ESTIMATES OF COMET FRAGMENT MASSES FROM IMPACT CRATER CHAINS ON CALLISTO AND GANYMEDE

Chains of impact craters, or catenae, have been identified in Voyager images of Callisto and Ganymede. Although these resemble in some respe cts secondary crater chains, the source craters and basins for the cat enae cannot be identified. The best explanation, proposed by Melosh an d Schenk, is a phenomenon similar to that displayed by former comet P/ Shoemaker-Levy 9: tidal (or other) breakup close to Jupiter followed b y gradual orbital separation of the fragments and collision with a Gal ilean satellite on the outbound leg of the trajectory. Because the tra jectories must pass close to Jupiter, this constrains the impact geome try (velocity and impact angle) of the individual fragments. For the d ominant classes of impactors, short-period Jupiter-family comets and a steroids, velocities at Callisto and Ganymede are dominated by jovian gravity and a satellite's orbital motion, and are insensitive to the p re-fragmentation heliocentric velocity; velocities are insensitive to satellite gravity for all impactor classes. Complex crater shapes on C allisto and Ganymede are determined from Voyager images and Schmidt-Ho lsapple scaling is used to back out individual fragment masses. We fin d that comet fragment radii are generally less than similar to 500 m ( for ice densities), but can be larger. These estimates can be compared with those for the Shoemaker-Levy 9 impactors.