Abstract
We use Monte Carlo methods to simulate impacts of ecliptic comets
on the synchronously rotating satellites of giant planets. We reconfirm
the long-standing prediction that the cratering rate should be much
higher on the leading hemispheres than on the trailing hemisphere;
indeed we find that previously published analytical formulations
modestly underestimate the degree of apex-antapex asymmetry to be
expected. We then compare our results to new mapping of impact craters
on Ganymede, Callisto, and Triton. Ganymede reveals a pronounced
apex-antapex asymmetry that is nonetheless much less than predicted.
All of Triton's confirmed impact craters are clustered toward the
apex of motion, far exceeding the predicted asymmetry. No asymmetry
is observed on Callisto. In each case at least one of our basic assumptions
must be wrong. Likely candidates include the following: (i) the surfaces
of all but the most sparsely cratered satellites are saturated or
nearly saturated with impact craters; (ii) these satellites have
rotated nonsynchronously over geological time; (iii) most of the
craters are made not by heliocentric (Sun-orbiting) comets and asteroids
but rather by planetocentric (planet-orbiting) debris of indeterminate
origin; or (iv) pathological endogenic resurfacing has created illusions
of structure. Callisto's surface is readily classified as nearly
saturated. Ganymede's bright terrains, although less heavily cratered
than those of Callisto, can also be explained by crater densities
approaching saturation on a world where endogenic processes were
active. The leading alternative is nonsynchronous rotation, an explanation
supported by the distribution of catenae (crater chains produced
by impact of tidally disrupted comets). Triton's craters can be explained
by planetocentric debris or by capricious resurfacing, but both hypotheses
are inherently improbable. (C) 2001 Academic Press.
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