Published online 5 September 2005 | Nature | doi:10.1038/news050905-4

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Saturn's rings more placid than thought

Chunks of ice are more like mini moons than wild spinning rings.

The Cassini-Huygens mission has taken the temperature of Saturn's rings.The Cassini-Huygens mission has taken the temperature of Saturn's rings.© AIP

The icy particles of Saturn's rings rotate slowly like miniature moons, rather than spinning around wildly as scientists had once thought.

Astronomers had assumed that chunks of ice and rock in the rings, ranging in size from dust grains to mountains, were bouncing around like pinballs, frequently slamming into each other. "They ought to be spinning out of control," says Kevin Baines, a planetary scientist from NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California. Some rings are packed so tightly that sunlight cannot break through, so collisions should be inevitable.

But the Cassini probe, now in orbit around Saturn, has revealed the particles' stately motion by measuring their temperature as they orbit the planet.

They find that a particle has a ‘hot’ side, of about -163 ºC, and a dark side a few degrees cooler. As the particle orbits the planet once every 10 hours or so, the temperature of each face drops by about 15 ºC at the farthest point from the Sun.

If the particles were spinning much faster than the time they take to make a complete orbit of the planet, they would experience less of a change in temperature, explains Linda Spilker, deputy project scientist for the Cassini-Huygens mission at JPL. The rapid rotation would give all parts of the particle an equal exposure to the Sun, she says, like a well-cooked rotisserie chicken.

Dark side of the ring

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This temperature profile suggests that the particles may make just one rotation for every orbit, just like most of Saturn's proper moons, and indeed our own Moon. This would mean that one face of the particle is always pointing towards the planet.

It also means that the particles must warm and cool quite quickly, says Spilker, something that shards of ice simply don't do. "They're more like fluffy, porous snowballs than a solid ice cube," she says. This soft surface might act as a damper on any collisions, explaining why the particles have maintained their gentle motion.

Another explanation might be that the particles are charged, suggests Baines, who was not part of the research team. This would make them repel each other and reduce the number and strength of collisions.

Mission scientists have had a glut of results about Saturn's rings since Cassini started to orbit the poles of the planet. The discovery comes from the first close-up views of Saturn's rings at infrared wavelengths, using the CIRS (Composite Infrared Spectrometer) instrument on board Cassini. Spilker will unveil the results at the American Astronomical Society's planetary sciences meeting in Cambridge, UK, on 9 September.