“Is it deranged to build a good part of a career studying something that may or may not have been?” asks David Grinspoon at the beginning of the chapter 2 of “Alien Seas”, the one about the oceans of Venus.
His chapter in Alien Seas explains the two main pieces of evidence of oceans on Venus in the past, namely
– the excess of deuterium in the present atmosphere, that implies that a lot of water, at least in vapour form, was present in the early atmosphere of Venus (now bone-dry)
– the more circumstantial evidence from our improved understanding of how planets form. We expect the surface early Venus to have been water-rich, like Earth, given how violent and random is the formation of small rocky planets.
Neither argument is entirely fool-proof. We are like detectives who arrive at the crime scene years after the events, and have no direct clue to work with, only a senile eyewitness with severe memory lapses and the charred remains of a burned note left by the victim.
David Grinspoon makes three points: the first is that Earth-style life could have developed in Venus’s atmosphere, because conditions were very similar to early Earth, with water oceans, volcanoes to provide the carbon and lightnings to provide the energy. The second is that Earth’s version of life could even be Venusian, or the other way around, because exchange of flying rocks was frequent between the planets in the turbulent early days of the Solar System (rocks from Mars still regularly reach Earth today). The third is that Venusian life could have migrated in the cloudtop when the atmosphere got caught in runaway greenhouse and evolved into the present hellish conditions.
You can see a webcast of the talk of David Grinspoon on the topic at Exoclimes 2010 here.