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K. Ohtsuki (U. Colorado)
Planetary rings such as Saturn's rings are composed of a number of particles and moonlets, and these small bodies undergo mutual collisions. Although physical properties of these small bodies are poorly understood, they might be inferred from their dynamical behavior such as random velocity evolution, structure formation, and rotation rates. In many of theoretical studies on planetary rings, it is often assumed that ring particles are smooth spheres. However, it is not realistic, and particles should have rough surface, which should cause rotation of particles as a result of oblique impacts. We examine the rotation of a moonlet embedded in planetary rings caused by impacts of ring particles, based on the results of analytic calculation and numerical orbital integration for the three-body problem. Taking into account the Rayleigh distribution of orbital eccentricities and inclinations of particles, we evaluate both systematic and random components of rotation, the former one arising from an average of a number of impacts and the latter one being contribution from large impacts. Calculations for parameter values corresponding to Saturn's C ring show that a moonlet would spin slowly in the prograde direction if most of impactors are small particles whose velocity dispersion is much smaller than the moonlet's escape velocity. However, we also find that the effect of the random component can be significant, if the velocity dispersion of particles is as large as the escape velocity and large impacts are common; in this case, slow rotation in both prograde and retrograde directions would be expected. We will also discuss other related issues, such as rotation and accretion of ring particles.
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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.