Asteroid impact

Recycling

Space science robots

Saturn and its rings in a mosaic of images taken by NASA's Voyager 1 in 1980 from a distance of 18 million kilometers. Features at least 350 kilometers across are visible. The rings are about 10,000 kilometers from edge to edge, but only a few meters thick. Photo: NASA/JPL

This 1981 image by Voyager 2 shows Saturn's thin F-ring bracketed by its two "shepherding satellites." These satellites (moons), which orbit near the ring, may provide both a source of debris in the ring, and a temporary resting place for debris accreted from the ring. The shepherds were then less than 1,800 kilometers apart. Photo: NASA/JPL

This computer-enhanced photo of Saturn was taken by Voyager 1 in 1980 from a distance of 1,570,000 kilometers. Brightness of the skinny F-ring (at outer edge) varies because of uneven distribution of rock and ice. Photo: NASA/JPL

Internal collisions and attack by meteorites are slowly grinding the ice and rock in the rings into ever-smaller bits. Eventually, the laws of physics require that these fragments will be slowed down by Saturn's atmosphere, until they drop out of orbit and fall to the frigid, gassy planet.

Do the math: Within 100 million years, the rings should disappear.

But if Saturn's rings are so durn temporary, why are they still around? Larry Esposito, a professor astrophysics and planetary science at the University of Colorado, has been gnawing at this issue for 20 years. Either the rings are quite young, he figures, or they are not disappearing as fast as expected.

Later this week, in a talk to the American Geophysical Union, Esposito will square the circle by explaining how the rings could be grinding themselves to death and still outlive the prediction of premature demise.

Cling to the ring
The answer, he says, is found in the same moons that apparently supplied the rock and ice in Saturn's rings in the first place. (Saturn has at least 31 moons, including many mini-moons, with diameters ranging from 20 to 200 kilometers.)

To understand the analysis, you have to recognize that, eventually, ring material must either drop out of orbit, or glom onto small moons near the rings. According to conventional wisdom, this accretion could only occur outside a certain orbital radius. But when Esposito's students factored in the relative sizes of the particles, they "showed that accretion was important" inside this radius, he says. "Small particles could stick on larger particles," largely due to gravity.

In that picture, the moons are frosted with a layer of light debris accreted from the rings.

The process of accretion would counteract the fragmentation of the moons. "While some are being broken up, other are recollecting," says Esposito.

Call it recycling: "In our previous calculations, we'd not considered recycling," Esposito says.

In the new view, moons are more than the source of rocky debris in the rings -- they are also a temporary warehouse for debris that's already served time out in the rings. Eventually, but over a much longer schedule, the ring junk should still be pulled out of orbit.

Recycling would explain how things can change -- and remain the same. "It's as if somebody walked down a street in New York City and saw a bunch of people," Esposito says. "If you went back a century later, you'd see more people. All the old individuals would be dead, but there would still be a bunch of people on the street."

A closer look at the unevenness of Saturn's F ring:
Photo: NASA's Cassini-Huygens-Operations

Bring back the ring
When translated to Saturn's rings, Esposito concludes that we have been watching a play with a rotating cast of characters. "I think I, and many others, have confused the lifetime of individual rings with the lifetime of the whole ring system."

The finding may also describe events on the fainter rings visible on Jupiter, Uranus and Neptune, other gas giant planets with many similarities to Saturn.

This baby's bound for Saturn! This Ultraviolet Imaging Spectrograph will examine Saturn's atmosphere and rings starting in 2004. Photo: NASA Cassini mission.

Esposito should know soon enough if he's right about Saturn. He is chief scientist on the Ultraviolet Imaging Spectrograph, one instrument on spacecraft Cassini that's due to enter Saturn orbit in July. By updating Voyager data, now more than 20 years old, Cassini should help prove whether a ring is a temporary thing.

-- David Tenenbaum

Bibliography

Laboratory for Atmospheric and Space Physics, University of Colorado