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Originally Posted by Eroica
That is indeed the crux of the matter. How quickly did conditions on Venus become inimical to the appearance of life?
Anyone know what the current thinking on this question is?
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This
link might help.
The main points for our discussion are:
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Originally Posted by David Grinspoon
Jim Kasting is the best in the business, and his models are state of the art. But the state of the art is not that good.
If you read Kasting's paper, there are these huge uncertainties in the time scale. He's had to make many simplifying assumptions to try and solve the problem of the loss of oceans on a planet like Venus. When you include all these assumptions, the real range of uncertainty in his model is longer than the age of the solar system. In other words, Venus could have lost its oceans in 10 million years, or retained them for longer than the age of the solar system.
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We decided, and Jim Kasting agrees, that the major uncertainty in the models is the role of clouds. Kasting's models did not include clouds...
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When we put in clouds in our model, we found that the clouds act to cool the planet significantly during that greenhouse phase. Temperatures are significantly lower.
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If these results pan out, it might lead to the conclusion that liquid water on the surface of Venus lasted significantly longer. I can't put a precise number on it yet, but it may go from hundreds of millions of years to billions of years.
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The article is from 2004. So back than the situation was quite unclear, but with a tendency to longer times with liquid water.
On earth life began 3.8 billion years ago. the formation of oceans and continental crust should have begun about 4.2 billion years ago. So everything that makes the venusian oceans last longer then about 400-600 million years should give hope that life developed in them.
For having life on Venus today, this early life would have had to be sophisticated enough to take the jump into the atmospheric clouds. This would have taken further time. But I think an ocean lasting between 1 and 2 billion years should sufffice.