Hedman, M., Burns, J., Tiscareno, M., Nicholson, P., Porco, C., Showalter, M., Bosh, A. (2006). "Remarkably Regular Ripples in Saturn's D Ring." American Astronomical Society, DDA meeting #37, #47.05.

Cassini observations have revealed that Saturn's innermost ring, the D ring, is extremely complex and dynamic, changing significantly over time scales as short as decades. Here we focus on structures in the outermost D ring (73,200 - 74,000 km), just interior to the C ring. A regular pattern with a wavelength of 30 km is observed across this region. Observations at small opening angles hint that this pattern is due to a vertical corrugation in the ring with a wave height of order a few km. A similar pattern was observed by HST during a shallow-elevation (B=2.7 degrees) stellar occultation on 21-22 November 1995, but at that time the wavelength was 60 km. The wavelength of this corrugation thus seems to shorten with time. Furthermore, the HST and Cassini data suggest that the wave number increases linearly at a rate (2.54 +/- 0.02) x 10-5 /km/day. This rate is consistent with differential nodal regression of an initially thin, slightly inclined ring, further supporting our interpretation of this pattern as a vertical corrugation. Given the proximity of this structure to Saturn and its radial extent, one should be able to place strong constraints on a combination of Saturn's J2n, including terms with n > 10, by watching the wave pattern tighten during the remaining years of the Cassini mission. Unwinding the corrugated spiral backwards in time indicates that this region of the D ring was a simple inclined sheet in the year 1984. We propose that a comet or meteoroid may have collided with the D ring at this time, disrupting a D-ring parent body and slightly tilting the angular momentum of the resulting debris.