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[39.21] Modeling Crater Populations on Venus and Titan

D. G. Korycansky (CODEP/IGPP UC Santa Cruz), K. J. Zahnle (NASA Ames Res. Cntr.)

We describe a model for crater populations on planets and satellites with dense atmospheres, like those of Venus and Titan. The model takes into account ablation, pancaking, and fragmentation. Fragmentation is assumed to occur due to hydrodynamic instabilities. Surviving fragments that hit the ground make craters or groups thereof. We estimate crater sizes using standard laws in the gravity regime, accounting for impactor disruption. Distributions of impactor mass, angle, and velocity are used in a Monte Carlo method. Good fits to the Venus crater populations (including multiple crater fields) can be found with reasonable values of model parameters. The model reproduces the dearth of small craters on Venus, due to a cutoff on crater formation when the expected crater is smaller than than the (dispersed) object that would make it. Hydrodynamic effects alone (ablation, pancaking, fragmentation) in the atmosphere cannot explain the lack of small craters. In our favored model, the observed number of craters (940) is produced by ~5500 impactors with masses > 10¹⁵ gm, yielding an age of 730±110 Myr for the the venusian surface.

We apply the model to Titan, assuming that the impactors are made of 50% porous ice. Predicted crater production rates are ~190 craters per 10⁹ year.The smallest craters on Titan are predicted to be ~2 km in diameter. If the impactors are composed of solid ice crater production rates increase by ~70%.We also explicitly address leading-trailing hemisphere asymmetries that might be seen if Titan's rotation rate is strictly synchronous: in that case, the ratio of crater production on the leading/trailing hemispheres is ~4:1.

Support for this work was provided by NSF Planetary Astronomy grant 0098787, NASA Cooperative Agreement NCC25428, and the NASA Exobiology Program.

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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.