What happened to the “spokes” in Saturn’s rings?

In 1979 the Voyager spacecraft revealed “spokes” in Saturn’s rings. Yet recent images from Cassini have shown no spokes. What happened to them?

Spokes in Saturn's rings

Spokes in Saturn's rings, as seen by Voyager 2 (August 1981).

Those spokes have kept planetary scientists scratching their heads for decades. It’s thought that the spokes are radial fingers of dark, extremely fine dust particles that become electrostatically charged by pulses of energy emanating from Saturn’s interior. Once charged, the dust particles are levitated out of the ring plane by Saturn’s strong magnetic field. They seem to form only in the shadowed (nighttime) section of the ring system, then come into view as the ring particles rotate into sunlight.

But that’s only an educated guess. “This is one of the biggest mysteries of the solar system,” admits Cassini investigator Alison Farmer. It could be related to the fact that Voyager visited Saturn when the planet’s rings were nearly edge-on to the Sun, whereas now they are tipped wide open. But it may be many years before the phenomenon is completely understood.

— J. Kelly Beatty