Planetary Acoustic Mode Seismology: Saturn's Rings

We examine the hypothesis that the rings of Saturn may act as a seismograph, recording gravitational perturbations associated with acoustic oscillation modes of the planet. The resonant interaction between planetary oscillation modes and ring particle orbits is similar to that between external satellites and the rings. However, the only strong interactions between planetary normal modes and the rings fall at the outer vertical and outer Lindblad resonances of low degree fundamental (f-mode) saturnian oscillations in the D-ring and inner C-ring. The predicted resonance locations fall near several ring features currently identified in the Voyager RSS occultation profile and a high-phase-angle D-ring image which are unassociated with any known satellite resonances. Constraints on the azimuthal wavenumber of these features are generally consistent with the proposed planetary forcing mechanism. Surface displacement amplitudes of ∼1 m for low degree l<5 f-modes should be sufficient to produce detectable features in the rings. However, resonant locations and ring feature attributes currently cannot be determined accurately enough to test rigorously the ring seismology hypothesis. Improved constraints on interior models and more accurate determination of ring feature characteristics may ultimately allow us to read the seismological record contained in Saturn's rings.