Kronoseismology V: A panoply of waves in Saturn’s C ring driven by high-order internal planetary oscillations

Abstract

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 m=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 f-mode oscillations. As classified by their spherical harmonic shapes, the modes in question range in azimuthal wavenumber from m=8 to 14, with associated resonance orders ℓ−m ranging from 0 to 8, where ℓ is the overall angular wavenumber of the mode. Our suite of detections for ℓ−m=4 is now complete from m=8 near the inner edge of the C ring to m=14 near 81,300 km. Curiously, detections with ℓ−m=2 are less common. These newly-identified non-sectoral (ℓ>m) 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 f-modes suitable for constraining interior models to 34.