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This is how Saturn's rings roll

NASA / JPL / SSI

Spiky vertical structures rise as high as 1.6 miles (2.5 kilometers) above the plane of Saturn's rings, as seen in an image captured by the Cassini orbiter two weeks before the planet's equinox in August 2009. Scientists believe the spikes are the result of a "splash effect" created by moonlets on the outer edge of Saturn's B ring.

The scientists behind the Cassini mission to Saturn say they have figured out the reasons behind the irregularities in the behavior of the most dynamic regions in Saturn's rings. They're due to a combination of natural oscillations that are amplified by the motions of the ring particles themselves -- plus an extra disturbance created by the moon Mimas.

The scientists also have discovered two regions within the rings that are the likely homes of moonlets yet to be discovered.

The lessons gained by watching the rings can also be applied to understanding how planetary systems and galaxies work, said Carolyn Porco, leader of Cassini's imaging team at the Colorado-based Space Science Institute.

"We have found what we hoped we'd find when we set out on this journey with Cassini nearly 13 years ago: visibility into the mechanisms that have sculpted not only Saturn's rings, but celestial disks of a far grander scale, from solar systems like our own all the way to the giant spiral galaxies," Porco said in a news release issued today.

Porco and Joseph Spitale, another member of the imaging team, are the authors of a report detailing the findings, published online today in The Astronomical Journal. The report is based on more than four years' worth of observations from the Cassini spacecraft, which has been orbiting Saturn since 2004.

What's causing the waves?
Those observations tracked the in-and-out oscillations of the planet's massive B ring, which you can see in this video on the imaging team's website, as well as this one. The shape of the B ring is controlled to some extent by Mimas -- but there are some extra wave patterns that weren't previously explained. Spitale and Porco say no fewer than three of the wave patterns spontaneously arose in part because the ring is dense enough, and the edge of the ring is sharp enough, for "free" waves to grow on their own and then reflect at the edge.

"These oscillations exist for the same reason that guitar strings have natural modes of oscillation, which can be excited when plucked or otherwise disturbed," Spitale said in the news release. "The ring, too, has its own natural oscillation frequencies, and that's what we're observing."

Such oscillations are thought to play a role in the motions of spiral galaxies as well as the dusty disks that give rise to planets, but because the oscillations can't be observed directly in those disks, they could only be inferred on the basis of computer simulations. Now astronomers have actually spotted large-scale wave patterns at work in the cosmos.

Peter Goldreich, a planetary ring theorist at Caltech, said the new findings show how the material in the densest parts of Saturn's rings can amplify oscillations and explain the "mysterious grooves first seen in images taken by the Voyager spacecraft" in the  1980s.

What's causing the spikes?
In addition to the self-excited oscillations, Cassini's scientists noticed disturbances in two regions on the B ring's outer edge, including spiky vertical structures that rise as much as 1.6 miles (3.5 kilometers) above the ring plane. One of the perturbed regions, measuring 12,000 miles (20,000 kilometers) in length, can be seen rolling around the edge of the B ring about halfway through this video clip.

NASA / JPL / SSI

A chevron-shaped disturbance can be seen along the outer edge of the B Ring, toward the top of this picture from the Cassini orbiter. Click to watch a QuickTime video of the disturbance rolling along the ring's edge.

The two disturbed areas -- known as Region A and Region B -- are not thought to be caused by the natural oscillations or by a previously known pattern linked to Mimas. Instead, the best explanation is that the regions contain moonlets measuring as much as a half-mile (1 kilometer) wide, or even wider.

"These objects may have been strewn across the outer B ring in the past, but migrated across the rings to become trapped in the Mimas resonance that maintains the B ring's outer edge," Porco told me in an e-mail.

As icy particles in the B ring pass by the moonlets, they "splash" upward from the closely packed ring plane to form the spiky peaks. The moonlets themselves have gone largely unseen. But the Cassini team did spot a moonlet embedded in the B ring last year, thanks to its shadow, and "propeller moons" have been detected in Saturn's A ring, which is outside the B ring. So scientists surmise that more moonlets should exist in Regions A and B.

In today's "Captain's Log" for the Cassini mission, Porco said the migration of the moonlets within the rings may mimic "the migration of the planets across the solar nebula in the early dawn of our solar system."

"All in all, we have here a fascinating story of physical mechanisms at work in Saturn's rings that are at work today, and have been in the past, in other disk systems throughout the cosmos," she said. "In other words, we have uncovered one single physical mechanism that has the power to explain simultaneously a host of seemingly unrelated phenomena ... just the kind of discovery we scientists love to make."

Correction for 4 p.m. ET Nov. 2: An earlier version of this posting erroneously said that the paper by Spitale and Porco appeared in The Astrophysical Journal instead of The Astronomical Journal.


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