Formulation and analytic calculation for the spin angular momentum of a moonlet due to inelastic collisions of ring particles

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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.
公益社団法人日本地震学会、地球電磁気・地球惑星圏学会、特定非営利活動法人日本火山学会、日本測地学会、日本惑星科学会の論文