For 3 decades, Saturn's broadest, brightest, and most massive ring has also been its most mysterious. The B ring is etched with darker "grooves," giving it the appearance of an old-time vinyl phonograph record. But no one could see what might be doing the etching. Now researchers see signs that the grooves are due not to outside forces but to a natural tendency of the densest parts of rings to clump into denser, brighter bands. Similar clumping is probably at work shaping galaxies and nascent planetary systems.
Astronomers have long known about processes that shaped Saturn's other rings. Imaging by the Voyager 1 and 2 spacecrafts in the late 1970s had shown how the gravity of distant moons and nearby moonlets can sculpt the planet's vanishingly thin swarm of trillions of orbiting icy bits into four main rings—and within them thousands of ringlets, ring gaps, and spiraling waves. But the B ring remained an enigma. No one could find any orbiting bodies that would gravitationally cut grooves across the 7.5-million-kilometer breadth of the B ring, which, like the rest of the rings, is at most a few tens of meters thick.
Part of the problem was that astronomers didn't have enough data on the B ring. Previous studies focused on a mere few days of observations by the Voyagers, but Joseph Spitale and Carolyn Porco took a much longer look. In a paper published this week in The Astronomical Journal, the pair of ring dynamicists at the Space Science Institute in Boulder, Colorado, studied 4 years of observations from the orbiting Cassini spacecraft. They identified three subtle rhythmic pulsations in the position of the B ring's outer edge. The oscillations carry the edge in and out by about 75 kilometers with the same timing, predicted a long-hypothesized mechanism called "overstability."
Overstability can occur in the densely packed B ring, and only there, because particles there are so close together that they behave collectively more like a liquid than like a gas, says Spitale. Random disturbances nudging them even closer can then set off waves of densely packed ring particles with the same frequencies that Spitale and Porco see at the ring's edge. Gentle collisions of ring particles then amplify these waves until they are powerful enough to show up as pulsations in the B-ring edge. The same spontaneously generated waves are likely creating the grooved appearance seen across the B ring, says Spitale. The process could also be shaping spiral galaxies and disks of dust and gas-forming planets around other stars, although making the leap to such grand scales would be difficult.
The Cassini discovery "shows that even in a simple system you can get remarkable behavior," says physicist Peter Goldreich of the California Institute of Technology in Pasadena. "This is going to lead to a lot more understanding about rings."