31st Annual Meeting of the DPS, October 1999
Session 22. Pluto and Triton
Contributed Oral Parallel Session, Tuesday, October 12, 1999, 8:30-9:10am, Sala Pietro d'Abano

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[22.02] Cratering Rates in the Outer Solar System

K. Zahnle (NASA Ames Research Center), H. Levison, L. Dones (S.W.R.I.), P. Schenk (L.P.I.)

We use numerical simulations of the orbital evolution of stray Kuiper Belt objects to relate the number of comets striking the planets to the number of Jupiter-family comets observed in the inner solar system. Cratering rates are obtained by accounting for gravitational focusing, cratering efficiency, and an intuitive average of the various available calibrations of cometary mass. The most telling craters are those of Triton, a retrograde moon in a prograde system. It is well-known that much of Triton's surface is relatively young. Less well-known is that Triton features the most startling hemispheric cratering asymmetry in the solar system: fresh impact craters are almost exclusively limited to the leading hemisphere. It would seem that Triton has been colliding almost exclusively with planetocentric debris. If so, then we conclude that Triton's trailing hemisphere is less than 10 million years old. Recent too must be the event that cratered the leading hemisphere. Once admitted we must consider planetocentric cratering of other, prograde satellites. In particular, the lack of a strong apex-antapex asymmetry on Ganymede is not as good an argument for nonsynchronous rotation as we once thought. Rather, many or most of Ganymede's craters might prove to be secondaries, most likely made by ejecta launched into orbit about Jupiter, only to return not too much later, like the insatiable shards of Texas in Armageddon II: The New Millenium.

The author(s) of this abstract have provided an email address for comments about the abstract: kzahnle@mail.arc.nasa.gov

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