**DPS Meeting, Madison, October 1998**

*Session 48P. Venus*

Contributed Poster Session, Thursday, October 15, 1998, 5:00-6:30pm, Hall of Ideas
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## [48P.05] Numerical simulations of large impacts in thick atmospheres

*K. Zahnle (NASA Ames), D. Korycansky (UCSC)*

We present numerical simulations of asteroids and comets
striking the atmosphere of Venus. We consider a wide variety
of objects, both comets and asteroids, ranging in size from
1 to 4 km diameter, striking at impact angles ranging from
the vertical to 60 degrees, at velocities from 20 to 90
km/s, in a variety of shapes (including some lumpy
spheroidal things), and in 2 and 3 dimensions. Our purpose
is to determine the permeability of atmospheres to impacting
bodies; i.e., to define the sheltering sky. We find that
details of the destruction, deceleration, and dispersal of
impactors depend sensitively on initial conditions, but that
the ensemble of models produces a relatively robust
distribution of atmospheric permeabilities that is
asymptotically insensitive to numerical resolution.

The approximately 940 impact craters on Venus provide an
excellent dataset against which to test our model. We
compute model crater diameters using Schmidt-Housen
gravity-scaling, using mass and momentum fluxes passing a
fiducial altitude (e.g., the surface) computed from the
numerical results. The absolute cratering efficiency (i.e.,
the crater diameter) is obtained as a function atmospheric
permeability. The more permeable the atmosphere, the larger
the crater must be for a given impactor. Thus by determining
the atmospheric permeability and using the crater
distribution observed on Venus, we obtain an independent
calibration of Schmidt-Housen scaling at the scale of 20 km
craters.

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