Article,
ESTIMATES OF COMET FRAGMENT MASSES FROM IMPACT CRATER CHAINS ON CALLISTO AND GANYMEDE
Geophysical Research Letters, 22 (13): 1829--1832 (1995)
Abstract
Chains of impact craters, or catenae, have been identified in Voyager
images of Callisto and Ganymede. Although these resemble in some
respects secondary crater chains, the source craters and basins for
the catenae cannot be identified. The best explanation, proposed
by Melosh and Schenk, is a phenomenon similar to that displayed by
former comet P/Shoemaker-Levy 9: tidal (or other) breakup close to
Jupiter followed by gradual orbital separation of the fragments and
collision with a Galilean satellite on the outbound leg of the trajectory.
Because the trajectories must pass close to Jupiter, this constrains
the impact geometry (velocity and impact angle) of the individual
fragments. For the dominant classes of impactors, short-period Jupiter-family
comets and asteroids, velocities at Callisto and Ganymede are dominated
by jovian gravity and a satellite's orbital motion, and are insensitive
to the pre-fragmentation heliocentric velocity; velocities are insensitive
to satellite gravity for all impactor classes. Complex crater shapes
on Callisto and Ganymede are determined from Voyager images and Schmidt-Holsapple
scaling is used to back out individual fragment masses. We find 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.
