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Last Updated: Thursday, 13 December 2007, 02:29 GMT
Saturn's rings 'may live forever'
By Jonathan Amos
Science reporter, BBC News, San Francisco

Saturn's rings (Nasa)
The UVIS instrument sees the rings in the ultraviolet
Saturn's iconic rings may be much older than we thought, scientists say.

Data from the Cassini probe shows these thin bands of orbiting particles were probably there billions of years ago, and are likely to be very long-lived.

It means we are not in some special time - the giant planet has most likely always provided a stunning view.

Previous data had led researchers to believe the rings were created just 100 million years ago, when a huge moon or comet shattered in Saturn's vicinity.

Professor Larry Esposito told the American Geophysical Union Fall Meeting that Cassini had completely changed that view.

"Despite what was thought after the [1970s] Voyager investigations of Saturn - that Saturn's rings might be very youthful, perhaps only as ancient as the dinosaurs - we have results that show the rings could have lasted as long as the Solar System and maybe will be around for billions of years," he said.

Mini-moons

Cassini has been studying the rings with its Ultraviolet Imaging Spectrograph (UVIS). It has looked at light reflected off and passing through the ring particles, which range in size from grains of sand to boulders.

It has concluded there is far more clumpiness in the water-ice particles than was previously thought - that there may actually be three times the mass than was assumed from the Voyager observations.

We are able to reach the paradoxical conclusion - because the rings appear so young, they may actually be as old as the Solar System
Prof Larry Esposito
University of Colorado at Boulder

Cassini sees features that suggest the rings cannot have formed in a recent one-off cataclysmic event because they display a range of ages - some of them very young.

To explain this, Professor Esposito and colleagues have put forward the idea that material is constantly coming together to form small "moonlets" and that these aggregations are then breaking up in what is a seemingly perpetual process.

In other words, there is a major recycling process going on.

"Although the Voyager observations indicated Saturn's rings were youthful, Cassini shows even younger ages; and because we see such transient, dynamic phenomena in the rings we are able to reach the paradoxical conclusion - because the rings appear so young, they may actually be as old as the Solar System," the University of Colorado at Boulder researcher said.

Scientists had previously believed that really ancient rings should be quite dark due to ongoing pollution from the "infall" of meteoric dust. But if there is recycling going on, this would explain why the rings overall appear relatively bright to ground-based telescopes and spacecraft.

Late collisions

"The more mass there is in the rings, the more raw material there is for recycling, which essentially spreads this cosmic pollution around," Professor Esposito said.

"If this pollution is being shared by a much larger volume of ring material, it becomes diluted and helps explain why the rings appear brighter and more pristine than we expected."

Artist's impression of Saturn's clumpy rings (Nasa)
The rings are a dynamic place - material clumps and breaks up

The question is when did the rings actually form? No-one can say for sure.

The scientists still hold to the idea that the rings resulted from a collision event - but it must have been a long time in the past.

There is enough mass in the rings to make a moon with a diameter of 300km.

"To break up an object that big is really difficult," explained Professor Esposito. He suggested the last obvious time to consider was the so-called Late Heavy Bombardment, when the Solar System experienced its last period of concentrated impacts.

This was about four billion years ago.

Jonathan.Amos-INTERNET@bbc.co.uk

VIDEO AND AUDIO NEWS
Professor Larry Esposito on Saturn's rings



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