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Where did Saturn's rings come from? Mystery gets a new clue.

Saturn's rings are one of the most recognized features of the solar system, but scientists don't know how they got there. New data suggest they're older than some theories suggested.


This image of Saturn and its rings was captured by the Cassini spacecraft.

Space Science Institute/JPL-Caltech/NASA/Reuters

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New evidence from the US-European Cassini mission to Saturn suggests a very early birth for ices in Saturn’s spectacular system of rings and moonlets, dating back to shortly after the planet itself formed.

The results deepen a mystery that has bedeviled Saturn watchers since Galileo first spotted what later would be interpreted as rings in 1610: How did the rings form? And, more recently, what sustains the ring system?

“No one actually knows why the rings can survive for 4.5 billion years,” says Scott Kenyon, an astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. “At the moment, we don’t have a good model” that explains this longevity.

The apparently implausible life span of the ring system has led some researchers to propose that the system didn’t form shortly after the planet did. 

Instead, it might have formed perhaps 100 million years ago. The raw material for the rings and moonlets could have come from the debris spawned by a collision between close-in moons, or between a close-in moon and a comet.

But the recent-ring scenario has had a troubled existence. 

In 2007, for example, scientists reported evidence from Cassini’s Ultraviolet Imaging Spectrometer indicating that the rings had significant age differences and that the material in the rings was constantly being recycled as moonlets collided. Some of the debris later would form into new moonlets. 

That evidence didn’t support a single, recent violent encounter between objects as a source of material for the ring system.

Now, researchers using another of Cassini’s instruments, the Visual and Infrared Mapping Spectrometer (VIMS), have uncovered further evidence for this recycling as they have mapped changes in the composition of the ring material and moonlets that form a 40,800-mile-wide band around the planet.

Perhaps more important, Cassini has uncovered far more water ice in the system than comets could deliver.

The system “is very ice rich,” says Bonnie Buratti, a researcher at NASA’s Jet Propulsion Laboratory in Pasadena, Calif., and a member of the VIMS team.

To Dr. Kenyon, the results showing ices throughout the system speak to a primeval origin.


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