Cassini sees collisions of moonlets in Saturn’s ring
BY DR EMILY BALDWIN
ASTRONOMY NOW
Posted: June 9, 2008

The rapid changes in Saturn’s F ring can be attributed to small moonlets embedded within the ring causing perturbations and collisions, say a team of UK-led scientists who published their findings in Nature last week.

The dynamic structure of Saturn's F ring. It is thought that nearby "shepherd" moon Prometheus, as well as objects embedded within the ring itself, cause these interesting features through gravitational pertubations and collisions. Image: NASA/JPL/Space Science Institute.

Saturn’s dynamic F ring, which lies 3,400 kilometres beyond the edge of the main ring system, has long been of interest to planetary scientists because its features change on timescales ranging from just a few hours to several years. It is also probably the only location in the Solar System where large scale collisions happen on a daily basis, and studying these processes gives insight into the early stages of planet formation when collision events were rife.

Images shot by Cassini in 2006 and 2007 show the evolution of a series of jet-like structures emanating from the F-ring as a result of collisions between small nearby moonlets within the core of the ring. A 5 kilometre object discovered by Cassini in 2004 is the best candidate to explain some of the largest jets seen in the images.

“Previous research has noted the features in the F ring and concluded that either another moon of radius about 100 kilometres must be present and scattering the particles in the ring, or a much smaller moonlet was colliding with its constituent particles,” says Professor Carl Murray of Queen Mary London. “We can now say that the moonlet is the most likely explanation and even confirm the identity of one culprit.”

Evidence for embedded satellites in the core of Saturn's F ring. These images were taken between September 2006 and May 2007. A fan shaped structure can be seen in the large image (a). Image: from Murray et al (Nature 453, 5 June 2008).

The F ring and all the nearby objects are being continually perturbed by encounters with the shepherding moon Prometheus, and the existence of sheared jets of material produced by collisions between objects embedded within the

F ring allows the Cassini scientists to detect the event even if the collisions cannot directly be seen. An incredible array of patterns is produced by these collisional events, including a new set of features nicknamed “fans” – a sequence of sheared channels with a common point of intersection – produced by the gravitational effect of small (~1 kilometre) satellites orbiting close to the F ring core.

“Large scale collisions happen in Saturn’s F ring almost daily – making it a unique place to study,” says Dr Sébastien Charnoz of Université Paris 7 / CEA Saclay. “We can now say that these collisions are responsible for the changing features we observe there.”

Cassini is now preparing for an extended two year mission (see 'Cassini primed for extended tour of Saturn'), adding to its previous successful four year odyssey in the Saturnian playground.

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