Resonance structures in Saturn's rings probed by radio occultation: II. Results and interpretation

Abstract

Twenty-nine wave features in the Voyager 1 radio occultation profiles of Saturn's rings are examined, individually and comparatively, by the wavelength dispersion and waveform modeling methods described by Rosen et al. (1991, Icarus, 93, 3–24) . The wave regions have a surface mass density of 30 ⪅ σ ⪅ 70 in Ring A, σ ⪆ 65 in Ring B, and σ − 1 g cm −2 in Ring C. Surface density estimates from waveform modeling are 20 to 40% larger than dispersion-derived values, suggesting accumulation of mass near resonance. The average offset from theoretical resonance of wave locations derived from waveform modeling in Ring A is about 4 km. Fitted wave amplitudes imply new mass estimates for Pandora and Prometheus of 2.3 ± 1.2 × 10 −10 and 2.6 ± 1.2 × 10 −10 Saturn's mass, respectively. Model waveforms of overlapping waves excited by the satellites Janus and Epimetheus agree well with observed morphologies in the linear region near resonance, and imply new mass estimates for Janus and Epimetheus of 2.3 −0.6 +3 × 10 −9 and 0.76 −0.2 +0.3 × 10 −9 Saturn's mass, respectively. Using the satellite volumes of Thomas (1989), the density of Janus is 0.45 −0.08 +0.65 g cm −3, consistent with Yoder et al. (1989) as underdense relative to water ice. Epimetheus, Pandora, and Prometheus are also underdense, with densities of 0.49 −0.15 +0.23, 0.42 −0.24 +0.28, and 0.27 −0.14 +0.16 g cm −3, respectively. A density wave is excited at the Mimas 4:1 inner Lindblad resonance (ILR) and possibly at the Atlas 2:1 ILR, both in Ring C. The locations of all identified Ring C resonances stronger than the Mimas 4:1 ILR coincide with gap-ringlet features. The regions of Ring C resonances weaker than the Atlas 2:1 ILR are featureless. If gap-ringlet features are created by the resonances, the required deficit of viscous torque relative to satellite torque limits the ring thickness to 2 H ⪅ 1 m. From wave damping lengths in Ring C, 2 H ⪅ 5 m at least locally. Eight new wave features in Ring C are unique to the radio observations and remain unexplained; however, bounds are placed on their strengths and azimuthal symmetry if resonantly driven: each experiences a satellite torque comparable to the Mimas 4:1 ILR if a density wave or the Mimas 3:1 inner vertical resonance if a bending wave and has only a few spiral arms. The difference between σ estimated here and by Zebker et al. (1985) suggests Ring C particles are also underdense, with density 0.38 −0.23 +0.43 g cm −3.