Cosmic murder created Saturn’s rings

By Seth Borenstein AP/AG Staff 13 December 2010
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The death throes of a nascent moon of Saturn may have created the gas giant’s rings.

ONE OF THE SOLAR SYSTEM’S most enduring 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 rings are the only evidence left; as the doomed moon made its death spiral, Saturn robbed it of its outer layer of ice, which then formed rings, according to a new theory published on Sunday in the journal Nature.

“Saturn was an accomplice, and that produced the rings,” says study author Dr Robin Canup, an astronomer at the Southwest Research Institute in the US city of Boulder, Colorado.

Crash debris

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

Something had to have stripped away the outer ice of a moon, she says. Her theory starts billions of years ago when the planet’s 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, jostled by the disk of gas.

These death spirals each took about 10,000 years and the key to understanding the rings’ origins is what happened to them during that time. According to Robin’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.

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, Robin says. So what began as moons has become rings and then new moons. This helps explain Tethys, an odd inner moon that doesn’t quite fit other moon formation theories. Saturn has 62 moons – 53 of them have names. New ones are discovered regularly by NASA’s Cassini probe.

Celestial recycling

The rings and ice-rich inner moons of Saturn are the last surviving remnants of this lost moon, “which is pretty neat”, Robin says. 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.

The new theory helps explain the mystery of Saturn’s rings which “has puzzled people for centuries”, says Cornell University astronomer Professor Joe Burns, who wasn’t involved in the study. He says that it explains the heavy ice components of the rings better than other possibilities.

Professor Larry Esposito, an astronomer at the University of Colorado 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,” he says, 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.”