AN INSTABILITY IN PLANETARY RINGS DUE TO BALLISTIC TRANSPORT

Ballistic transport in planetary rings is the net radial transport of mass and angular momentum due to exchanges of meteoroid impact ejecta between neighboring ring regions. The detailed linear stability analys is in this paper demonstrates that ballistic transport causes wavelike disturbances to grow and propagate in an otherwise uniform ring. The growth is strongest for intermediate values of the normal ring optical depth tau = 0.1 to 1.0 and goes to zero as tau --> 0 and tau --> infi nity. For nominal values of various physical parameters, the minimum e -folding time is similar to 10(5) years for tau similar to 0.4. The di rection of propagation is opposite to the sense of any asymmetry that may exist in the ejecta direction distribution (inward for prograde ej ecta and outward for retrograde ejecta). The additional effect of visc ous transport tends to damp wavelike perturbations strongly at short w avelengths and at high values of tau. The quantitative agreement betwe en this analytic work and numerical simulations reported elsewhere is generally quite good. As applied to Saturn's rings, the results in thi s paper strengthen the earlier conclusion from numerical calculations that the 100-km structure in the inner B Ring is caused by ballistic t ransport. However, it is also dear that ballistic transport cannot pro duce the complex structure seen in the outer two-thirds of the B Ring where tau greater than or similar to 1.5. Wavelike structures in the C Ring might also be attributed to ballistic transport; but this requir es further study. (C) 1995 Academic Press, Inc.