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R. Morishima, H. Salo (U. Oulu, Finland)
Simple theoretical arguments and numerical simulations suggest the formation of semi-permanent gravitational particle aggregates in Saturn's A ring: Cassini mission will clarify if these aggregates exist or not. Besides its internal density, the spin rate of a forming aggregate is an important factor affecting its stability against subsequent break up. In the rings, spin rates of such aggregates or small moonlets are determined mainly by collisions with the surrounding ring particles. Spin state of the satellite Pan in the Encke gap might also be affected by collisions, although it exists in a gap and is very massive compared to the bulk of the ring particles.
In order to investigate spin rates of embedded moonlets, we perform three-body integrations including friction and spins of small particles colliding with a spherical moonlet: we follow all successive impacts of the particle, including a possible sliding phase. Two types of impact models are applied, one of which assumes instantaneous changes in the perpendicular and tangential relative velocities of the impacting bodies, while the other utilizes the force model introduced in Salo (1995, Icarus 117, 287). Spin rate of the moonlet is treated as a constant in each set of orbital calculations, and the torque exerted on the moonlet is followed through changes of spins of impacting particles. Conducting a series of integrations with various moonlet spin rates, we determine the equilibrium spin rate for which the averaged torque vanishes. We find that if the initial spin velocities of small particles are ignored and the effect of friction is not too small, the resulting equilibrium spin rate of the moonlet is faster than the synchronous rotation rate for rp < 1, where rp denotes the radius of the moonlet normalized by its Hill radius. This rate is much larger than that obtained by considering only the first hits, as would be appropriate in the case of instantaneous sticking. This rapid spin may make the low internal strength moonlets more unstable. In future our studies will be extended to N-body simulations, taking into account also the mutual interaction of ring particles accumulating on the moonlet.
This study is supported by the Academy of Finland and the Oulu University special research-unit grant.
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Bulletin of the American Astronomical Society, 34, #3< br> © 2002. The American Astronomical Soceity.