Burns, J. A., Hedman, M. M. Showalter, M. R. (2010). "Tilting at Tilted Rings" American Astronomical Society, DPS meeting #42, #19.02.
Saturn's D (Hedman et al. 2007) and C (Hedman et al. 2010) rings display sinusoidal brightness variations in which the radial wavelength increases smoothly with distance from the planet. These structures likely represent 2-200m vertical corrugations that arose due to differential nodal regression (driven by Saturn's higher-order gravity harmonics) of an initially flat, tilted ring. Jupiter's rings exhibit similar features (Showalter & Hedman, this meeting). The spatial and temporal behaviors of these patterns imply that the ring's angular momentum vector shifted by about 10-7 radians relative to that of the planet in mid-1984; after that date, the once-flat ring subsequently wound up.|
We consider scenarios in which the inner parts of Saturn's ring system were tilted relative to the planet's equator by the impact of external material moving at several tens of km/sec. In order for the angular momentum to be distributed across many thousand kilometers, the interplanetary projectile must first fragment (tidally or collisionally) into many pieces. This permits a wide swath of ring to be eventually struck by smaller debris. Even though much of the ring is hit, planetary shadowing and gravitational focusing allow angular momentum to be emplaced non-uniformly in longitude, such that the net angular momentum has a component in the ring plane. To produce the necessary tilt, the impactor must be >/~ 1 km in radius, the size of a small comet nucleus. We estimate that comets this big could pass close enough to Saturn for disruption at ~2% of Jupiter's impact rate. Accordingly, Saturn's rings could potentially be tilted every 500-10,000 yrs.
We have explored other possible scenarios for tilting the rings (e.g., direct hits of large ring particles, atmospheric break-up, and changes in Saturn's mass distribution that shift the planet's inertia tensor). Currently, they seem even less plausible.