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**Session 5 - Solar System Objects.**

*Display session, Monday, June 09*

*South Main Hall, *

## [5.03] Numerical Simulations of Structure in Saturn's Rings due to Ballistic Transport

*H. M. Pickett, R. H. Durisen, R. Tripoli (Indiana U.)*
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.

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