A photometric study of Saturn's Ring

Abstract

Constraints on the particle properties in Saturn's F Ring have been derived from Voyager images and occultation data. We have measured the ring's radially integrated brightness over a wide range of phase angles (7° to 156°) from the Voyager images. Whenever possible, measurements have been repeated in multiple images over a wide range of longitudes in order to average out the ring's intrinsic brightness variations. To model the resultant phase curve we have devided the ring population into two regimes: dust, of size comparable to or smaller than the wavelength of light (0.5 μm), and larger bodies. We model the single scattering properties of the small particles using a semiempirical theory for scattering by randomly oriented, nonspherical particles; scattering by the large bodies is based on the photometric behavior of satellites. We apply a doubling algorithm to solve the multiple scattering problem and to include the contribution of Saturn-shine to the incident radiation field. The free parameters in our models include the power law index q of the dust size distribution and the fractional contribution f of the dust to the total optical depth. Least-squares fits of this model to the imaging phase curve yield q = 4.6 ± 0.5 and ⨍ ≥ 98% . Comparison of optical depth profiles across the F Ring at wavelengths of 0.264 μm, 3.6 cm, and 13 cm indicates that centimeter-sized particles are the dominant source of opacity in a core ∼ 1km wide, while the micrometer-sized dust dominates in a much wider “envelope” that extends inward from the core. We suggest that the dust in the envelope arises from micrometeoroid impacts into the larger core particles and then migrates inward under Poynting-Robertson drag.