Cassini shows Saturnian roller derby

The seemingly serene orb of Saturn is in fact a gas giant with extraordinary patterns of charged particles and rough and tumble roller derbies for rings. Such are the findings of NASA's Cassini spacecraft since its arrival at Saturn in 2004 - they are combined in two review papers to be published in the March 19 issue of the journal Science.

'This rambunctious system gives us a new feel for how an early solar system might have behaved,' said Linda Spilker, a planetary scientist and the new Cassini project scientist at NASA's Jet Propulsion Laboratory in Pasadena, Calif. 'This kind of deep, rich data can only be collected by an orbiting spacecraft and we look forward to the next seven years around Saturn bringing even more surprises.'

In the paper describing the elegant mess of activity in the rings, lead author Jeff Cuzzi, Cassini's interdisciplinary scientist for rings and dust who is based at NASA Ames Research Centre, Moffett Field, Calif., describes how Cassini has shown us that collisions are routine and chunks of ice leave trails of debris in their wake. Spacecraft data have also revealed how small moons play tug-of-war with ring material and how bits of rubble that would otherwise join together to become moons are ultimately ripped apart by the gravitational pull that Saturn exerts.

During equinox, the period when sunlight hits the rings exactly edge-on, Cassini witnessed rings that are normally flat - about tens of metres thick - being flipped up as high as the Rocky Mountains.

The spacecraft has also shown that the rings are composed mostly of water ice, with a mysterious reddish contaminant that could be rust or small organic molecules similar to those found in red vegetables on Earth.

'It has been amazing to see the rings come to life before our very eyes, changing even as we watch, being colourful and taking on a tangible, 3-D nature,' Cuzzi said. 'The rings were still a nearly unstructured object in even the best telescopes when I was a grad student, but Cassini has brought us an intimate familiarity with them.'

Cuzzi said Cassini scientists were surprised to find such fine-scale structure nearly everywhere in the rings, forcing them to be very careful about generalising their findings across the entire ring disk. The discovery that the rings are clumpy has also called into question some of the previous estimates for the mass of the rings because there might be clusters of material hidden inside the clumps that have not yet been measured.

Cassini's intense investigation has opened up new mysteries. It has, for example, revealed images of occasional cannon-ball-like objects that rocket across one of the outer rings known as the F ring, without many clues about where they came from or why they quickly disappear. Carl Murray, Professor of Mathematics and Astronomy at Queen Mary, University of London, is a member of the Cassini Imaging Science Subsystem Team and a co-author of the paper. He has used Cassini images to try to make sense of the F ring. 'Rapid change is the order of the day at the F ring with bizarre structures created by the combined effect of gravity and impacts from nearby objects. Each Cassini image gives us another piece of the F ring jigsaw puzzle and gradually a complete picture of this strange ring is starting to emerge.'

'Cassini has answered questions we were not even smart enough to ask when the mission was planned and raised a lot of new ones,' Cuzzi said. 'We are hot on the trail, though.'