SAN FRANCISCO, CALIFORNIA— Saturn's rings, like any aging beauty, are keeping their age mysterious. Before the first spacecraft reached Saturn, planetary scientists assumed its rings formed along with the planet 4.5 billion years ago. But the Voyager spacecraft flying by in the early 1980s sent back evidence that the rings could be just a few hundred million years old. Now, the still-orbiting Cassini spacecraft is getting a longer look and finding signs of old age. “Right now, we still don't know whether it's old or new,” concludes ring specialist Joseph Burns of Cornell University. But if the rings are indeed old, some researchers think they see ways that the rings make themselves look forever young.
The latest sign of the rings' antiquity is a tendency to put on weight—at least in the eyes of planetary scientists. Last month at the fall meeting of the American Geophysical Union here, Larry W. Esposito of the University of Colorado (UC), Boulder, and colleagues discussed results from Cassini's Ultraviolet Imaging Spectrograph (UVIS). In one mode of operation, UVIS can precisely and quickly record the brightness of a star as it appears to pass behind the rings, twinkling as the metersize particles that make up the rings block varying amounts of starlight. Voyager recorded a single such stellar occultation, which Esposito used to estimate the mass of the rings. But in the dense B ring, Cassini's occultation results have yielded at least three times the mass of Voyager's. “I hit myself on the head,” said Esposito. “‘I must have made a mistake with Voyager,’ I thought.”
Once Cassini observed a number of occultations, the reason for the discrepancy became clear: Mass in the rings is clumped, not spread uniformly. As a result, starlight passes through the rings mostly by way of nearly empty gaps, and some angles offer clearer passage than others. Esposito compares it to a car's headlights viewed through a picket fence, shining brightly viewed head-on but dimly when viewed at a steep angle. Because Voyager was looking through the rings at a higher angle, it caught more starlight and underestimated the mass.
Given the new big ring mass, planetary scientists Glen Stewart, Stuart Robbins, and Joshua Colwell, all of UC, argue that the B ring, at least, could not have formed in recent eons. A moon massive enough to form the ring is unlikely to have broken up during the past 4 billion years, they argue; collisions able to do that kind of damage were much more common during the solar system's early days, when lots of big lingering debris was still winging around.
Even if Saturn's rings formed long ago, Esposito sees reasons they might appear young. The ice in the rings' particles seems too pure to have spent eons accumulating dark, dirty meteoritic material, and moonlets in the rings are too small to have survived that much time under cometary bombardment. But a larger mass of ring ice could have diluted the pollution, Esposito argues, and the moonlets might be recycling themselves—breaking up and reassembling by agglomeration.
Researchers are comfortable with a heftier B ring but not necessarily with Esposito's take on the implications. “There's a bunch of caveats in all of this,” says ring dynamicist and Cassini camera principal investigator Carolyn Porco of the Space Science Institute in Boulder, Colorado. “Very little in this area is definite. Each part of the rings may have a different age.” The rings—like scientists' understanding of them—may be a work in progress.