Moons work together to hold Saturn's largest ring in place

Rodiano Bonacci
Ottobre 18, 2017

In the most recent analysis of Cassini data, Cornell astronomers have determined that a three-decades-old assumption about the relationship between Saturn's moon Janus and the planet's A ring is just plain wrong. In 2009, for example, Saturn's largest ring was discovered lying beyond the inner rings; it was formed from debris from its moon Phoebe. During a news conference at the American Astronomical Society Division for Planetary Science meeting on October 16, the team working on Cassini's data shared their latest finding - Saturn's rings rain down methane on to its atmosphere. Without forces to hold the A ring in check, the ring would keep spreading out and ultimately disappear.

For years, astronomers have charted the relationship between the Saturnian rings and the planet's moons.

As revealed in high-definition by Cassini's cameras, Saturn's A ring features density waves, which look like the groves in a vinyl record.

There are hundreds of density waves spread over the A ring that are generated by different moon resonances. The waves are caused by the orbital resonances of nearby moons.

The push and pull exerted by the gravity of various smaller moons works to slow the momentum of the material in the A ring, so much so that it enhances the orbital resonance of Janus, whose influence on the A ring form's the ring's outer edge. Without the help of its neighbors, Janus alone couldn't contain the ring. "Janus has been getting all of the credit for stopping the A ring, which has been unfair to the other moons".

"This was exactly the sort of information we had hoped the Cassini mission would provide, and by doing so it has allowed us to solve this puzzle", says senior author Joe Burns, a professor of engineering and of astronomy at the university.

"That's the novelty of this idea", said Tajeddine.

"The density waves created by moons are handsome to look at, but they actually participate in confining the ring", said Tajeddine.

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