005.
Surprisingly, instruments aboard Cassini measured an absence of electrons around the moon where astronomers expected the charged particles to swarm.
"This showed that there was something unique going on," said Geraint Jones, a Cassini scientist. "We haven't seen anything like this at any of the other moons. The only thing we can come up with that fits what we see is that maybe there is some debris around Rhea. If it is correct this would be the first moon where we have evidence of rings."
Since Rhea lies within Saturn's magnetosphere, which traps ions and electrons, scientists expected the moon to be awash in these particles. Instead, they measured a gap in electrons in a swath of space surrounding Rhea.
A set of rings could explain the disappearing electrons because the material making up the rings — most likely chunks of ice up to less than an inch to over three feet in diameter — would absorb electrons.
Rhea, named after the classical Greek titan goddess, the mother of Zeus, Hades, Hera and other Olympian deities, is a barren, icy world covered in craters. Its diameter is a little less than half that of our moon.
So far, Cassini has not been able to see the rings. If the spacecraft's mission is extended, the researchers hope it might glimpse them on a future close flyby of Rhea.
Saturn has dozens of moons; to date, 52 of them have been named.
Before now, the Saturnian moons Enceladus, Titan and Iapetus have stolen most of the fame, Jones said. "Rhea seemed to be one of the least interesting of the moons, but now it looks like it might be interesting for its own special reasons," he said.