Migration of small moons in Saturn's rings

摘要

The motions of small moons through Saturn's rings provide excellent tests ofradial migration models. In theory, torque exchange between these moons andring particles leads to radial drift. We predict that moons with Hill radii r_H~ 2-24 km should migrate through the A ring in 1000 yr. In this size range,moons orbiting in an empty gap or in a full ring eventually migrate at the samerate. Smaller moons or moonlets -- such as the propellers (e.g., Tiscareno etal. 2006) -- are trapped by diffusion of disk material into corotating orbits,creating inertial drag. Larger moons -- such as Pan or Atlas -- do not migratebecause of their own inertia. Fast migration of 2-24 km moons should eliminateintermediate-size bodies from the A ring and may be responsible for theobserved large-radius cutoff of r_H ~ 1-2 km in the size distribution of the Aring's propeller moonlets. Although the presence of Daphnis (r_H ~ 5 km) insidethe Keeler gap challenges this scenario, numerical simulations demonstrate thatorbital resonances and stirring by distant, larger moons (e.g., Mimas) may beimportant factors. For Daphnis, stirring by distant moons seems the mostpromising mechanism to halt fast migration. Alternatively, Daphnis may be arecent addition to the ring that is settling into a low inclination orbit in~10^3 yr prior to a phase of rapid migration. We provide predictions ofobservational constraints required to discriminate among possible scenarios forDaphnis.