  Voyager images and Cassini occultation data 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 with Mimas. 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 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 250 km of the ring, 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
presence of a shadow-casting moonlet ~ 0.3 km wide near the ring's edge.

Earth and Planetary Astrophysics