Spokes in Saturn's rings: Dynamical and reflectance properties

Abstract

Orbital velocities and relative reflectances of a preliminary sample of 15 Saturnian spokes recorded in the Voyager 2 low-resolution ring movie have been examined. While 13 spokes exhibit the expected Keplerian velocities, 2 anomalous spokes deviate from this motion. For approximately 2 hr after their formation these spokes exhibit corotational motion and, only then, accelerate to Keplerian speeds. Only 1 of the 2 accelerating spokes is within view of the Voyager cameras throughout its lifetime. When first seen this spoke appears on the morning ansa of the B ring with a 0.02 contrast; it gains in contrast throughout its corotational phase, reaching a maximum of 0.09 during its velocity transition. The spoke then loses contrast during its Keplerian phase, dropping to 0.02 in the last visible image. Thus a correlation between the contrast and the anomalous dynamical phases of this feature is observed. The radial reflectance profile, measured when the spoke is approximately 1 hr old, suggests discrete sources for spoke material in regions of maximum contrast within the B ring; a lower limit of 3 × 10 11 g can be established for the mass at this point. The behavior of this spoke seems to be explained by the plasma cloud model of Goertz and Morfill (1983, Icarus 55, 111–123). The atypical dynamics of these 2 spokes suggest that they are generated by plasma clouds of unusually high charge density, while the contrast of these features appears to depend more on ring particulate concentration than on plasma cloud density.