Characterizing Particle Size Distributions and Outlier Populations in Saturn's Rings using UVIS Occultation Statistics

The unocculted time series data from the Cassini Ultraviolet Imaging Spectrograph (UVIS) High Speed Photometer (HSP) follow Poisson counting statistics, such that the second and third central moments of the unocculted data are expected to equal the mean unocculted photon rate of the star. Upon occultation, these central moments deviate from their expected values; the nature of these deviations changes with the observed optical depth. The normalized difference of the variance over the mean can be used to infer an effective particle size (Showalter and Nicholson 1990, Icarus, 87, 285; Colwell et al. 2018, Icarus, 300, 150). The normalized difference of the third central moment, μ3, over the mean can also provide another estimate of particle size. Furthermore, the introduction of outlier features, namely small gaps (dubbed 'ghosts' by Baillie et al. 2013, AJ, 145, 171) and clumps, produces variable effects on the behavior of both moments. By exploiting these phenomena, we can characterize both the local particle size distributions and outlier populations within low-optical depth regions of the rings. We compare the higher-order moments from Monte Carlo simulations of a simplified ring system to those of the Cassini UVIS data to gain insight on such features in the C Ring and Cassini Division.