Kronoseismology III: Waves in Saturn’s inner C ring

Abstract

We investigate waves driven in Saturn’s rings by planetary normal modes, concentrating on the previously unexplored inner C ring. In this region, the known waves all have exceptionally short wavelengths, which has proven problematic for previous studies that depended on the accurate estimate of wave phases for wave identification. However, detailed study of over 200 stellar and radio occultations observed by Cassini, involving measurements of ∼ 65 circular ring features, has resulted in a radius scale for the rings which has absolute errors of ∼ 250 m and internal (relative) uncertainties of ∼ 150 m (French et al., 2017). This permits us to measure the phases of waves with wavelengths as short as ∼ 1 km, or even less, which has enabled the identification of additional waves in the inner C ring. Using a variety of wavelet-based approaches and employing stellar occultation data acquired by the Visual and Infrared Mapping Spectrometer (VIMS) instrument, we have identified three new Saturn-driven outer Lindblad resonance density waves: W76.02 (radius near 76020 km, wavenumber m=−9), W76.44 (m=−2), and W76.46 (m=−7). We also identify a new class of waves driven by Saturn’s internal oscillations: vertical (bending) waves W74.67 (m=−7), W74.93 (m=−4), W74.94 (m=−9), and W76.24 (m=−8). All of these identifications are based on the three observational parameters that can be derived from the wave profiles: their m-values (or number of spiral arms), their pattern speeds ΩP, and their directions of propagation. These results, when combined with previous identifications of Saturn-driven waves, provide significant constraints on Saturn’s internal structure.