Saturn's G ring, a faint band of material near the outer bounds of the planet's famed ring system, hosts a bright arc about 90,000 miles (150,000 kilometers) long. The arc, or partial ring, which stretches through about a sixth of the G ring's length, is believed to provide the rest of the ring with dust and ice, but its evolution has remained a mystery.

Recent images from the Cassini spacecraft (at left) point to a moonlet embedded in the G ring. The moonlet, the Cassini team speculates, might help to repopulate the arc, and then the ring as a whole, with material as it suffers collisions with meteoroids or other small bodies within the ring.

The moonlet, which showed up as a bright speck in the ring's arc, appears to be approximately 0.3 mile (0.5 kilometer) in diameter, based on its reflectance. The finding was reported today in a circular from the International Astronomical Union.

"Before Cassini, the G ring was the only dusty ring that was not clearly associated with a known moon, which made it odd," Matthew Hedman, an astronomy research associate at Cornell University and a member of the Cassini imaging team, said in a statement. Data from the same mission in 2007 indicated that the ring-feeding arc could stem from an orbital relationship with Mimas, one of Saturn's proper moons, but the arc's composition remained a puzzle. "The discovery of this moonlet, together with other Cassini data," Hedman said, "should help us make sense of this previously mysterious ring."

Photo credit: NASA/JPL/Space Science Institute