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A Close Look at the C Ring Plateaus from Cassini Stellar and Radio Occultations

Presentation #301.03 in the session Ring Systems: Planetary Rosetta Stones.

Published onOct 20, 2022
A Close Look at the C Ring Plateaus from Cassini Stellar and Radio Occultations

Saturn’s C ring plateaus are bands a few hundred km in radial extent with average normal optical depths that are roughly a factor of four larger than the surrounding background C ring. While their origin and confinement are still poorly understood, stellar and radio occultations provide important clues. The C ring plateau edges are all resolved at the resolution of Cassini stellar and radio occultations. When viewed near normal incidence, plateaus typically are more opaque near one or both of their edges than at their centers giving a “U”-shaped or ramp-shaped appearance in normal optical depth profiles. Over the 13-year extended mission, stellar occultations measured by the Ultraviolet Imaging Spectrograph (UVIS) and Visual and Infrared Mapping Spectrometer (VIMS) as well as Radio Science Subsystem (RSS) radio occultations of the rings probed a wide range of viewing geometries and with 5 wavelengths spanning ~0.15 μm – 13 cm. For RSS radio occultations, the diffraction corrected signal was determined for simultaneous Ka-band (λ = 0.94 cm), X-band (λ = 3.6 cm), and S-band (λ = 13 cm) observations. Differential optical depth measurements place tight constraints on the particle size distribution parameters, and S-band observations do not show the characteristic peaks at the plateau edges that X-band, Ka-band, UVIS or VIMS observations show, indicating that the peaks are largely due to an increased abundance of 4 – 13 cm-sized particles near the plateau edges. Furthermore, stellar and radio occultation normal optical depths measured at very low ring opening angles (B < 5°) are inverted compared to observations at large B, with lower optical depth near the plateau edges suggesting the particle size distribution varies with distance from the ring plane.

Streaky texture seen in Cassini Imaging Subsystem (ISS) images and holes dubbed “ghosts” observed by UVIS stellar occultations show that the plateaus consist of irregularly-spaced, azimuthally-limited semi-transparent regions ~ 20 – 30 m in radial extent presumably due the depletion of particles in the vicinity of 5 – 10 m propeller moonlets. We present a wavelength and viewing geometry dependent model to constrain the vertical distribution of particle sizes near the plateau edges. We constrain the average vertical thickness of the cm-sized particles in the plateaus to less than ~ 40 m, consistent with dynamical models of 5 – 10 m propellers.

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