Nick said:

>before 3 Ga, Venus was probably very
>Earth-like, if a bit warm. Plate tectonics, oceans, continents,
>life, everything.

Given he early Sun was fainter and Venus is the Earth's twin in terms of size and mass, I've been attracted to the idea that Venus was once a living planet. It's more plausible than Mars ever having had biology.

When would be the latest time that Venus could still have supported life before it went into thermal run away?

Assume for a moment that Venus had its equivalent of a Devonian era with ferns, insects, ocean life, etc. As in our own Devonian, some of the ancient Venusian life would have fossilized.

Is it credible given the conditions on Venus today, that these hypothetical fossils could be recognizable or would any surface fossils simply dissolve away under the sulfuric acid, super critical carbon dioxide, etc.?

Unfortunately Venus is a hard sell for lander missions. I've been involved in more than one failed Venus lander proposal and it's frustrating. The argument against Venus landers is they have an intrinsically short life span. As MER so wonderfully demonstarted, a lander or rover on Mars can keep running long past its expiration date, providing excellent science return for the investment (BTW, we need to send more MERs to Mars). However sending something to the surface of Venus costs almost as much as a Mars lander but it's lucky if the hardware will run for more than a couple of hours. I'm partial to the "crazy idea" of having a floating lighter-than-CO2 platform on Venus with a long cable that could reach down to the surface. This cable would have a grapple on the end of it that could be used to fish for random rocks. These rocks would be reeled up to the platform for inspection and analysis. The whole thing would just float around willy-nilly grabbing random samples. With some added smarts, it might be possible to design a sample return based upon such a platform. It might make more sense to get our feet wet with a Venus sample return before attempting a Mars sample return with all the attendent biological back contamination nonsense. Along these lines it might even make sense to have a manned Venus orbiter. Venus is closer than Mars. Getting a crew to Venus would not take that long in terms of heliocentric transfer (fewer issues with GCR dosage). Aerobraking to a Venusian orbit would be easier than with the tenuous martian atmosphere. Once on orbit, the flight crew could command Venusian robots on the surface or on a floating platform in real time without dealing with speed-of-light issues. The flight crew might even descend to the floating platform if it was floating high enough in the Venusian atmosphere. There could be something like Bespin from the Star Wars episode "The Empire Strikes Back". However the platform would have to be above the sulfuric acid clouds which is probably the "gotcha" with this particular nutty idea.

Gary