Some of Saturn's rings have constantly changing structures, making it difficult for researchers to decipher the processes that shape them. Yet such knowledge could provide a window on the formation of the Solar System. Data from Cassini — the first spacecraft to enter into orbit around Saturn — have allowed Carl Murray at Queen Mary, University of London, and his team to study Saturn's outermost ring, the F ring, in unprecedented detail. They find that constant collisions and the gravitational pull among a population of small satellites give the ring its form (see page 739).

Saturn's narrow F ring was discovered from images captured by the Pioneer 11 spacecraft in 1979. Later data from Voyagers 1 and 2 revealed the ring as having a core about 1 kilometre wide surrounded by a 50-km-wide envelope, both supposedly held in place by the gravitational pull of Saturn's shepherding moons, Prometheus and Pandora.

Cassini entered into orbit around Saturn in July 2004, and detected additional and somewhat puzzling features of the F ring. For example, outward and inward extensions of material called 'jets' project from the ring. Particles further away from Saturn move more slowly than those closer to the planet — a process known as keplerian shearing — which causes the jets to appear distorted, like smoke from a chiminey being blown by a gentle breeze. Moreover, some of these sheared jets aggregate together, forming arrangements known as fans.

To explain these structures, Murray and his colleagues took advantage of movie footage of the F-ring captured by Cassini between December 2006 and May 2007. The researchers took stills from these movies, each containing a part of the ring, and used them to compile a series of 'mosaics' that provided a complete view of the ring. “We unwrapped the ring to get a complete 360° panorama,” says Murray, explaining that the photos from the previous missions by Voyagers 1 and 2 and Pioneer 11 were simply “snapshots over time”. With Cassini in orbit around Saturn, the team had a chance to view the entire F ring at many different times. “This is something we couldn't have done with a fly-by spacecraft,” Murray says.

Originally, the team was looking for particles that might, through collisions with one another, cause the jets and fans to appear. “It's like a crime scene,” says Murray. “You're trying to figure out what the evidence is telling you, and then trying to come up with some suspects. Prometheus and Pandora are the usual suspects, but we think they have some interesting accomplices.”

Cassini detected 13 large objects — with diameters ranging from 27 metres to 9 km — lying within about 10 km of the F ring's core, providing direct evidence of a 'moonlet belt'. Murray's group determined that a population of larger (5-km diameter and more) moonlets continuously collide with material within the core, producing new particles that are then pulled by the gravitational force of smaller, nearby moonlets. The combination of these two processes determines most of the ring's morphology.

The F ring is possibly unique in the Solar System as a location where large-scale collisions happen almost daily. “These processes are similar to those going on in a protoplanetary disc and in basic planet formation,” says Murray. “The F ring has a lot of secrets. We need to continue looking at it.”