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Though no one is sure how the rings formed, some of the material is almost certainly the remains of small pulverized moons that were destroyed either by a cataclysmic meteor hit or when they wandered too close to a gravitational danger zone known as the Roche limit: the altitude above a planet at which the difference in gravity between the end of an object closest to the planet and the end farthest from the planet is great enough to pull the object apart while not pulling the remains out of orbit. Instead, the rubble disperses around the planet. Photographs of the debris could help confirm this phenomenon and could even turn up smaller, still undiscovered moons hiding within the rings.
Scientists already know of some moonlets that orbit inside the rings, sweeping areas clean of debris and accounting for at least two conspicuous gaps. Other tiny moons move along the outer rim of rings; these are the so-called shepherding moons that groom the ring edges and keep them sharp.
If Cassini-Huygens traveled all the way to Saturn and returned nothing but data on the planet and its rings, the mission would probably still be judged a success. Yet the true scientific goods will come when the spacecraft trains its instruments on the swirl of Saturnian moons. It would be nearly impossible for one ship to visit all 31 known satellites in Saturn's litter, so NASA has selected nine of them, both for their scientific promise and their comparatively convenient locations. The exotic names of the chosen moons--Phoebe, Titan, Iapetus, Enceladus, Mimas, Tethys, Hyperion, Dione and Rhea--hint at the exotic science that awaits.
Iapetus, for example, is a two-toned world, its leading edge dark, its trailing edge white. There are many theories advanced for this--including the possibility that there are hemisphere-wide volcanoes or that the moon is picking up dust as it moves through its orbit, staining its face and leaving the other side clean. "We have all kinds of questions," says Cassini physicist Larry Soderblum. "Were there volcanoes? Were there oceans of some mystical hydrocarbon that froze?"
Enceladus holds mysteries of its own. A bright white world with a relatively smooth face, it appears to have been repeatedly resurfaced by some kind of underground slurry or perhaps by ice volcanoes. In some places, once deep crevasses have been largely filled in and craters have been cut neatly in half, leaving one side deep and raw and the other covered, as if by snowdrifts. The area of the Saturnian ring that follows in the wake of Enceladus is slightly thicker than the rest, as if the moon were pumping out some kind of frozen exhaust, leaving a plume in its wake like the smoke from a steamship.
Other questions should be answered when Cassini flies by Hyperion, a tumbling moon that appears to have been knocked off its pins by a collision eons ago and has never regained its footing; and Tethys, a moon that bears such a massive impact scar that only the barest geological margin keeps it from shattering altogether.