Saturn’s moons band together to corral its rings NASA / JPL-Caltech / Space Science Institute

Tidying up Saturn’s rings takes teamwork. One ring in particular owes its manicured appearance to no fewer than seven moons working together, according to new results released this week at the American Astronomical Society’s Division of Planetary Sciences meeting in Provo, Utah.

We already knew that Saturn’s biggest and brightest ring, the B ring, is kept in check by gravitational nudges from the large moon Mimas. Left alone, friction inside the ring would cause its icy particles to spill out both on the inner and outer edges. Eventually, the ring would broaden and disperse. But the presence of Mimas trims the B ring’s outer edge by pushing wayward particles back inside.

Scientists long thought that the A ring, which orbits further out, was hemmed in the same way by the smaller moon Janus. But this was giving Janus too much credit, says Radwan Tajeddine at Cornell University in New York. “I realized it can’t be holding the edge,” he says, because it isn’t massive enough.


Redirecting the flow

Tajeddine and his team created computer models based on data from Cassini, the probe that ended its 13-year mission to the Saturn system by plunging into the giant planet’s atmosphere in September.

Their modelling suggests that besides just Janus, a sextet of other moons also pitch in: Pan, Atlas, Prometheus, Pandora, Epimetheus and Mimas.

“If these moons weren’t working together, the A ring would have spread out over hundreds of millions of years,” says Cassini member Linda Spilker at the Jet Propulsion Lab in Pasadena, California, was not part of the research team. They accomplish this like pebbles in a streambed piling up and redirecting the stream’s flow, she says.

In Tajeddine’s model, small gravitational tugs from each moon create density waves in which thicker material piles up inside the ring at specific locations. Those pileups then absorb angular momentum from the particles in the ring, stealing enough that little Janus on the A ring’s outer edge can hold the line.

“It’s almost certainly right,” says Joe Spitale of the Planetary Science Institute in Tucson, Arizona, who also did not participate in the research. “It’s one of those things we’ve been staring at for years, and it should have been obvious.”