Saturn's Rings

"Some of the waves have a spiral shape, like the spiral arms of galaxies," says Cuzzi. To an astronaut floating among the rings, such waves would appear to be gentle swells, a few kilometres high and hundreds of kilometres wide. They move around the rings every few days or weeks. "We understand these spiral waves," he added. They're triggered by gravitational tugs from Saturn's moons - the same ones that are sapping the rings' angular momentum.

Other structures, like spokes and irregular ripples, are puzzling. Some of them might be signs of space rocks plunging through the ring system. Others might be spawned by tiny moonlets, as yet undiscovered, plowing through Saturn's rings. "Cassini, which will orbit Saturn for years, should provide some answers," he says.

Voyager 2 spacecraft images of "spokes" and other irregular features in Saturn's rings.

In many science fiction tales, alien visitors are amazed by Saturn as if there were no ringed planets back in their own solar system. According to Cuzzi, Saturn's rings might be rare indeed. "If they are as short-lived as we think, we're lucky to be here at just the right time to see them."

Actually, other giant planets in our solar system do have rings, but they are very dark and millions of times less massive than the rings of Saturn. Jupiter's rings are made of bits of dust that fly off Jupiter's moons when they are struck by meteorites. No one is sure what made the black rings of Neptune and Uranus, although Cuzzi notes they could be debris from kilometre-sized moonlets that were struck by asteroids.

In another few hundred million years, if Cuzzi is right, Saturn's rings will sag inward and our solar system will become a little more ordinary. Perhaps by then star-faring humans will have seen countless ringed planets elsewhere in the Galaxy and won't care much what happens to Saturn. On the other hand, maybe Saturn's rings really are a Galactic wonder, and super-engineers of the distant future will take measures to preserve them.

No one knows.

We can only be sure that Saturn's rings are lovely now. And if they are indeed fleeting, as such ages are reckoned for stars and planets, their short life makes them even more wonderful. Don't miss them!