G Ring Particle Sizes Derived from Ring Plane Crossing Observations

The Saturn ring plane crossings in 1995{96 allowed observers using the Hubble Space Telescope and the W. M. Keck telescope to image the planet's di use rings from 0.3 m { 2.2 m at a scattering angle 175 . We calculate the G ring re ectance for size distributions of dust to km-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 (Canup & Esposito 1997), 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 di erential power law exponent qdust, in the range 1.5{3.5. A quasi-Gaussian size distribution centered at 15 m also matches the observations, although is not predicted by the evolutionary model. Distributions with qdust> 4, such as that proposed by Showalter & Cuzzi (1993) based on Voyager G ring photometry, are too blue to match the spectrum. In order to t 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 unaccountedfor dust sources, or that the ring is not in steady-state and we are seeing it at a particularly bright moment.