Saturn ring particles 'fluffy'

By Paul Rincon
BBC News science reporter, Cambridge

Scientists determined the spin rate by studying the temperature profiles of the ring particles

The particles that make up Saturn's rings are more like "fluffy" snowballs than hard ice cubes, as some scientists had previously described them.

And these grains have been found to be spinning more slowly than thought, according to new data from the US-European Cassini space probe.

This is even the case in parts of the rings that are densely packed and where there should be many collisions.

Details were presented at an American Astronomical Society meeting in the UK.

"The ring particles, in our conclusion, are rotating slowly. This tells you something about the physical particles themselves," said Linda Spilker, deputy project scientist for the Cassini mission.

"It would be wonderful to take a butterfly net and scoop up the ring particles, bring them back and look at them. But we have to indirectly infer what a ring particle might look like, and they're probably more like a fluffy snowball."

Scientists determined the spin rate by studying the temperature profiles of the ring particles. They thought that collisions in the dense A and B rings would have resulted in these particles having a more uniform temperature.

We have never seen anything like this before Dr Carolyn Porco, Space Science Institute

Instead, they found that the temperature of the particles drops by about 15 degrees (Kelvin) when they are not exposed to the Sun. This suggested they were spinning slowly enough for them to cool off when they were not in sunlight.

Observations from Cassini also show several faint, thread-like ringlets, or minor rings, around Saturn's F ring in fact form a single spiral arm that encircles the planet.

The spiral is constantly being created and destroyed through its interactions with one of Saturn's moons.

"We have never seen anything like this before. There are spiral structures in the rings, there are gravity and density waves, but they are completely different things," said Dr Carolyn Porco, of the Space Science Institute in Boulder, Colorado.

When a small moon of Saturn gets close enough to the ringlets, or strands, the gravitational force it exerts scatters the particles. After about a year, these organise themselves into a single spiral arm. But successive interactions with the satellite destroy the spiral after two years.

The results were presented at the American Astronomical Society Division of Planetary Sciences meeting in Cambridge, UK.