No. 14: Winter 1981 Supplement
Among all the recently discovered complexities of Saturn's rings, the dark spokes are perhaps the most challenging to astronomers. These dark areas seem to rotate with the rings and are likely regions nearly devoid of the particles that constitute the rings. The "normal" annular gaps between the rings can be explained in part as due to the gravitational influences of Saturn's moons. The dark spokes, however, do not succumb so easily. There are no obvious gravitational nuances that can sweep particles selectively from radially aligned areas.
(Anonymous; "Voyager Discovers Spokes in Saturn's Rings," New Scientist, 88: 276, 1980.)
Comment. Any theory accounting for radial gaps may also explain the "knots" of brightness occasionally seen through the telescope down the years. Incidentally, a few observers in the past have also claimed to have seen radial gaps in the rings, so the dark spokes are not exactly new.
Reference. Spoke phenomena of Saturn's rings are cataloged at ARL5 in The Moon and the Planets. A description of this book is located here.