The radial motions of small moons in Saturn’s rings can provide excellent tests of satellite migration models. In theory, radial drifts arise from a torque exchange between these moons and ring particles. We predict that moons of radius 2–20 km in the A ring are massive enough to clear a gap in the ring, yet are light enough to migrate through the ring in ~1000 yr. Smaller moons, such as the propellers, are trapped by the inertia of ring particles diffusing onto corotating orbits. Larger moons, such as Pan and Atlas, have too much of their own inertia for torque-induced migration to operate efficiently. The fast migration of the 2-20 km moons may explain the observed large-radius cutoff around 1-2 km in the size distribution of A ring moonlets. However, Daphnis, a ~4 km moon in the A ring, is not evidently experiencing radial drift, thus challenging this picture. Possibly, Daphnis may not migrate because it is being stirred by distant, massive moons (e.g., Mimas). Or it may be a recent addition to the ring that is orbitally settling into the ring plane prior to the onset of fast migration. We use numerical simulations to make measurable predictions for Daphnis to distinguish between these scenarios. Our simulations can be applied to satellite migration in general, so we conclude by extending the lessons from the A ring to the growth of planets in the early Solar System and in protoplanetary disks around other stars.
Published on March 4, 2013
Speaker:Benjamin Bromley (University of Utah)