17.04

Dynamics of Clumps in Saturn's F Ring

M. R. Showalter (Stanford)

An exhaustive analysis of the Voyager image data sets reveals the F Ring to be the most dynamic ring in the solar system. Principal properties are as follows. (1) At any given time, the ring holds 2-3 extremely bright clumps (each several times brighter than the local average for the ring) and perhaps 20-40 identifiable smaller clumps. In practice, the closer one looks at the rings, the finer detail one finds. (2) No major clumps persist for the nine months between the Voyager encounters, but most survive for the weeks they can be detected during a single encounter. (3) A few major clumps are seen to appear or disappear very quickly, on time scales as brief as days. One clump appears to spread longitudinally after it first appears, but other clumps stay roughly fixed in longitudinal extend. The processes behind clump formation and destruction are unknown. (4) Clumps propagate at different mean motions; there is no evidence for a discrete set of rates that might correspond to the individual ``strands'' described in some models. Motions range from /day to /day, implying that the F Ring's clumps span a semimajor axis range of  km. (5) Some sections of the ring show a distinct periodicity in clump spacing, as expected from the gravitational perturbations by Prometheus. Others do not. (6) In the Voyager 2 images, a single prominent clump seems to eject smaller clumps behind it on time scales of weeks. However, nothing analogous is observed in Voyager 1 data.