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Session 8 - Planets and Comets.
Display session, Monday, June 08
Atlas Ballroom,
The brief and dramatic appearance of Comet Shoemaker--Levy 9 (S--L 9) has punctuated the notion that many small members of the solar system might be strengthless `rubble-piles'. Models of the S--L 9 encounter indicate that only a relatively strengthless rubble-pile is able to catastrophically disrupt into a cloud of debris that later condenses into twenty or so gravitating `clumps' having the S--L 9 `string-of-pearls' morphology (e.g., Asphaug and Benz 1996). These models show that gravitational condensation of the debris into distinct clumps depends sensitively on the progenitor's density as well as its orbit. This phenomenon is re-examined by tracking the orbital motion andthe local mass density of a tidally disrupted projectile (see also Rettig et al. 1996). By employing elementary orbit mechanics, it is shown that when the debris' gravitational contraction timescale becomes shorter than its orbital spreading timescale, the debris breaks up into n L/D distinct clumps, where L is the debris length and D is the progenitor's diameter. Without appealing to a CPU--intensive calculation, we extend the available N--body simulations to unexplored regions of parameter space and reveal how the number of clumps n depends upon the progenitor's density, its periapse distance, and velocity at infinity (the problem is generalized to hyperbolic encounters as well). These findings also provide an additional constraint for the Galilean crater chain problem. We find that the projectile's responsible for the Gomul and Gipul crater chains on Callisto likely had comet--like densities of p<1 gm/cm^3. However we are unable to distinguish between cometary and asteroidal impactors for the remaining chains which have fewer numbers of craters.
