Migration of a moonlet in a ring of solid particles : Theory and application to Saturn's propellers

Abstract

Hundred meter sized objects have been identified by the Cassini spacecraft in Saturn's A ring through the so-called "propeller" features they create in the ring. These moonlets should migrate, due to their gravitational interaction with the ring ; in fact, some orbital variation have been detected. The standard theory of type I migration of planets in protoplanetary disks can't be applied to the ring system, as it is pressureless. Thus, we compute the differential torque felt by a moonlet embedded in a two-dimensional disk of solid particles, with flat surface density profile, both analytically and numerically. We find that the corresponding migration rate is too small to explain the observed variations of the propeller's orbit in Saturn's A-ring. However, local density fluctuations (due to gravity wakes in the marginally gravitationally stable A-ring) may exert a stochastic torque on a moonlet. Our simulations show that this torque can be large enough to account for the observations, depending on the parameters of the rings. We find that on time scales of several years the migration of propellers is likely to be dominated by stochastic effects (while the former, non-stochastic migration dominates after ~ 10^{4-5} years). In that case, the migration rates provided by observations so far suggests that the surface density of the A ring should be of the order of 700 kg/m^2. The age of the propellers shouldn't exceed 1 to 100 million years, depending on the dominant migration regime.

17 pages, 5 figures, submitted to Astronomical Journal on february, the 23d

References in corpus (7)

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• Did Saturn's rings form during the Late Heavy Bombardment ?
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• On the evolution of mean motion resonances through stochastic forcing: Fast and slow libration modes and the origin of HD128311
• The population of propellers in Saturn's A Ring
• Physical characteristics and non-keplerian orbital motion of "propeller" moons embedded in Saturn's rings

Cited by in corpus (15)

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• Symplectic integrators in the shearing sheet
• The propeller and the frog
• Care and feeding of frogs
• Migration of small moons in Saturn's rings
• Planetary Rings
• Stochastic orbital migration of small bodies in Saturn's rings
• A Pluto-Charon Concerto: An Impact on Charon as the Origin of the Small Satellites
• Stochastic flights of propellers
• A modified "Type I migration" model for propeller moons in Saturn's rings
• Incomplete cooling down of Saturn's A ring at solar equinox: Implication for seasonal thermal inertia and internal structure of ring particles
• Mean Motion Resonances With Nearby Moons: An Unlikely Origin For The Gaps Observed In The Ring Around The Exoplanet J1407b
• Hydrodynamic Simulations of Asymmetric Propeller Structures in Saturn's Rings