The B-ring’s surface mass density from hidden density waves: Less than meets the eye?

Abstract

Saturn's B ring is the most opaque ring in our solar system, but many of its fundamental parameters, including its total mass, are not well constrained. Spiral density waves generated by mean-motion resonances with Saturn's moons provide some of the best constraints on the rings' mass density, but detecting and quantifying such waves in the B ring has been challenging because of this ring's high opacity and abundant fine-scale structure. Using a wavelet-based analyses of 17 occultations of the star γ Crucis observed by the Visual and Infrared Mapping Spectrometer (VIMS) onboard the Cassini spacecraft, we are able to examine five density waves in the B ring. Two of these waves are generated by the Janus 2:1 and Mimas 5:2 Inner Lindblad Resonances at 96,427 km and 101,311 km from Saturn's center, respectively. Both of these waves can be detected in individual occultation profiles, but the multi-profile wavelet analysis reveals unexpected variations in the pattern speed of the Janus 2:1 wave that might arise from the periodic changes in Janus' orbit. The other three wave signatures are associated with the Janus 3:2, Enceladus 3:1 and Pandora 3:2 Inner Lindblad Resonances at 115,959 km, 115,207 km and 108,546 km. These waves are not visible in individual profiles, but structures with the correct pattern speeds can be detected in appropriately phase-corrected average wavelets. Estimates of the ring's surface mass density derived from these five waves fall between 40 and 140 g/cm 2 , even though the ring's optical depth in these regions ranges from ∼ 1.5 to almost 5. This suggests that the total mass of the B ring is most likely between one-third and two-thirds the mass of Saturn's moon Mimas.

Density waves and the viscous overstability in Saturn’s rings

“…For reference, the first row shows a density wave in a homogeneous ring. The second case, with a region of increased τ 0 , bears some similarities with Figures 4 and 5 in Hedman and Nicholson (2016), showing profiles of the Mimas 5:2 density wave in Saturn's B ring which passes through a region of radial width ∼ 60 km where the normal optical depth increases sharply from about 1.5 to values 3 − 5. In the region of enhanced surface density in Figure 20 the wave damping is reduced due to its decreased wavenumber.…”

Axisymmetric density waves in Saturn’s rings

“…We analyze these occultation data using wavelet-based tools developed in Hedman & Nicholson (2016) for isolating wave signatures in Saturn's B ring. These tools are designed to take multiple occultation profiles and combine the data in a manner that isolates signals with pattern speeds and mvalues consistent with specified density waves.…”

“…The theory behind these patterns is very well developed (Goldreich & Tremaine 1982;Shu 1984), enabling ring parameters like the local surface mass density to be derived from observable wave properties (Cuzzi et al 1981;Esposito et al 1983;Tiscareno et al 2007;Esposito 2010). At the same time, the predictable properties of these waves allow wavelet-based filtering techniques to identify wave-like structures that are not apparent in individual observations (Hedman & Nicholson 2016). These tools are also starting to reveal unexpected waves with unusual properties, including several features that appear to be axisymmetric density waves (i.e.…”

Mean motion resonances with nearby moons: an unlikely origin for the gaps observed in the ring around the exoplanet J1407b

“…Orbital resonances may also exist with larger moons located external to the rings. Here, ring particles experience a gravitational tug at the same location in their orbit when the semimajor axis of ring particles and moon result in integer multiples of their respective orbital periods (Goldreich & Tremaine 1978;Lissauer & Cuzzi 1982) Hedman & Nicholson 2016). Spiral density waves in planetary rings from an outer moon, also known as Lindblad resonances, are formed when a particle in the ring has a radial epicyclic frequency integer multiples of the forcing frequency (Nicholson et al 2014;Shu 2016).…”