Here’s a recent picture of spokes in Saturn’s rings. It’s the dark radial smudge in the bright B ring (and a couple of fainter ones).
Image Credit: NASA/JPL/Space Science Institute
We know the particles making up the spokes are dust (micrometers in size) due to the way they scatter light (in the case of the picture above, they are not reflecting as much light as the larger meter-sized particles that make up the main rings). Small particles are easily pushed around by electric and magnetic fields, and this in some way probably explains the radial shape of the spokes. However, what triggers their formation is still unknown. We had hoped that the UV instrument on Cassini would be able to detect the flash of light produced when a meteoroid hit the rings (one theory for the mechanism that triggers spoke formation), but in a paper published earlier this year we showed that our non-detection is an unavoidable consequence of the nature of the expanding cloud of plasma produced in an impact. Spokes will get increasing scrutiny in the Cassini Equinox Mission (now underway) and hopefully in the proposed Cassini Solstice Mission. High resolution movies should help reveal the nature of these intriguing phenomena.