Young and restless

Cuzzi says there are two reasons to believe the rings are young:

First, they are bright and shiny like something new. It's no joke, he assures. The wide-spanning rings sweep up space dust (bits of debris from comets and asteroids) as Saturn orbits the Sun. Rings much older than a few hundred million years would be darkened by accumulated dust.

"The fact that they're bright suggests they're young," he says.

Second, small moons that orbit through the outermost regions of the ring system are gaining angular momentum at the expense of the rings.

"During the next few hundred million years," explains Cuzzi, "the outer half of the rings will fall toward the planet, and the little moons -- called shepherd satellites -- will be flung away. This is a young dynamical system."

The first argument (shiny rings) is less certain than the second (angular momentum), he cautions, "because we're not sure there's enough dust at the orbit of Saturn to pollute and blacken the rings."

NASA's Cassini spacecraft will measure the dust population when it reaches Saturn in 2004. Then, perhaps, there will be no doubt.

Other mysteries

Cuzzi hopes Cassini will solve other ring-mysteries, too.

"In the early '80's," he recalls, "the Voyager spacecraft visited Saturn and took close-up pictures that revealed many strange things in the rings, including spokes, braids and waves.

"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 kilometers high and hundreds of kilometers 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.

Lucky us

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 kilometer-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!

See Saturn, and its rings, disappear behind the Moon tonight.