Regular ArticlePlanetary Acoustic Mode Seismology: Saturn's Rings
Abstract
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.
References (0)
Cited by (97)
We calculate Saturn's quasi-toroidal normal modes of azimuthal order three for several candidate models for the interior stratification. We propose that these oscillations are responsible for exciting a class of slowly propagating spiral density waves observed in the C ring, possessing the same azimuthal symmetry and pattern speeds, that cluster near the rotation rate of the planet. Both prograde and retrograde propagation relative to the planetary rotation are observed. We find that we can construct interior models for which the pattern speeds of the normal modes align with those of a majority of the slow density waves. Excitation of the retrograde density waves requires retrograde toroidal modes with matching pattern speeds. We find that in order to support such modes, the interior model must include a layer of positive static stability in the molecular hydrogen envelope. The model that performs best places the peak of the static stability at a fractional radius of 0.85, near the radius of the semi-conductive layer where Saturn's zonal jets are inferred to decay. The properties of the toroidal modes are less sensitive to the stratification in the deep interior, hence our results neither confirm nor contradict the presence of a dilute core. When differential rotation is included in the form of zonal winds based on analysis of Cassini gravity and magnetic field data, we find that several toroidal modes become inertially unstable, raising the possibility that this instability is the principal mechanism for their excitation. However, while we are able to construct interior models that align the pattern speeds of a few unstable retrograde modes with those of prominent density waves in the ring, the same models fail with respect to the unstable prograde modes. Consequently, the significance of the instability mechanism remains an open question. Our results suggest that the interaction of Saturn's quasi-toroidal modes with the C ring provides an important source of new, complementary constraints on the planet's internal stratification and that further study of these modes is warranted.
Saturn’s rings act as a system of innumerable test particles that are remarkably sensitive to periodic disturbances in the planet’s gravitational field. We identify 15 additional density and bending waves in Saturn’s C ring driven by the planet’s internal normal mode oscillations. The collective response of the rings to Saturn’s oscillations results in a host of inward-propagating density waves at outer Lindblad resonances (OLRs) and outward propagating bending waves at outer vertical resonances (OVRs). In the emerging field of Kronoseismology, nearly two-dozen OLRs and OVRs have previously been identified in high-resolution radial profiles of the rings obtained from Voyager and Cassini occultation observations (see Hedman et al. (2019) and references cited therein for a recent summary). Here we apply similar wavelet techniques to extract and co-add phase-corrected waveforms from multiple Cassini VIMS stellar occultations. Taking advantage of a highly accurate absolute radius scale for the rings (French et al., 2017), we are able to detect weak, high-wavenumber (up to =14) waves with km-scale radial wavelengths. From a systematic scan of the entire C ring, we report the discovery and identification of 11 new OLRs, two counterpart inner Lindblad resonances (ILRs), and two new OVRs. The close agreement of the observed resonance locations and wave rotation rates with the predictions of models of Saturn’s interior (Mankovich et al., 2019) suggests that all of the new waves are driven by Saturnian -mode oscillations. As classified by their spherical harmonic shapes, the modes in question range in azimuthal wavenumber from to 14, with associated resonance orders ranging from 0 to 8, where is the overall angular wavenumber of the mode. Our suite of detections for is now complete from near the inner edge of the C ring to near 81,300 km. Curiously, detections with are less common. These newly-identified non-sectoral () waves sample latitudinal as well as radial structure within the planet and may thus provide valuable constraints on Saturn’s differential rotation. Allowing for the fact that the two ILR-type waves appear to be due to the same normal modes as two of the OLR-type waves, the 13 additional modes identified here bring the number of distinct -modes suitable for constraining interior models to 34.
Tidal Dissipation in Stably Stratified and Semiconvective Regions of Rotating Giant Planets: Incorporating Coriolis Forces
2024, Astrophysical JournalTidal Dissipation in Stratified and Semi-convective Regions of Giant Planets
2023, Astrophysical Journal