Exploring Overstabilities in Saturn's a Ring Using Two Stellar Occultations

Abstract

Certain regions of Saturn's rings exhibit periodic opacity variations with characteristic radial wavelengths of up to a few hundred meters that have been attributed to viscous overstabilities. The Visual and Infrared Mapping Spectrometer (VIMS) onboard the Cassini spacecraft observed two stellar occultations of the star γ Crucis that had sufficient resolution to discern a sub-set of these periodic patterns in a portion of the A ring between 124,000 and 125,000 km from Saturn center. These data reveal that the wavelengths and intensities of the patterns vary systematically across this region, but that these parameters are not strictly determined by the ring's average optical depth. Furthermore, our observations indicate that these opacity variations have an azimuthal coherence scale of around 3000 km.

Density waves and the viscous overstability in Saturn’s rings

“…We applied our scheme to investigate the damping behavior of spiral density waves in a planetary ring which is subject to viscous overstability. The results of our large-scale hydrodynamical integrations confirm the observation that resonantly forced spiral density waves can co-exist with short-scale waves generated by the viscous overstability (Hedman et al (2014)), an aspect not taken into account in existing models for the damping of density waves. Due to our approximation of the azimuthal derivative terms the free short-scale overstable modes appearing in our hydrodynamical integrations are also non-axisymmetric with the same azimuthal periodicity m as the spiral density wave.…”

“…The Cassini mission to Saturn has revealed a vast abundance of structures in the planet's ring system, spanning a wide range of length scales. The finest of these structures have been detected by several Cassini instruments (Colwell et al (2007); Thomson et al (2007); Hedman et al (2014)) and are periodic and quasi-axisymmetric 1 with wavelengths of some 100 m. It is generally accepted that this periodic micro structure originates from the viscous overstability mechanism which has been studied so far only in terms of axisymmetric models Tscharnuter (1995, 1999); Spahn et al (2000); Salo et al (2001); Schmidt and Salo (2003); Latter and Ogilvie (2008; Rein and Latter (2013); Lehmann et al (2017)). On much greater scales, typically 10's to 100's of kilometers, numerous spiral density waves propagate through the rings, as these are excited at radii where the orbiting ring particles are in resonance with the gravitational perturbation of one of the moons orbiting the ring system.…”

A Weakly Nonlinear Model for the Damping of Resonantly Forced Density Waves in Dense Planetary Rings

“…Overstability in the inner B ring might explain the remarkable length (> 500 km) of the Janus 2:1 wave train (Colwell et al (2009b)). On the other hand, visible wave signatures extend to less than about 200 km for two waves propagating in the overstable region of the A ring (Atlas 7:6 and Pan 10:9, Hedman et al (2014) Figure 5). This appears surprising in view of the results from our weakly nonlinear model and from the BGT model.…”

Planetary Rings and Other Astrophysical Disks

“…But density waves have been observed on much shorter scales as well. Both axisymmetric and non-axisymmetric density structures appear with roughly 100 m wavelengths in RSS, UVIS, and VIMS observations (Thomson et al, 2007;Colwell et al, 2007;Hedman et al, 2014). The small-scale non-axisymmetric wakes, in particular, give rise to a striking large-scale effect, the azimuthal brightness asymmetry (Camichel, 1958;Colombo et al, 1976;Thompson et al, 1981;Salo, 1992).…”

“…The fastest growing modes favour scales of some 100 m, a tiny fraction of r Tscharnuter, 1995, 1999;Salo et al, 2001;Schmidt et al, 2001), and as a consequence, the saturation mechanism is controlled by nonlinear travelling waves (Schmidt and Salo, 2003;Ogilvie, 2009, 2010;Rein and Latter, 2013). It is generally accepted that the fine-scale axisymmetric density waves in Saturn's A and B-rings are generated and sustained by viscous overstability (Thomson et al, 2007;Colwell et al, 2007;Hedman et al, 2014).…”