A vertical rift in Saturn's inner C ring

In 1988 Rosen and Lissauer identified an unusual wavelike feature in Saturn's inner C ring as a bending wave driven by a nodal resonance with Titan (Science 241, 690). This is sometimes referred to as the −1:0 resonance since it occurs where the local nodal regression rate is approximately equal to −nT, where nT=22.577∘d−1 is Titan's orbital mean motion. We have used a series of 44 stellar occultation profiles of this wave observed by the Cassini VIMS instrument to test their hypothesis. We find that, as predicted, this wave is an outward-propagating m=1 spiral with a leading orientation and a retrograde pattern speed approximately equal to −nT. But the most intriguing feature associated with the wave is a narrow gap that lies ∼7 km outside the resonance. This gap varies noticeably in width and is seen in roughly 3/4 of the occultation profiles, appearing to rotate with the wave in a retrograde direction. We have developed a simple kinematical model that accounts for the observations and consists of a continuous but very narrow gap (radial width ≃ 0.6 km), the edges of which are vertically distorted by the propagating bending wave as it crosses the region. Differences in viewing geometry then largely account for the apparent width variations. We find vertical amplitudes of 3.8 km for the inner edge and 1.2 km for the outer edge in 2008, with nodes misaligned by ∼110°. Moreover, both edges of the gap are slightly eccentric, with pericenters aligned with Titan, suggesting that the eccentricities are forced by the nearby Titan apsidal resonance. We hypothesize that the gap forms because the local slopes in the bending wave become so great that nonlinear effects result in the physical disruption of the ring within its first wavelength, beyond which point the wave re-establishes itself with a reduced amplitude.