New evidence has been obtained that Saturn's rings have formed as a result of a collision between a comet and one of its moons. This cataclysmic event, which happened no more than a few hundred million years ago, produced the debris that eventually turned into the famous set of rings.

Scientists have long suspected that Saturn's rings were not as old as the planet itself because they are not a stable system - they tend to dissipate and it is unlikely there is sufficient material constantly added to the rings to maintain a sort of dynamical equilibrium. Some material is added to the rings because meteorites strike Saturn's moons and expel debris into space.

Computer simulations of how these rings could have formed out of a collision between an icy comet and a small moon have indicated that certain features should be present in today's rings. In particular the simulations pointed to the existence of various "moonlets", small moons inside the rings, that aren't sufficiently large to clean up the debris around them, but are sufficiently large to create a partial gap centered on themselves and shaped like an airplane propeller.

Astronomers have now identified exactly such propeller like shapes in the photographs taken by Cassini from under the rings in July 2004, thus adding more weight to the idea that the rings have indeed been relatively recently formed after a violent impact.

Saturn also has larger moons, Pan (30-kilometer-wide) and Daphnis (7-kilometer-wide) orbiting inside the rings - these are large enough to clear the space around them. The two pairs of dashes now found mark the first evidence of objects of about 40 to 120 meters in diameter. "The discovery of these intermediate-sized bodies tells us that Pan and Daphnis are probably just the largest members of the ring population, rather than interlopers from somewhere else," said Matthew Tiscareno, an imaging team research associate at Cornell. Tiscareno and his colleagues explained that such a continuum of sizes from small and abundant to large and rare is just what a collision would produce. Thus, Pan, Daphnis and the moonlets are most likely simply the largest leftovers of the collision. The team estimates that throughout the ring system there could be about ten million such moonlets.

These photographs of moonlets surrounded by an abundance of small particles also have another importance: they offer the opportunity to observe the processes by which planets form in disks of material around young stars, including our own early solar system. "The structures we observe with Cassini are strikingly similar to those seen in many numerical models of the early stages of planetary formation, even though the scales are dramatically different," said co-author Carl Murray, an imaging team member at Queen Mary, University of London. "Cassini is giving us a unique insight into the origin of planets."



Photo credit: JPL/NASA