Saturn's F-ring is being twisted into its contorted shape by the gravitational effect of the moon Prometheus. The F-ring is riddled with unusual structures and distortions, like knots, kinks and clumps, but now astronomers have shown how some of these effects can be explained by the gravity of the small moon.

Prometheus orbits just inside the F-ring. Although it never actually intersects with the ring, its eccentric orbit means that it approaches the ring every 15 hours or so and its gravitational effects create a gore in the ring each time.

Writing in the journal Nature the Cassini imaging team describes developing a computer model of how the moon might cause the patterns.

In the model, as the moon recedes from the ring, it drags strands of particles behind it. The particles never actually escape the ring, but instead spring back and forth across the ring, carving a dark channel that becomes visible about one orbital period later.

The model results in dark channels and drape-like structures in the ring, as can be seen in the image of the real F-ring above.

Dr. Carl Murray from Queen Mary, University of London, lead author of the paper explains that Prometheus was always considered the most likely culprit for changing the ring like this. What the model does is provide a "plausible mechanism" by which it creates the channels and streamers "and a variety of other phenomena can all be understood in terms of the simple gravitational effect of a satellite on ring particles", he says.

Over the next four years, Prometheus' orbit is expected to take it deeper into the F-ring, and the Cassini team says it is looking forward to seeing what effect this will have. ®