G Ring Particle Sizes Derived from Ring Plane Crossing Observations

Abstract

The Saturn ring plane crossings in 1995–1996 allowed observers using the Hubble Space Telescope and the W. M. Keck telescope to image the planet's diffuse rings from 0.3 to 2.2 μm at a scattering angle θ ∼ 175°. We calculate the G ring reflectance for size distributions of dust to kilometer-sized bodies derived from a physical, evolutionary model. The model tracks the evolution of the G ring from its initial formation following the disruption of a progenitor satellite (R. M. Canup and L. W. Esposito, 1997,Icarus126,28–41) until a steady state distribution is reached. We calculate the total particle scattering from contributions due to Mie scattering, isotropic scattering, and Lambert scattering and compare the spectra, phase curves, and RMS particle mass from our physical model to that observed by HST, Keck, and Voyager. A range of particle size distributions from the models are consistent with the observations. These distributions have a dust component that can be described by the differential power law exponentqdust, in the range 1.5–3.5. A quasi-Gaussian size distribution centered at 15 μm also matches the observations, although is not predicted by the evolutionary model. Distributions withqdust≳ 4, such as that proposed by M. R. Showalter and J. N. Cuzzi (1993),Icarus103,124–143) based on Voyager G ring photometry, are too blue to match the spectrum. In order to fit the visible optical depth, many of the models require longer particle lifetimes against plasma drag than Voyager plasma measurements imply. This may suggest that plasma densities are overestimated, that the ring has unaccounted-for dust sources, or that the ring is not in steady-state and we are seeing it at a particularly bright moment.