Characterization of Particle Sizes and Local Gaps in Saturn’s C Ring from Stellar Occultation Data

The Ultraviolet Imaging Spectrograph High-Speed Photometer (UVIS HSP) aboard the Cassini spacecraft collected stellar occultation data from stars as they were occulted by Saturn’s rings. The variance of the data exceeds that expected from Poisson statistics due to the finite sizes of clumps and particles in the rings, and this has been used to study variations in mean particle size (Colwell et al. 2018, Icarus, 300, 150). The skewness of the data is related to both particle size and the presence of outliers (unusually high or low counts) that can be caused by big clumps in low optical depth regions or gaps or holes in moderate to high optical depth regions (Green et al. 2019, AGU #P23C-3506). We created a library of occultation statistics from simulated occultations of Monte Carlo simulations of a two-dimensional ring system composed of circular particles with interspersed clumps and small gaps or “ghosts”. We vary the effective particle radius (R), the probability of a measurement encountering a ghost (GP), and the probability of encountering a clump (CP). We calculate the skewness (S) and the excess variance (E) from the simulated occultations, over a range of optical depths. We find that ghosts are necessary to simultaneously fit E and S in the C ring plateaus. We test different occultations across a range of inclination angles and star brightnesses and find effective particles sizes on the order of 1 m, and a small population of ghosts, from 0.1-2% of measurement samples. In the background C ring, we find that the abundance of ghosts is less than in the plateaus while the mean particle size is larger, consistent with Colwell et al. (2018). Additionally, we generate three-dimensional, collisional, self-gravity n-body simulations, with embedded moonlets that produce localized openings referred to as “propellers” (Tiscareno et al. 2006, Nature, 440, 648). Simulated occultations of these n-body simulations produce enhanced skewness like that seen in the UVIS data, confirming that large embedded objects may be the cause of the ghosts in the C ring plateaus revealed by the skewness of the UVIS occultation data. Future work will extend these analyses to other ring regions such as the A and B rings.