Sizes of Particles, Clumps, and Holes in Saturn's Rings from Cassini UVIS Stellar Occultation Statistics
Abstract
The high speed photometer (HSP) aboard Cassini's Ultraviolet Imaging Spectrograph (UVIS) observed more than 170 ring stellar occultations between 2004 and 2017. Because photon counts are described by Poisson statistics, we expect our data to exhibit a variance equal to the mean when there is no ring material obstructing the view of the instrument. Large ring particles obstruct the field of view in an irregular manner, causing an excess variance related to the size of the largest particles or clumps in the rings. Showalter and Nicholson (1990, Icarus, 87, 285) analyzed the Voyager 2 stellar occultation of dScorpii to obtain an effective particle size across the rings except for the densest regions of the B ring. Colwell et al. (2018, Icarus, 300, 150) analyzed the excess variance across the rings in two UVIS occultations. Here we remove secular trends in the data to allow for a determination of the excess variance due to finite particle or clump sizes within regions where there is structure in the rings, such as within density waves and at the edges of ringlets and plateaus. We determine a best fit by testing varying degrees of polynomial fits to the data. After removing the trend due to underlying ring structure we will explore variations in particle or clump size across small spatial scales that were not included in Colwell et al. (2018). In particular, this trend removal allows for examination of the particle properties within density wave troughs, within the Mimas 5:3 bending wave, and across the relatively broad edges of C ring plateaus where Colwell et al. (2018) found transitions in particle sizes. Examination of the next higher moment of the data, the skewness, provides information on the distribution of localized holes, nicknamed "ghosts" (Baillie et al. 2012, Astron. J., 145, 171), in the rings which are necessary to explain the skewness observed in the C ring. We will present results from the Cassini data with comparisons to Monte Carlo simulations of the distribution of ring particle sizes and holes in the rings.
- Publication:
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AAS/Division for Planetary Sciences Meeting Abstracts #50
- Pub Date:
- October 2018
- Bibcode:
- 2018DPS....5011704E