Formulation and analytic calculation for the spin angular momentum of a moonlet due to inelastic collisions of ring particles
© The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences. 2004
Received: 6 February 2004
Accepted: 28 July 2004
Published: 24 June 2014
Small moonlets embedded in planetary rings acquire spin angular momentum by inelastic collisions of a number of ring particles. We obtain analytic expressions for the mean and mean square spin angular momenta delivered to a moonlet in the high-velocity case where mutual gravity between the moonlet and particles can be neglected. We find that the mean angular momentum brought by a large number of small impacts would result in an equilibrium rotation of a moonlet in the prograde direction that is slower than the synchronous rotation, while large impacts would significantly affect the rotation when the mass of largest impactors is comparable to the moonlet’s mass and/or the velocity dispersion of particles is larger than the Kepler shear across the moonlet’s radius. We present a new formulation that allows a unified analysis of these two components of moonlet rotation, and confirmed the validity of this formulation and the above analytic calculations using N-body simulation.