NASA/JPL/Space Science Institute

The actions of small moonlets often force geysers of material to shoot out from Saturn's F ring.

Odd Ring Out

Saturn's spectacular ring system appears to be the picture of serene beauty. But the narrow F ring is actually a chaotic place, one that is constantly disturbed and reshaped by its resident population of small moonlike objects. Now scientists are finally shedding light on what makes the ring so unruly.

Ever since its discovery in 1979, the F ring--one of seven and fifth in distance from Saturn--has fascinated scientists with its dynamic personality. The two Voyager missions in the 1980s found that, in contrast to the very smooth and nearly two-dimensional nature of the main rings, the F ring is a clumpy mass of twisted braids. Further observations in this decade by the Cassini spacecraft revealed that the ring's core is full of rocks up to several kilometers in diameter. These objects are far bulkier than the smoky dust particles that constitute the great inner rings, and they constantly bend and stretch the F ring in three dimensions. Even larger objects, known as moonlets, were suspected to be disrupting the ring in such a way as to cause periodic jets of dust to shoot hundreds of kilometers into space. But astronomers didn't know how this happened, and only one moonlet had been positively identified.

Now a European team has caught this moonlet in the act. Analyzing all available images of the F ring and Saturn's nearby moons, the team uncovered evidence for several more moonlets and observed the previously discovered moonlet creating a geyser as it plowed through the F ring. As for the shifting braids, the researchers conclude that they result from the motion of as-yet-identified moonlets through the ring--a hypothesis confirmed by computer models--and the occasional influence of Prometheus, a potato-shaped moon that orbits in the gap between the F ring and Saturn's much bigger A ring. Prometheus follows an elliptical orbit that takes it through the F ring. Each pass carves a channel through the ring material, the team found, and the resulting gravitational disturbance contributes to the clumps and twists. The F ring's gravity maintains enough hold on the moonlets to keep them from drifting away but not enough to prevent their continual caroming and reshuffling, the researchers report today in Nature.

The paper "is a wonderful and gratifying confirmation of what we found several years back," says planetary scientist and Cassini team member Carolyn Porco of the Space Science Institute in Boulder, Colorado, who worked on the earlier examination of the F ring. The research "gives ironclad proof" of the existence of moonlets in the F ring, says astronomer Douglas Hamilton of the University of Maryland, College Park. He adds that the F ring might serve as a model for other narrow rings around the gas-giant planets of the solar system "and perhaps even for rings found around extrasolar planets."

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