Published online 5 October 2010 | Nature | doi:10.1038/news.2010.515

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Model explains icy composition of planet's spectacular halo.

Saturn's icy rings could be the leftover mantle of a huge moon. NASA/JPL

Saturn's rings may have been formed from the death of an early Titan-sized moon whose upper layers were ripped off as it spiralled into the infant Saturn.

One of the problems in working out where Saturn's rings came from is their composition, says planetary scientist Robin Canup of the Southwest Research Institute in Boulder, Colorado. The rings are 90%–95% water ice — odd because the primordial solar system would have been comprised of about equal parts ice and rock.

Furthermore, the rings have been collecting interplanetary dust ever since they were formed. "So they must have formed as essentially pure ice," she said in a presentation at a meeting of the American Astronomical Society's Division for Planetary Science in Pasadena, California.

Saturn's inner moons are also anomalies, with low enough densities that they too must be comprised of nearly pure ice.

Prior theories suggested that the rings were produced by the breakup of a small moon that fell too far into Saturn's mammoth gravity or by the breakup of a very large comet that suffered the same fate. But the small-moon theory begs the question of why there is so little rock. And comet disruptions should have been much more common at Saturn than at other outer planets, Canup said in her abstract.

Canup's hypothesis is that the rings were formed when a Titan-sized moon with a rocky core and an icy mantle spiralled into Saturn early in solar system history. Tidal forces ripped off part of the icy mantle, distributing it into what would become the rings. But the rocky core was made of tougher stuff. "It hits the planet's surface before it disrupts," she said. "The end result is a pure ice ring."

Afterward, some of that ice recondensed into new moons, she added. But due to changes in the evolving Saturn system, these "spawned" moons now spiralled outward rather than inward.

Warm reception

"It is a very clever new idea," says Joseph Burns, a planetary scientist from Cornell University in Ithaca, New York. "One of the things it can do is produce rings made out of quite pure water ice, which has been a problem in the past."

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And, he adds, because Canup's theory indicates that large satellites will tend to fall inward toward the planet, there might have been a chain of Titan-sized satellites spiralling to their deaths. Titan is the one that remained when the system stabilized.

If Canup's theory is correct, Saturn would originally have had a ring much more massive than it is now. That would mean the ring contained enough material to spawn icy moons like Enceladus, Dione and Tethys.

"This occurs very naturally as part of another origin process — the process by which the rings themselves were made," Burns says. "She has a pretty convincing story, I would say."

Larry Esposito, a planetary scientist at the University of Colorado, agrees. "I am comfortable with her conclusions and believe other ring specialists will consider her idea seriously," he says, adding that tests will come when the Cassini spacecraft, now orbiting Saturn, measures the rings' mass later in its mission.