Images of Saturn's narrow and contorted F ring returned by cameras onboard NASA's Cassini spacecraft have revealed phenomena not previously detected in any planetary ring. The findings are reported in Nature (27th October 2005).
The F ring is notorious for exhibiting unusual structures, like "knots," "kinks," and "clumps" that continue to puzzle astronomers. However, Cassini images have shown that the gravitational effect of the inner shepherding satellite, Prometheus, appears to produce regular patterns on the ring including a series of channels or gores and 'streamers' of particles that temporarily link the ring to the satellite. As an example of a satellite that enters a ring on a regular basis, the phenomena posed unique challenges to the understanding of ring-satellite interactions.
The findings of the Imaging Science Subsystem team show that channels and streamers can be understood in terms of a simple gravitational interaction as Prometheus approaches and recedes from the F ring every 14.7 hours. Using Cassini data the team developed a model that shows the mechanism by which Prometheus, as it recedes from its closest approach to the F ring, gravitationally extracts material from the ring. The affected particles do not escape the F ring - the changes to their orbits produced by Prometheus cause them to oscillate back and forth across the ring. One orbital period after the encounter the effect is visible as a channel - in excellent agreement with the Cassini images. In this way Prometheus leaves its mark on the F ring long after it has moved on.
Professor Carl Murray from Queen Mary, University of London is the lead author of the paper and member of the Cassini Imaging Science Subsystem team said, "As the closer and more massive of the F ring's two shepherding satellites Prometheus was always the likely culprit for causing changes to this narrow ring. Our model provides a plausible mechanism for the origin of intricate structures detected in the F ring and suggests that streamers, channels and a variety of other phenomena can all be understood in terms of the simple gravitational effect of a satellite on ring particles."
Over time Prometheus is expected to drive deeper into the F ring - with more extreme perturbations - culminating in December 2009 when the two orbits approach their minimum separation.
Professor Joseph Burns, an imaging team member from Cornell University, Ithaca, New York and also one of the papers co-authors said, "We're eager to learn what the satellite will do to this narrow, already contorted ring, and in turn whether the ring particles will strike Prometheus, changing its surface."
Professor Murray added, "We see the model we have developed very much as a first step in understanding the processes at work. There are many features of the F ring that we have yet to explain but at least we have uncovered one of its secrets. Ultimately this type of research will help us to understand how planets form and evolve."
The work described in the Nature paper is a collaboration between Cassini imaging scientists at Queen Mary, University of London, Cornell University and the Space Science Institute.
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