Saturn's C ring and Cassini division: Particle sizes from Cassini UVIS, VIMS, and RSS occultations
Saturn's C ring and Cassini Division share many morphological traits: both contain numerous opaque, sharp–edged ringlets and gaps, broader low optical depth “background” regions, larger-optical depth regions that rise abruptly from the background (known as the C ring's plateaus and the Cassini Division's triple band feature), and linear ramps in optical depth up to the abrupt inner edges of the B ring and A ring, respectively. Throughout the majority of both regions, the surface mass density of the rings is small enough that the Toomre critical wavelength (most unstable wavelength for gravitational collapse) is comparable in size to, or smaller than, the largest individual ring particles. Thus, self-gravity wakes do not form in these regions, unlike the A and B rings where the critical wavelength is tens of meters and the self-gravity wakes introduce strong dependence of the observed optical depth on viewing geometry. In the absence of self-gravity wakes, we model the ring particle size distribution with a simple power-law, where the number of particles per unit area of the rings in the size range [a, a + da] is given by n(a)da = Ca-qda between amin and amax. We fit normal optical depths derived from the power-law size distribution parameters and the thin-layers ring model of Zebker et al. (1985) to the wavelength-dependent optical depth profiles obtained by 3 Cassini Instruments: UVIS at λ = 0.15 μm, VIMS at λ = 2.9 μm, and RSS Ka-band (λ = 9.4 mm), X-band at (λ = 3.4 cm), and S-band (λ = 13.0 cm). We find that the C ring is best characterized by five or more thin layers of particles with a mean power-law index of q ~ 3.16 in the C ring background and q ~ 3.05 in the C ring plateaus, in the rings. We find a minimum particle radius of amin ~ 4.1 mm in the background C ring and plateaus and amin ~ 6 mm in the plateaus. The cross-section-weighted effective particle radius determined using the excess variance of UVIS signal beyond Poisson counting statistics by Colwell et al. (2018) constrains the size of the largest particles in the rings. We find the largest particles contributing to the power-law size distribution, and thus to the optical depth, are amax ~ 10–15 m in the background C ring and amax ~ 5–6 m in the C ring plateaus. This substantial difference in the sizes of the largest ring particles together with the overall shallower power law index in the plateaus explains their optical depth difference relative to the background C ring. Additionally, Baillié et al. (2013) used UVIS stellar occultations to find a distribution of small-scale, low optical depth gaps in the plateaus. These regions, with radial widths of <100 m and dubbed “ghosts”, also appear in Cassini ISS images (Tiscareno et al., 2019) as “streaky” texture with an azimuthal length scale of many km but widths near the pixel scale of the images (<300 m). Baillie et al. (2013) proposed that the ghosts are propeller features (Tiscareno et al. 2006) like those found in the A ring, opened by particles or aggregates with Hill radii <20 m in radius. Best-fit values of amax confirm that particles between 5 and 20 m sparsely populate the C ring plateaus and do not to contribute significantly to the measured normal optical depth, but their presence in smaller numbers could account for the “streaky” texture as azimuthally limited gaps or propellers in the plateaus and in the Cassini Division's triple band feature. In the background C ring and background Cassini Division, where amax is ~12 m, these larger particles or aggregates are ubiquitous and do contribute significantly to the measured normal optical depth, but their abundance and closer spacing may disturb the formation of azimuthally limited gaps such as those seen in the C ring plateaus. In the Cassini Division background and ramp, we find a shallower power law index of q ~ 3.0, and in the triple band feature we find q ~ 2.9. We find a similar smallest particle size cutoff as in the C ring but larger largest particles in the Cassini Division ramp. We constrain the particle densities by dividing the surface mass densities determined from the dispersion of spiral density waves by the total particle volume integrated over a square meter of the ring slab. We derive low bulk particle densities of ρ ~ 0.1–0.3 g/cm3 except in the central background C ring where we find ρ ~ 0.9 g/cm3. These low derived densities may be due to inter-particle spaces within 1–20 m particle aggregates that contribute to the measured optical depth and thus to the upper end of the size distribution. Our results are consistent with Zhang et al. (2017) who reported particles with porosities >75% made of water ice with up to 11% silicate contaminate in the central background C ring.
National Aeronautics and Space Administration
Richard G. Jerousek, Joshua E. Colwell, Matthew M. Hedman, Richard G. French, Essam A. Marouf, Larry W. Esposito, and Philip D. Nicholson. "Saturn's C ring and Cassini division: Particle sizes from Cassini UVIS, VIMS, and RSS occultations" Icarus (2020). https://doi.org/10.1016/j.icarus.2019.113565