Particle Size Distribution in Saturn’s C Ring and Cassini Division from VIMS and UVIS Stellar Occultations

Abstract:

More than sixty VIMS (Visual and Infrared Mapping Spectrometer) and more than one hundred UVIS (Ultraviolet Imaging Spectrograph) photometric measurements of stellar occultation profiles have been collected since Cassini’s orbit insertion in June, 2004. The UVIS HSP (High Speed Photometer) collects photons with an effective wavelength λ_eff = 0.15 μm within a 6.4 mrad x 6.0 mrad field of view while, when operating in occultation mode, the VIMS instrument collects photons with λ_eff = 2.9 μm incident on a single pixel of angular dimensions 0.5 mrad x 0.25 mrad. Starlight is diffracted through an angle of θ ~ 1.22λ_eff/2a by ring particles of radius a. Due to the smaller angular dimensions of the VIMS pixel and longer measured wavelength, starlight can be diffracted out of its field of view by small particles (a~ 9 mm) and not replaced by diffraction from neighboring ring particles. This reduction in the measured intensity of starlight can lead to higher optical depths in VIMS occultations than in UVIS occultations. Because self-gravity wakes introduce additional differences in measured optical depths due to differences in viewing geometry, here we compare measurements in the C ring and Cassini Division where self-gravity wakes are not present. We find evidence of sub-cm particles in both the C Ring and Cassini Division. Minimum particles in the C Ring are relatively constant with radial distance from Saturn, with a mean of ~6mm. There is an increasing trend in minimum particle size outward through the Cassini Division.