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D. E. Dunn (University of California at Berkeley), L. A. Molnar (Calvin College), I. de Pater (University of California at Berkeley), J. J. Lissauer (NASA Ames)
We have developed a Monte Carlo simulation for detailed modeling of the radio emission from Saturn's rings. We treat the ring as a layer of spheres whose scattering properties are determined by a linear combination of Mie and isotropic phase functions. Other adjustable parameters include the particle size distribution, layer optical depth, and dust fraction, but we generally fix these parameters at values determined by the Voyager observations. In our radiative transfer model, we include multiple scattering and a complete description of the geometry. Multiple scattering becomes important because of the high albedo of the particles and the high optical depth in some portions of the ring.
Also included in the model are nonuniform spatial particle distributions such as wakes (e.g., Daisaka and Ida, Earth Planets Space, 51, 1195 [1999]). The wakes are idealized, consisting of parallel, rectangular structures of uniform number density. We will also model the wakes to have constant and nonconstant interwake spacing. Though simplistic, this allows us to easily explore the full range of parameter space (wake width, height, and spacing as well as the relative density). We can also parameterize realistic wake calculations into our model.
We present results from these models as applied to Saturn's rings. With the inclusion of the wakes, an E/W asymmetry can arise both on the ansae and in front of the planet. This effect is more or less pronounced depending on what wake parameters are chosen. Furthermore, the variable wake spacings allow direct planetary emission through the ring at ring inclinations far to shallow to permit such emission through a uniform ring. We compare our results with an analytic geometric model which ignores multiple scattering. This allows us to understand the physical phenomenon underlying the E/W asymmetry (e.g., the relative importance of multiple scattering).
We also compare preliminary runs of the model to Very Large Array microwave data. These maps range both in inclination and wavelength. We demonstrate the sensitivity of the model to the various parameters of the rings.
This project is funded by NASA's Planetary Astronomy Program under grant NAG5-6544 and by Research Corporation.