To keep Saturn’s A Ring contained, its Moons Stand United

This image from NASA’s Cassini mission clearly show the ring’s density waves created by the small moons. The waves look like the grooves in a vinyl record. Credit: NASA

For 3 decades, astronomers thought that only Saturn’s moon Janus confined the planet’s A ring – the largest and farthest of the visible rings. But after poring over NASA’s Cassini mission data, Cornell astronomers now conclude that the teamwork of seven moons keeps this ring corralled. Without forces to hold the A ring in check, the ring would keep spreading out and ultimately disappear. “Cassini provided detail on the mass of Saturn’s moons and the physical characteristics of the rings, so mathematically speaking, we concluded that the moon Janus alone cannot keep the rings from spreading out,” said Radwan Tajeddine, a research associate in astronomy and lead author of the new research.

The scientists discovered that confinement of the A ring is shared among the moons Pan, Atlas, Prometheus, Pandora, Epimetheus, Mimas and Janus. “All of these moons work as a group to contain the ring. Together they are strong. United they stand,” said Tajeddine. Cassini, which crashed into Saturn Sept. 15 at the mission’s end, provided valuable data and detailed images of the planet’s rings. The A ring looks similar to a vinyl record; it has “density waves” that resemble a record’s grooves that are created by what astronomers call moon resonances. These resonance markers enabled scientists to deduce that the moons’ gravitational influence help to slow and reduce the spreading ring’s momentum.

There are hundreds of density waves spread over the A ring that are generated by different moon resonances. Tajeddine compares it to tug of war with many knots along the gravitational rope. All of these gravitational pushes by these moons slow the ring down and pull momentum from it. So much momentum is lost by the time the ring gets to Janus that the forces create the edge of the A ring.

Tajeddine said scientists are still not sure how the rings formed, but the mechanism of their confinement is finally understood. “That’s the novelty of this idea. No one imagined that rings were held by shared responsibility,” he said.
http://news.cornell.edu/stories/2017/10/keep-saturns-ring-contained-its-moons-stand-united