THE PROPELLER AND THE FROG
- Department of Astronomy, University of California, Berkeley, CA 94720 (United States)
'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 Bleriot in Saturn's A ring, our theory predicts a libration period of {approx}4 years, similar to the {approx}3.7 year period over which Bleriot's orbital longitude is observed to vary. These librations should be subtracted from the longitude data before any inferences about moonlet migration are made.
- OSTI ID:
- 21452737
- Journal Information:
- Astrophysical Journal Letters, Vol. 722, Issue 2; Other Information: DOI: 10.1088/2041-8205/722/2/L178; ISSN 2041-8205
- Country of Publication:
- United States
- Language:
- English
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