Over the last year, NASA's Cassini spacecraft has repeatedly spotted a debris pocket bulging out of one of Saturn's rings. This bright blob of material sits on the inner edge of the tenuous G ring, fattening one segment of it into a feature that planetary scientists call a ring arc — the first ever seen around Saturn.
Ring arcs are rare, since orbital dynamics usually force particles to spread out into a nearly uniform disk. The Voyager 2 spacecraft imaged a set of arcs bulging from one of Neptune's rings, and mission scientists concluded that they were held there by the gravitational shepherding of the small moon Galatea.Cassini scientists suspect that Saturn's newly discovered arc is confined by a similar process involving the moon Mimas. The small icy moon is in a 6:7 resonance with the arc, meaning it completes six orbits of Saturn in the time it takes the arc to do seven. Such a gravitational set-up creates pockets inside the ring where clumps of rock and ice can build up.
But like any unsightly bulge, Saturn's ring arc needed more than just a physical predisposition. It also needed material. Matthew Hedman (Cornell University), who is leading the work on the ring arc, speculates that a recent collision between two moonlets could have broken up large chunks of rock and ice, which were then scooped into the gravitational pocket."There are six spots around the ring where the arc could have formed," says Hedman. "We do not know why only one of these regions contains the arc. We will have to make more observations and do some detailed modeling to know for sure. Fortunately, Cassini provides an unprecedented opportunity to study this system in detail."
Curiously, Saturn's ring arc might explain the strange appearance of the G ring, one of the planet's faintest, narrowest rings. The Cassini team speculates that tiny grains of dust and ice might be leaking away from the bright, thick ring arc, smearing out to form the diffuse ring.
"The G ring is the least understood ring in the Saturn system," says Hedman. "Once we saw the arc, we at least may have a reason why the G ring is where it is and why it is narrow."
The ring arc is one of several new features that the Cassini spacecraft has spotted in Saturn's rings over the first two years of its orbit. In March, the Cassini team reported that they had found evidence for "moonlets" in the rings: 100-meter (300-foot) chunks of ice and dust that constitute a new class of ring objects. The vast majority of Saturn's ring particles are flakes of ice just a few centimeters across, and a few large moons many kilometers across. Researchers think these new mini-moons might be the leftovers of an ancient moon that broke to pieces, producing the material for Saturn's rings. It is possible that one of these mini-moons is responsible for the clod of material in the G ring.