* 13:10 06 September 2005
* NewScientist.com news service
* Maggie McKee
A spiral of powder-sized particles winds around Saturn’s outermost ring, according to new observations with the Cassini spacecraft. The new feature may be caused by small moons ploughing through the ring, but scientists are at a loss to explain the process.
Scientists led by Sebastien Charnoz of the University of Paris in France have used Cassini’s Imaging Science Subsystem to identify the spiral around the F ring, which lies beyond the planet's main ring system. It curls around and into the ring, and stretches about 400 kilometres to either side of the ring's outer edge.
“We’ve never seen anything like this before,” says Carolyn Porco, an imaging team member at the Space Science Institute in Boulder, Colorado, US. The spiral structure appears to be associated with clumps of matter seen occasionally in the ring and in particular with one small moon, called S/2004 S6, that has collided with the ring twice over the last year or so on its orbit around Saturn.
"It is very possible that the spiral is a consequence of moons crossing the F ring and spreading particles around," says Charnoz. "It may be telling us that the F ring might be a very unstable, or even an ephemeral, structure."
Prometheus in passing
The observation is surprising because a moon about 1000 times more massive than S6 is required to fling the particles out to the distance of the spiral using its gravity alone. But Charnoz says S6 may still be able to scatter the particles outward by simply bumping into them.
"We have a suspect, but we don't have the weapon," Charnoz told New Scientist. Once free of the ring, however, Saturn's gravity is thought to sculpt the particles into the spiral shape as the motes nearer the planet move more quickly than those farther away.
Astronomers are also puzzled over how the little moon S6 survives these brush-ups. That is because the F ring straddles the planet’s so-called Roche limit. This is the gravitational threshold outside of which a celestial body – such as a moon – will coalesce, but inside of which potential constituents of a body will collapse and contribute to a planet's rings.
Small objects that cross the Roche limit tend to have a difficult time holding themselves together. The F ring is in a region with a split personality, says Porco. “Something colliding with the ring should be ripped apart, but S6 isn’t.”
She described the research into the spiral on Monday at a meeting of the American Astronomical Society’s Division of Planetary Sciences in Cambridge, UK.
Warm and fluffy
Another team of researchers has found a mystery in Saturn's largest rings – the A, B, and C rings. Scientists had expected the particles to collide, spinning at different rates and showing a relatively even temperature. But Cassini’s composite infrared spectrometer (CIRS) has revealed the particles can vary by as much as 15°C.
They are warmer when Cassini viewed the rings’ day-lit side but cooler when Cassini viewed them from behind, where the ring particles were shaded from the Sun. The observations, by NASA’s CIRS team at the Goddard Space Flight Center in Maryland, US, suggest the particles rotate slowly enough to cool off when they are not directly heated - a rate that may mirror Saturn’s own rotation of about once every 10 hours.
This hints that the larger particles may always keep one side facing Saturn – a phenomenon seen in much larger objects, such as the Moon, which always shows the same face to the Earth. It is not clear what causes the slow rotation, but the observation does suggest the particles are fluffy rather than solid like ice cubes. That is because they seem to be able to heat up and cool off quickly, even with a range of spin rates.