[Previous] | [Session 16] | [Next]
R. Karjalainen, H. Salo (Univ. of Oulu)
The formation of semi-permanent particle groups in the outer parts of Saturn's rings is studied via local simulations including the mutual collisions and gravitational interactions between particles. Different factors, including the internal density of particles, their elasticity, and size distribution, are studied. Special attention is paid for removing the uncertainties caused by simulation-related parameters (e.g. number of particles, time step, method of force calculations). Also, for each distance and particle properties, typically 20 separate experiments are carried out, lasting 50 orbital periods. Simulations indicate that for 1-meter sized identical particles with solid ice density (\rho = 900 \ kg/m3), and Bridges et al. velocity-dependent elasticity, (\epsilonB(v)), the particle groups start forming at distance a = 136 \ 000 \ km. However, the transition from non-accretion to accretion takes place over a relatively wide transition zone: only beyond 145 \ 000 \ km are stable groups seen in practically all separate experiments. For constant \epsilon = 0.5 the behavior is about the same as with \epsilonB(v), whereas for \epsilon = 0.1 the transition zone is moved closer to planet by almost 10 \ 000 \ km's. In the case of power-law size distribution the transition to aggregate formation moves inward about 8 \ 000 \ km (for q = 3, and 0.5 < r < 5 m). For different internal densities these distances scale with \rho-1/3. The stable groups seen in simulations have typically retrograde rotation whereas groups with prograde rotation often dissolve in about 5 to 10 orbital revolutions. Preliminary studies of the effect of particle spins and surface friction is also in progress.
This study is supported by Väisälä foundation and the Academy of Finland.