It turns out Janus has help lassoing Saturn's A ring
The Cassini spacecraft may have been destroyed on its fiery plunge through Saturn's atmosphere last month, but the data it sent back to Earth before it met its end is still proving valuable. 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.
Working outward from the surface of Saturn, the major rings are C, B and A. Beyond A, there are three more fainter rings labeled F, G and E (they are named in the order they were discovered.) According to NASA, Saturn also has 52 confirmed moons plus another eight provisional moons.
For years, astronomers have charted the relationship between the Saturnian rings and the planet's moons. In 2009, for example, Saturn's largest ring was discovered lying beyond the inner rings; it was formed from debris from its moon Phoebe. There are also two gaps in Saturn's inner ring system known as the Encke Gap and Keller Gap. These are formed by the orbit of two tiny moons: Pan and Daphnis, respectively.
Another long-held belief was that Saturn's moon Janus, which orbits the gas giant just outside the A ring, was singlehandedly responsible for keeping that ring constrained. Without the gravitational force it asserts on the ring, went the thinking, the tiny particles comprising it would shoot out into space, eventually depleting it.
While the assertion about the constraining influence of the moon on the rings is correct, the new data suggests that Janus doesn't do its ring-corralling all by itself. The nearby moons Pan, Atlas, Prometheus, Pandora, Epimetheus, Mimas also chip in by helping Janus steal some of the A ring's momentum and thereby keeping it from spiralling out into space.
"All of these moons work as a group to contain the ring," said Radwan Tajeddine, a Cornell research associate in astronomy and lead author of the new research. "Together they are strong. United they stand."
In making their determination about the role of the other moons, Tajeddine and his associated examined the Cassini data regarding density waves in the A ring, which are similar to grooves in a vinyl record. The waves are created by a phenomenon known as moon resonances in which orbiting objects exert influence on each other.
"That's the novelty of this idea," said Tajeddine. "No one imagined that rings were held by shared responsibility. The density waves created by moons are beautiful to look at, but they actually participate in confining the ring. Janus has been getting all of the credit for stopping the A ring, which has been unfair to the other moons."
The new finding joins other recent Cassini revelations including the fact that Saturn's magnetic field has a surprising lack of tilt; that there are much fewer dust particles between the planet and its first ring than expected; and that there are dunes made of hydrocarbon sands on the moon Titan that can help explain its winds.
"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," said senior author Joe Burns, professor of engineering and astronomy, about the recent findings, which will be published in the the Astrophysical Journal on October 18.
Source: Cornell University