Imaging scientists working on the data from the Cassini spacecraft have identified ghostly spokes in Saturn's rings.

These faint, radial lines were first observed by Voyager, 25 years ago, and have also been seen in pictures from Hubble. But there had been no sign of them in pictures from Cassini's cameras until now.

Originally, researchers had expected that the rings of Saturn would be uniform in structure, that collisions between the particles would smooth things out.

It is not clear exactly how the spokes form, but they are relatively short-lived structures. The spokes can be seen to form and dissipate within hours.

Scientists suggest electrostatic repulsion between ring particles may play a role, perhaps levitating finer particles above the main ring structure. Gravitational interactions may also contribute.

In the new images, the spokes are about 3,500 kilometres long and about 100 kilometres wide. They are on the unilluminated side of the rings (most of Voyager's observations were on the sunlit side), and are in the same region as spokes that have previously been observed.

Time lapse photography from Voyager is probably the most dramatic illustration of the phenomenon. ®