There is the 'thin crust' theory and the 'thick crust' theory. What I know about these ideas is the following.
If Venus has a thin crust, its internal heat escaped into space long ago and the crust is now frozen in time with no tectonic activity. This doesn't explain why the crater distribution is so random and why there are seemingly no old craters. If Venus has a thick crust, this is ultimately unstable because the internal heat of the planet cannot easily escape into space. In this scenario, every 3/4 of a billion years, the surface turns itself inside out in a catastrophic episode. Then, after Venus has lost its load of trapped heat, the crust begins to slowly thicken again until at long last the internal heat convection process again drives the crust to break up and turn inside out. All the while, the radioactive decay of elements in the core continue to generate heat inside the planet. This periodic turnover may take a million years or less, followed by nearly a billion years of nothing happening. Except that meteors will continue to fall and produce craters. What we now see are the craters generated in the crust since the last turn over.
Personally, it is an intriguing theory, and a simple seismic measurement will tell us whether Venus is in this regime or not. If its crust is thin, say 20 - 30 kilometers, then this periodic turnover cannot happen and we will have to look elsewhere for explaining the weird cratering record. If it is thicker than, say 50 - 100 kilometers, then episodic turnover may be favored.
Copyright (C) 1997 Dr. Sten Odenwald
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