HR: 09:30h
AN: P31D-07    [Abstracts]
TI: Where Did the Spokes Go?
AU: * Mithchell, C
EM: Colin.Mitchell@lasp.colorado.edu
AF: LASP and Department of Physics, U. of Colorado, Boulder, CO 80309 United States
AU: Horanyi, M
EM: horanyi@colorado.edu
AF: LASP and Department of Physics, U. of Colorado, Boulder, CO 80309 United States
AB: The formation of radial dust clouds over the B ring of Saturn, observed by the Voyagers about 25 years ago represented a major puzzle. There are several competing theories explaining the spatial and temporal evolution of these features. Hence, new observations by Cassini were expected to resolve many of the outstanding questions. Contrary to expectations Cassini did not observe a single spoke to date. According to the most promising model, spokes are formed when small dust grains are lifted off the main ring in a region of enhanced plasma density due to a meteor impact. The dense plasma quickly charges the background ring material as well as the small dust particles negatively, which causes the dust particles to be lifted off the ring. These particles move through the impact plasma, charging more negatively until they leave, at which time they enter the ambient plasma environment of the rings. We work with this model for spoke formation, and include the seasonal variation of the solar illumination due to the obliquity and the orbital motion of the planet. Currently, the solar elevation is high and the rings are expected to charge positively. Negatively charged grains emerging from the impact generated plasma are expected to be swiftly pulled back down onto the ring, possibly preventing spoke formation. We have calculated the lifetime of spoke particles for various plasma parameters and particle sizes and find that under the current conditions spokes cannot form. This mechanism provides a possible explanation of their absence in the Cassini observations to date and allows for a prediction that spokes will return toward the end of the main mission of Cassini.
DE: 5759 Rings and dust
DE: 6213 Dust
DE: 6275 Saturn
SC: Planetary Sciences [P]
MN: Fall Meeting 2005