Spinning spirals. Strands in Saturn's narrow F ring are actually part of a Tightly wound spiral, possibly caused by the gravitational interactions of tiny moons or clumps of dust.

Ring Riddles Baffle Saturn Scientists

CAMBRIDGE, U.K.--Saturn stole the show at the start of the 37th annual meeting of the Division of Planetary Sciences of the American Astronomical Society, held here this week. Scientists discussed the latest mysteries uncovered by NASA's Cassini spacecraft, including unexplained structures in the ring system, strange properties of its constituent particles, and a ring that is out of place.

One of the most dramatic changes involves the planet's D ring. In just 25 years, one particular ringlet moved 200 kilometers inward and became 10 times dimmer, reported Carolyn Porco of the Space Science Institute in Boulder, Colorado. "That's radical," she says, adding that no one has a clue about the cause of the shift. Porco, who heads the imaging team of NASA's Cassini spacecraft, and colleagues discovered the rapid change by comparing Cassini ring photos with images obtained by the Voyager spacecraft in 1980. The shift possibly suggests that this part of the ring system could be young and short-lived.

Other results are equally baffling. For instance, Cassini's temperature measurements indicate that ring particles are 15 degrees cooler on their night side than on their day side, according to a team led by Linda Spilker of NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California. This means that all particles--from a few centimeters to a few tens of meters across--rotate very slowly, otherwise they would bake evenly on all sides. "We always thought that mutual collisions would lead to a wide variety of rotation rates," says Spilker.

Weirdest of all is Saturn's thin, braided, and kinky F ring, lying just outside the main ring system. A careful analysis by the Cassini imaging team of dozens of F-ring photos reveals that various strands are actually a single, narrow dust ring, tightly wound like a spiral. This unique structure might be caused by gravitational disturbances of a small moonlet in an eccentric orbit that has apparently criss-crossed the F ring for almost 9 months. In fact, that's a mystery in itself: The F ring is believed to contain many large boulders and moonlets, which would make it hard for a small satellite to survive multiple crossings.

Future observations of Saturn will surely reveal more small satellites. For instance, newly found arcs in the remote and tenuous G ring, reminiscent of the ring arcs of the planet Neptune, are probably caused by the gravitational influence of a nearby moonlet, yet to be discovered. Says Cassini's project scientist Dennis Matson of JPL: "The complexity in the rings is just dumb-founding."

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