Spitale, J. N., Porco, C. C. (2010). "Detection of Free Unstable Modes and Massive Bodies in Saturn's Outer B Ring" American Astronomical Society, DPS meeting #42, #19.06.


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 non-resonant, ≥100-m-class structures throughout the high-mass-density B ring and in other high-mass-density regions in Saturn's rings. Analysis of the inferred waves yields a lower bound on the average surface mass density of ~44 g cm² for the outer 250 km of the ring, though the true surface density could be as high as 100 g cm² 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.