Planetary Rings
From Earth, the most striking planetary rings are the "main" rings of Saturn. These rings are the most massive, scatter the most light, and contain objects with sizes up to many meters. The main rings show a considerable degree of well-ordered structure, including large gaps named after Cassini and Encke as well as much fine structure making them appear like the groved surface of a vinyl, long-playing recording.
Some of the ring structures arise due to the gravitational fields of Saturn's moons. These features occur at radii at which the orbital frequency is a whole number or simple fractional multiple of the orbital frequency of one of the moons.
First Evidence for Electromagnetic Influence
Spoke features in the B ring viewed by the
Voyager 2 spacecraft.
"Spokes" in one of the main rings were discovered in the early 1980s when the Voyager spacecrafts flew by Saturn. Just like the spokes of a bicycle wheel, the ring spokes are thin, radially orientated structures. Each survives a good part of an orbital period. They are caused by the electrostatic support of micron-sized dust above the ring. Meteoretic impacts on the ring may be the source of plasma (ions and electrons moving together) that is involved in the charging of the large particles that remain in the ring and the fine ring dust that is lifted. The radial extent of a spoke is caused by the force arising from the azimuthal motion of the charged dust relative to the planet's magnetic field.
The radial motion of dust in spokes may have significant consequences for the transport of angular momentum in at least some of Saturn's rings. The rate of angular momentum transport is important for the rings' lifetimes and, thus, for theories of their origin. Angular momentum transport by dust particles may also play dominant roles in the establishment of the groove-like fine structure in the rings.
The Encke Gap viewed by the Cassini spacecraft.