The propeller and the frog

Abstract

"Propellers" in planetary rings are disturbances in ring material excited by moonlets that open only partial gaps. We describe a new type of co-orbital resonance that can explain the observed non-Keplerian motions of propellers. The resonance is between the moonlet underlying the propeller, and co-orbiting ring particles downstream of the moonlet where the gap closes. The moonlet librates within the gap about an equilibrium point established by co-orbiting material and stabilized by the Coriolis force. In the limit of small libration amplitude, the libration period scales linearly with the gap azimuthal width and inversely as the square root of the co-orbital mass. The new resonance recalls but is distinct from conventional horseshoe and tadpole orbits; we call it the "frog" resonance, after the relevant term in equine hoof anatomy. For a ring surface density and gap geometry appropriate for the propeller Blériot in Saturn's A ring, our theory predicts a libration period of ~4 years, similar to the ~3.7 year period over which Blériot's orbital longitude is observed to vary. These librations should be subtracted from the longitude data before any inferences about moonlet migration are made.

11 pages, 2 figures. Accepted to ApJ Letters

References in corpus (3)

• The population of propellers in Saturn's A Ring
• Physical characteristics and non-keplerian orbital motion of "propeller" moons embedded in Saturn's rings
• Migration of a moonlet in a ring of solid particles : Theory and application to Saturn's propellers

Cited by in corpus (10)

• Care and feeding of frogs
• Planetary Rings
• Migration of small moons in Saturn's rings
• 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
• The gravitational instability of a stream of co-orbital particles
• A Librational Model for the Propeller Blériot in the Saturnian Ring System
• Hydrodynamic Simulations of Asymmetric Propeller Structures in Saturn's Rings