The recent discovery of new rings around Saturn has prompted speculation that the Cassini mission will find previously unidentified moons orbiting the planet.

"Just like the old maxim that says 'where there's smoke there's fire', at Saturn, where there's a new ring there's bound to be a moon," said Jeff Cuzzi, Cassini interdisciplinary scientist at NASA's Ames Research Centre.

In mid-September, Cassini moved into Saturn's shadow and viewed the rings, backlit by the sun. As we reported, the researchers identified a new ring that shares its orbit with the moons Janus and Epimetheus, just inside the E and G rings.

A week later, another ring was discovered overlying the orbit of the moon Pallene. A third and fourth ring were also identified by Cassini in a gap in the Saturnian ring system. These latest rings were not imaged by the voyager spacecraft.

"We are hot on the trail of these possible elusive moonlets," said Joe Burn, Cassini imaging scientist at Cornell University.

"Finding the moons and learning about their interactions with the rings will help us understand how the moons formed and perhaps how the Saturn system formed."

The images taken while Saturn sheltered Cassini from the sun also revealed differences in the composition of the rings that have surprised astronomers.

The infrared images reveal striking colour differences in the rings, indicating variations in composition and in microscopic particles between the various rings. The main rings show a neutral color, while the C ring is reddish, and the D and E rings are quite blue.

Researchers are not sure what causes the difference in colour, but speculate that it could be to do with the size of the particles or the composition of the ring material. NASA says there are hints that materials other than the usual water-ice might be identified in the ring system. ®