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Mysteries of the rings

Cassini reveals strange phenomena in Saturn's rings.
September 19, 2005
During the Cassini spacecraft's first season of ring studies, the Saturn-circling observatory has yielded remarkable findings, from new structures to a rediscovery of the mercurial spokes the Voyager spacecraft spotted two decades earlier. Cassini imaging team member Joseph Spitale says, "We're seeing things that were theorized about but never seen. This stuff is right out of a textbook simulation."
Saturn spokes
These three images, taken over a span of 27 minutes, show a few faint, narrow spokes in the outer B ring. The spokes are about 2,200 miles (3,500 kilometers) long and about 60 miles (100 km) wide. The motion of the spokes here is from left to right. They are seen just prior to disappearing into the planet's shadow on the rings. Cassini's first sighting of spokes occurs on the unilluminated side of the rings, in the same region in which they were seen during the Voyager flybys. Although the most familiar Voyager images of spokes showed them on the sunlit side of the rings, spokes also were seen on the unilluminated side.
NASA / JPL / Space Science Institute
Spokes on Saturn
This sequence of images captured by the Voyager spacecraft shows dusky spokes on the sunlit side of Saturn's broad B ring in 1981.
NASA/JPL
The revelations paint a portrait of rings that's more dynamic and fluid than scientists thought. "The interesting part," says Spitale, "is that not long before we got into orbit around Saturn, simulations of large numbers of particles were becoming available, so people saw new dynamical effects in the models." These effects include the movement of ring boundaries, scalloped ring edges, and other phenomena that Cassini has now charted.

A quarter century ago, the Voyager spacecraft discovered streams of material within Saturn's rings that traveled around the planet like the spokes of a wheel (see animation at left). But when Cassini arrived at the ringed giant in 2004, the spokes were nowhere to be seen. Finally, Cassini spotted them on the rings' unilluminated side. The spokes are about 60 miles (100 kilometers) wide and span a radial length of up to 2,200 miles (3,500 km).

In another example of ring activity, the tenuous D ring has shifted locations since the Voyager flybys. The ring has migrated inward by roughly 125 miles (200 km). Another dynamic discovery lies in the tenuous G ring, which appears to have at least one ring segment. This arc of material is similar in structure to the ring arcs encircling Neptune.
Wavemaker moon
This Cassini image shows a new body, provisionally named S/2005 S1, orbiting within the Keeler gap in Saturn's rings.
NASA / JPL / Space Science Institute
Cassini has also imaged elegant waves spanning thousands of miles where small moons orbit close to ring boundaries. For example, scalloped edges bracket a small moon in the Keeler gap called S/2005 S1. From the size of the waves, scientists should be able to discern the mass of the tiny moon, which is roughly 4 miles (7 km) across.

The F ring continues to provide challenges for ring dynamicists. Cassini's sharp eye is untangling the ring's clumped nature, revealing a spiral ring just inside the main ring. This spiral defies explanation, although a hidden moon is the chief suspect. No such moons have been found in the F ring yet; instead, images have captured clumps of debris — perhaps harboring a moon just beyond the resolution of Cassini's imaging system — that seem to cross the ring. The disruptive object, provisionally dubbed S6, appears to plow through the F ring's core periodically.
Clumps in the F Ring
This Cassini image shows clumps seemingly embedded within Saturn's narrow, outermost F ring. The larger version, two images captured hours apart, shows these clumps as they revolve about the planet.
NASA / JPL / Space Science Institute
"How can you have a body crossing the ring without wrecking the whole thing?" Spitale asks. Cassini images are filled with small spots; some are real moons, and some are due to cosmic rays. It takes a series of observations to locate objects. The team has tracked at least 15 possible objects. "With all these sightings, it's easy to get coincidences that mean nothing. But now we have 3 or 4 that we're confident about."

Within these three or four moons may lie the answer to the F ring's twists and turns. But how does the ring interact with the elusive S6? Does the theorized satellite actually collide with the ring? Spitale and the Cassini team may know soon. "If our predictions are right, it will actually hit the ring in April."

The imaging team will be watching. But even if the moonlet shows up at its predicted location then, it is far too small to scatter as much material as the observations show. Says Spitale, "Even if it turns out to be S6, we still don't have the mechanism to explain it."
Michael Carroll is a science writer and astronomical artist, and author of nearly 20 children's science books. He lives with his family in Colorado.
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