31st Annual Meeting of the DPS, October 1999
Session 70. Ganymede and Callisto
Contributed Oral Parallel Session, Friday, October 15, 1999, 10:30-12:00noon, Sala Pietro d'Abano

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[70.02] Cratering Asymmetries on Ganymede and Triton: From the Sublime to the Ridiculous

P. Schenk, S. Sobieszczyk (Lunar and Planetary Institute)

A factor of > 20 difference in crater density between the apex and antapex of motion is predicted on Ganymede (Zahnle et al, Icarus, 136, 1998) for craters formed by Jupiter-family comets (JFCs). Less pronounced asymmetries are predicted if other bodies dominate the projectile flux. Voyager-based crater counts suggested little or no leading-trailing asymmetry, in part because resolvable imaging at the apex and antapex regions were not obtained by Voyager. Using Galileo regional scale images of these regions we have completed the crater counts of these regions down to 30-km diameters. To reduce the importance of stochastic variations in crater density, we bin the crater counts in 20-degree wide bands surrounding the apex and antapex. We also restrict our reported counts to younger bright terrain. Our counts indicate a difference in crater density between apex and antapex regions of 2 to 4. (Counts are being extended to 20-km diameter to further reduce stochastic variations.) Distributions of crater chains formed by tidally disrupted comets do not strongly support the idea of sustained nonsynchronous rotation of Ganymede after the time of bright terrain formation. Hence, the observed leading-trailing asymmetry suggests that the population of impactors since bright terrain formation is not dominated totally by JFCs. A contribution of 25-50 a uniformly distributed population (e.g., planetocentric) to the present crater population could explain the observed asymmetry on Ganymede. Triton, on the other hand, exhibits the strongest leading-trailing hemisphere asymmetry possible, with no craters larger than 4 km on the trailing hemisphere and > 150 on the leading hemisphere. Only a planetocentric population, with virtually no contribution from heliocentric bodies, can explain such a distribution on a retrograde body. If so, the lack of Kuiper-belt impacts on Triton points to an extremely young surface age for Triton, and may indicate that the satellite is geologically active today, and perhaps, as some have suggested, may possess an internal ocean like Europa.

The author(s) of this abstract have provided an email address for comments about the abstract: schenk@lpi.jsc.nasa.gov

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