The Collisional Cascade Model For Saturn's Ring Spokes

Abstract:

Dust particles are ubiquitous in the saturnian system, spewing forth 
from the geysers of Enceladus and lurking as mysterious wedge-shaped 
spokes in the planet's main rings. The smallest dust grains are 
strongly influenced by electromagnetic forces arising from the motions 
of charged dust particles relative to Saturn's rotating magnetic field 
while large dust grains follow Keplerian paths determined by the 
planet's gravity. The most interesting dynamics result when the two 
forces have similar strengths, typically for particles ~100 nanometer 
in size. Differences between the motions of dust grains and much 
larger ring particles provides a free energy source that powers spoke 
formation. 

Most observations of ongoing spoke formation can be understood in the 
context of a Collisional Cascade model in which a hail of 
rapidly-moving ~50nm dust grains rain down upon more massive ring 
particles. After leaving the ring plane en masse from the site of an 
initial disturbance, these mid-sized grains are accelerated by the 
magnetic field to high speeds relative to ring particles. When they 
return to the ring plane - nearly simultaneously over a large radial 
range - they strike dust-coated fluffy ring particles, freeing both 
visible 0.5 micron spoke particles and additional 50nm debris that 
goes on to continue the cascade. The Collisional Cascade model can 
account for the rapid onset of spokes, their hour-long active phases, 
and the propensity of spokes to prefer certain magnetic longitudes.