The source for Saturn's enigmatic G ring may be debris formed by collisions between micrometeorites and relatively large, icy particles that form an arc along the ring's inner edge, according to a series of images and data from NASA's Cassini spacecraft.

Cornell researcher Matt Hedman and colleagues report the finding in the Aug. 3 issue of the journal Science.

Saturn's faint, next-to-outermost G ring has long been a puzzle to researchers. Unlike its neighbors, the ring has no nearby moons to provide source material (as Saturn's moon Enceladus does for the E ring) or to shepherd its particles around their orbit (as moons Prometheus and Pandora do for the F ring). The nearest satellite to the G ring is the moon Mimas, which is over 15,000 kilometers away.

The G ring is unique in the Saturn system for the bright, narrow arc extending around about one-sixth of the ring's circumference, which makes the trip around Saturn seven times for every six passes by Mimas.

The arc has been observed at least six times since Cassini entered Saturn's orbit in 2004, indicating it is a relatively stable structure. Computer simulations show that a resonance effect with Mimas could be constraining particles to their place within the arc -- giving Mimas the same kind of gravitational shepherding function as that of Prometheus and Pandora on the F ring, but from a greater distance.

Plasma measurements by Cassini also show a depletion in charged particles near the arc, indicating the presence of additional, unseen mass -- approximately equivalent to that of a 100-meter-wide (328 feet) moonlet -- which absorbs the particles.

"From magnitude of absorption, we can tell how much matter is present," said Hedman, "and the mass we measured was more than we see in the images."

Since Cassini's cameras see light reflected by ring particles in proportion to the particles' surface area, the unseen mass is likely to be in larger chunks (with less surface area to deflect light for the same mass) -- rather than in fine grains.

When small meteorites collide with particles in the arc, Hedman said, the smaller particles of debris resulting from the collision are influenced not only by the gravitational pull of Mimas, but also by Saturn's magnetic field. "The big objects feel only gravity, but dust grains feel other forces," Hedman said. "The plasma is rotating faster than the ring particles, so the dust tends to get dragged out," to form the rest of the G ring.

Cassini is expected to pass near the G ring in 18 months for a closer look. In the meantime, the findings illustrate the complex, unique factors involved in each of Saturn's rings. "The dynamics are much richer than we imagined," Hedman said. "Each ring has its own character."

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission. The imaging team is based at the Space Science Institute, Boulder, Colo. Cassini's Magnetospheric Imaging Instrument is based at the Applied Physics Laboratory, Johns Hopkins University.