The mass per unit area of the Venusian atmosphere is ~106 kg/m2, equivalent to a 1-km-deep water ocean. An important effect of this ''ocean'' on cratering is pressure enhancement due to multiple reverberations during the compression stage. We consider impactors with diameters, D, less than a scale height (~15 km). Such impactors drive before them bow shocks with standoff distances of Δ~0.05 D into the lowermost atmosphere. Reflection of this shock at normal incidence from the basalt surface raises the pressure in the atmosphere and surface, but initially not as much as a direct impact would. Several shock reverberations should occur, however, as the shock crossing time in the trapped, compressed atmospheric layer is much less than either the shock crossing time in the impactor or the time for the impactor to penetrate the atmospheric layer. With each reverberation we estimate new Hugoniots for shocked atmosphere, impactor, and target. The first is treated as an ideal gas of &ggr;=1.2--1.3, and the latter are modeled with Murnaghan equations of state and Gr¿neisen &Ggr;S∝&rgr;-1. The asymptotic pressure enhancement for a 20 km/s carbonaceous chondrite impactor (modeled as serpentine) striking basalt at 20 km/s, relative to the atmosphereless 1-D impedance match pressure (~550 GPa), is a factor of ~1.4.

This maximum enhancement factor increases with velocity, reaching 1.6 for serpentine moving at typical Venus-impacting cometary velocities (~65 km/s), and is greater for more compressible impactors (comets). Each reverberation sends a shock pulse into the surface or impactor that overtakes the previous ones, hence at the end of the compression stage both impactor and target are shocked to a higher pressure than would occur in the absence of the atmosphere. The impactor is effectively denser or stiffer than in the atmosphereless case, but as the energy and momentum delivered are essentially the same, cratering efficiency should be unaffected to first order. However, fusion and vaporization efficiency should be enhanced for impactors whose impact velocities are below (whether initially or due to atmospheric drag) the threshold for complete melting or vaporization, respectively, in the absence of an atmosphere. ¿ American Geophysical Union 1992