Spitale, J. N., Porco, C. C. (2010). "Detection of Free Unstable Modes and Massive Bodies in Saturn's Outer B Ring" Abstract P33D-02 presented at 2010 Fall Meeting, AGU, San Francisco, Calif., 13-17 Dec..


Abstract
Voyager images (Porco et al. 1984; Icarus 60) and Cassini occultation data (Hedman et al. 2010; AJ 139) have previously shown that the behavior of the outer edge of Saturn's massive B ring is determined only in part by a static response to the 2:1 inner Lindblad resonance. In Cassini images of this region, we find, in addition to the expected wavenumber-2 forced distortion, evidence for unforced self-excited wavenumber-3, wavenumber-2, and wavenumber-1 normal modes. These are the first observations to suggest substantial wave amplification in Saturn's broad rings. Moreover, the presence of these free modes strongly implicates viscous overstability as their underlying cause (Borderies et al. 1985; Icarus 63) and, by inference, the cause for most if not all of the unforced structures throughout the high-mass-density B ring and in other high-mass-density regions in Saturn's rings. Analysis of each of the inferred waves reveals a consistent lower bound on the average surface mass density of ~ 44 g cm-2 for the outer zone of the ring to as deep as 250 km, though the true surface density could be as high as 100 g cm-2 or higher. Interference between the forced and free wavenumber-2 modes yields a total wavenumber-2 pattern that varies in amplitude and orientation with a characteristic period of ~5.5 years. We also find localized disturbances, including 3.5-km-tall vertical structures, that provide circumstantial evidence for embedded massive bodies in the Mimas resonance zone. The presence of such bodies is supported by the direct discovery of a moonlet ~ 0.3 km wide near the ring's edge.

The Huygens ringlet, located ~ 250 km exterior to the massive B-ring edge, also possesses a complicated structure superimposed on its Keplerian shape (Spitale and Porco 2006; LPSC #37), and may be influenced by the B-ring normal modes described above, as well as by the Mimas resonance (Porco 1990; Adv. Space Res. 10(1)). Using the parameters that we determined for the variation at the B-ring edge we hope to untangle the various perturbations on the Huygens ringlet in order to produce estimates for its physical properties.