Saturn's rings formed by moon's plunge

One of the solar system's most evocative mysteries the origin of Saturn's rings may be a case of cosmic murder, new research suggests.

The victim: an unnamed moon of Saturn that disappeared about 4.5 billion years ago.

The suspect: a disk of hydrogen gas that once surrounded Saturn when its dozens of moons were forming, but has now fled the crime scene.

The cause of death: A forced plunge into Saturn.

And those spectacular and colourful rings are the only evidence left. As the doomed moon made its death spiral, Saturn robbed its outer layer of ice, which then formed rings, according to a new theory published online Sunday in the journal Nature.

"Saturn was an accomplice and that produced the rings," said study author Robin Canup, an astronomer at the Southwest Research Institute in Boulder, Colo.

The mystery of Saturn's rings "has puzzled people for centuries," said Cornell astronomer Joe Burns, who wasn't involved in the study and said Canup's new theory makes sense.

One of the leading theories has been that either some of Saturn's many moons crashed into each other, or an asteroid crashed into some of them leaving debris that formed the rings. The trouble is Saturn's moons are half ice and half rock and the planet's seven rings are now as much as 95 per cent ice and probably used to be all ice, Canup said. If the rings were formed by a moon-on-moon crash or an asteroid-on-moon, there would be more rocks in the rings.

Something had to have stripped away the outer ice of a large moon, Canup said.

So her theory starts billions of years ago when the planets' moons were forming. A large disk of hydrogen gas circled Saturn and that helped both create and destroy moons. Large inner moons probably made regular plunges into the planet, pulled by the disk of gas.

These death spirals took about 10,000 years, and the key to understanding the rings' origins is what happened to them during that time. According to Canup's computer model, Saturn stripped the ice away from a huge moon while it was far enough from the planet that the ice would be trapped in a ring.

Rings coalesce as new moons

The original rings were 10 to 100 times larger than they are now, but over time the ice in the outer rings has coalesced into some of Saturn's tiny inner moons, Canup said. So what began as moons has become rings and then new moons.

This helps explain Tethys, an odd inner moon that didn't quite fit other moon formation theories, she said. Saturn has 62 moons 53 of them have names. New ones are discovered regularly by NASA's Cassini probe.

But this doesn't explain rings on other planets in our solar system, such as Jupiter, Neptune and Uranus, which probably formed in a different way, Canup said.

The rings and ice-rich inner moons are the last surviving remnants of this lost moon, "which is pretty neat," Canup said.

Burns said Canup's theory explains the heavy ice components of rings better than other possibilities. Larry Esposito, who discovered one of Saturn's rings, praised the new paper as "a very clever, original idea."

"I would call it more like cosmic recycling," Esposito said because the moon became rings which then became moons. "It's not so much a final demise, but a cosmic effort to reuse materials again and again."