Title: Cratering Rates on the Galilean Satellites

Authors: Kevin Zahnle, Luke Dones, & Harold F. Levison

Status: To appear in Icarus.

Abstract: We exploit recent theoretical advances toward a theory of the origin and orbital evolution of comets and asteroids to obtain revised estimates for cratering rates in the jovian system. We find that more than 90%, of the craters on the Galilean satellites are caused by the impact of Jupiter-family comets (JFCs). These are comets with short periods, in generally low-inclination orbits, whose dynamics are dominated by Jupiter. Nearly isotropic comets (long period and Halley-type) contribute at the 1-10% level. Trojan asteroids might also be important at the 1-10% level; if they are important, they would be especially important for smaller craters. Main belt asteroids are currently unimportant, as each 20 km crater made on Ganymede implies the disruption of a 200 km diameter parental asteroid, a destruction rate far beyond the resources of today's asteroid belt.

Twenty kilometer diameter craters are made by km-size impactors; such events occur on a Galilean satellite about once in a million years. The paucity of 20 km craters on Europa indicates that its surface is of order 10 Ma. Lightly cratered surfaces on Ganymede are nominally of order 0.5-1.0 Ga. The uncertainty in these estimates is about a factor of five. Callisto is old, probably more than 4 Ga. It is too heavily cratered to be accounted for by the current flux of JFCs. The lack of pronounced apex-antapex asymmetries on Ganymede may be compatible with crater equilibrium, but it is more easily understood as evidence for nonsynchronous rotation of an icy carapace.

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