Cassini Spots an Unusual Spoke
This picture of a spoke with a sharp bend in it at the location of synchronous rotation (the place where particles orbit at exactly the same rate that Saturn, and its magnetic field, rotates) may help unravel the cause of these ghostly phenomena in Saturn’s rings. After Voyager returned the first images of spokes in 1981 scientists scrambled to explain these features that seemed to appear within a matter of minutes, were thousands of km long, and dissipated after only one orbit around Saturn. Their light-scattering properties (dark against the bright rings when seen with the Sun behind the camera, and bright against the dark rings with the Sun on the opposite side of the rings (like in the current Cassini viewing geometry)) tell us that the particles making up the spokes are “dust”. This generic term is a bit over-used in astronomy and the size of a dust particle depends on whom you’re talking to. In this context it means about 1 micron (1 millionth of a meter), or about the size of particles in cigarette smoke. Their radial orientation provided another clue to their origin. Saturn has a co-rotational electric field that points radially away from Saturn. Plop a positively charged particle near the rings and that electric field will push it radially away from Saturn. That and the fact that micron-sized particles are particularly susceptible to the electric force suggest that the formation of spokes has something to do with clouds of charged particles, plasma, being created in the rings. The standard model for nearly 25 years now has been that the plasma is created by a meteoroid smacking into the rings, vaporizing itself and a ring particle, and producing a cloud of plasma in the process. An alternative was proposed by Cassini scientist Geraint Jones and colleagues that has the plasma produced by electrons travelling along magnetic field lines to the rings from the tops of thunderstorms. A key element of their argument is the unusual shape of some spokes seen in Voyager images. Emily Lakdawalla at the Planetary Society blog has an extensive discussion of Jones’ model. The new spoke observed by Cassini has a sharp kink which is not easily explained by the meteoroid impact model. Whether the thunderstorm model can explain it will require observations that show spokes and thunderstorms simultaneously at the right location in Saturn’s atmosphere.