SATURN has a celestial swirl: its outermost ring includes a spiral-shaped element that wraps around the planet three times.
The F ring has puzzled scientists since the 1980s, when the Voyager probes began beaming back pictures showing that it has multiple strands on either side of its circular central band or 鈥渃ore鈥�. These strands are sometimes woven into braids and populated by kinks and knots, all caused by gravitational tugs from the nearby small moons Prometheus and Pandora.
Now full images of the F ring from the Cassini probe show the strands are coils of a spiral that sweeps around the planet three times, crossing the ring鈥檚 core as it does so (Science, DOI: 10.1126/science.310.5752.1241a).
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S茅bastien Charnoz and colleagues at the University of Paris 7 in France studied the way the coils behaved between November 2004 and May 2005 and ran a computer simulation backwards from that point. They suggest the coils originated from a cloud of particles shed from the F ring鈥檚 core, perhaps after a meteoroid crashed into a large ring particle. The debris could have been strung out into a spiral by the gravitational pull of an eccentrically orbiting body called S/2004 S6, which is not yet classed as a moon, or other as yet unseen moons.
