The Cassini Ultraviolet Imaging Spectrograph (UVIS)’s High-Speed Photometer (HSP) observed numerous occultations of stars by Saturn’s rings over 13 years in orbit. The combined analysis of observations from multiple viewing configurations can be used to investigate different facets of the ring structure. Showalter and Nicholson (1990, Icarus, 87, 285) and Colwell et al. (2018, Icarus, 300, 150) interpreted excess variance above that predicted by Poisson counting statistics in terms of an effective particle size, RE, dependent on the length of shadows cast by the particles. However, their assumption of spherical particles is invalid in Saturn’s A ring, which is dominated by elongated clumps of particles called self-gravity wakes (e.g. Colombo et al. 1976, Nature, 264, 344; Colwell et al. 2006, Geophys. Res. Lett. 33, L07201; Hedman et al. 2007, Astron. J., 133, 2624). We present an analysis of the excess variance within three density waves in the A ring in terms of a “granola bar” model of self-gravity wakes that describes the wakes as rectangular blocks of width W, height H, and length L, with an average separation S. We find that regularly spaced wakes cannot match the observed statistics of the troughs and peaks of the waves simultaneously, suggesting that S and W vary on short timescales. Our results are consistent with Esposito et al. (2012, Icarus, 217, 103)’s predator-prey model which predicts that wakes grow and erode as they are perturbed by passing density waves, exhibiting a temporal response to forcing by nearby moons.

For more information: hdlander@eckerd.edu, mwriley@eckerd.edu