Saturn's Rings

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.

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.