Saturn's A ring contained by not one, but seven moons

Until recently, astronomers assumed Saturn's A ring was contained by a single moon, Janus, the gas giant's largest. But new research shows the A ring, the outermost of Saturn's large, bright rings, is confined by seven moons.

The orbital resonances of the seven moons, new research shows, prevents the A ring from diffusing into nothingness. Without the seven moons, the ring's material would spread out and dissipate entirely over time.

The revelation was made possible by observations recorded by the Cassini probe.

"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," Radwan Tajeddine, a research associate in astronomy at Cornell University, said in a news release.

Tajeddine and his research partners presented their analysis of Saturn's A ring to attendees of the American Astronomical Society's Division of Planetary Science meeting, held on Tuesday in Provo, Utah.

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. The waves are caused by the orbital resonances of nearby moons. Orbital resonance describes the push and pull of gravity of nearby bodies, like moons and planets.

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. Together, Janus and the other six smaller moons, maintain the ring's stable structure. Without the help of its neighbors, Janus alone couldn't contain the ring.

"The density waves created by moons are beautiful to look at, but they actually participate in confining the ring," said Tajeddine. "Janus has been getting all of the credit for stopping the A ring, which has been unfair to the other moons."

A paper describing the ring containment teamwork of the seven moons will be published in the Astrophysical Journal on October 18.

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