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Session 5 - Solar System Objects.
Display session, Monday, June 09
South Main Hall,
As first revealed in observations by the Voyager spacecraft, there are distinctive radial optical depth structures near the inner edges of Saturn's A and B Rings. Earlier work (for instance, Durisen et al. 1996 Icarus 124 220) has demonstrated that some of these features, including the edges themselves, can be produced or maintained by "ballistic transport," that is, radial transport of mass and angular momentum due to exchanges of ejecta from meteoroid impacts on ring particles. In the simulations published to date, the calculation of net exchanges of impact ejecta has involved accurate, computationally-intensive triple integrals over the ejecta velocity distribution for each radial zone at each time step. The computational cost of these integrations restricted studies to only relatively few calculations that spanned no more than 100 characteristic times t_c (where t_c \approx 10^5 yrs). Work by others (J.N. Cuzzi, private communication) suggests that meteoroid exposure ages for the rings are more like 300 to 1000 t_c. By making analytic fits to the detailed ejecta velocity distribution and simplifying other aspects of the exchange integrals, we have reduced the integrals to single integrals and have increased the speed of our computational algorithm by a factor of 10 to 100. Long evolutions with the older, more precise scheme were used to fine-tune the fits. Calculations of 300 to 1000 t_c on workstations are now routine. Results will be presented for a wide range of plausible input physics over these longer times. These simulations show structural features due to ballistic transport which were not evident in calculations at 100 t_c. The new fast code will also permit more complex and realistic mixes of ejecta types to be included in future calculations. This work was supported by NASA Grant NAGW-4587.
