Four hundred years after they were discovered, Saturn's breathtaking rings remain a mystery.

by Dr Tony Phillips

Galileo Galilei was accustomed to extraordinary discoveries. Using his primitive telescope he had found new worlds orbiting Jupiter, watched planet-sized spots crossing the Sun, and explored craters on the Moon. But when Galileo turned his telescope toward Saturn in 1610, even he was amazed.

The planet looked nothing like others in the solar system. Through 17th century optics, Saturn appeared to be one bright star closely flanked by two dimmer ones - a blurry suggestion of the planet's magnificent rings.

What Galileo did next was nearly as unusual as Saturn itself.

He wanted to tell everyone what he had seen, but he also wanted to keep his work secret while he studied the puzzling planet. So, he published his discovery in code: smais mr milmep oet ale umibunen ugttauir as. Unscrambled, the anagram means "I have observed the highest planet tri-form."

Nowadays anyone with a department store telescope can get a better view of Saturn's rings than Galileo did. Otherwise, matters stand much as they did four hundred years ago. First-time observers of the planet still step back from their telescopes speechless. And scientists are still puzzled.

"After all this time we're still not sure about the origin of Saturn's rings," says Jeff Cuzzi, a planetary scientist at the NASA Ames Research Centre. Astronomers once thought that Saturn's rings formed when Saturn did: 4.8 billion years ago as the Sun and planets coalesced from a swirling cloud of interstellar gas. "But lately," Cuzzi says, "there's a growing awareness that Saturn's rings can't be so old."

Credit: Humboldt State University [more] Saturn's rings might have formed only a few hundred million years ago when dinosaurs and their cousins roamed our planet.

Cuzzi speculates that some hundreds of millions of years ago - a time when the earliest dinosaurs roamed our planet - Saturn had no bright rings. Then, he says, something unlikely happened: "A moon-sized object from the outer solar system might have flown nearby Saturn where tidal forces ripped it apart. Or maybe an asteroid smashed one of Saturn's existing moons." The debris encircled the planet and formed the rings we see today.

Saturn's ring particles range in size from microscopic dust to barn-sized boulders. If you assembled them all in one place, notes Cuzzi, you would have enough material to make an icy satellite one or two hundred kilometres wide - much like Saturn's present-day moon Mimas.

The debris layer is extraordinarily thin, he marvels. "Saturn's rings are 250,000 km wide, but only a few tens of meters thick. A sheet of paper the size of San Francisco would have about the same ratio of width to depth." Indeed, if you made a 1-meter-wide scale model of Saturn, the rings would be 10,000 times thinner than a razor blade.

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.

Saturn's rings are very thin. Astronomers using the Hubble Space Telescope captured this image of the rings edge-on in 1995. Star-like objects in the ring plane are icy satellites.

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 dynamic 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.

Saturn's 200 km-wide moon Mimas, also known as the "Death Star" satellite because of its distinctive impact crater, is about as massive as 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.

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!