A new study in Geophysical Research Letters reports evidence of granite in the highlands of Venus, granite being a rock variety that needs water to form. This means water in Venus’s past as there’s not obviously water there now in sufficient amounts for granite making – the place is very dry. The question is: when did Venus dry out?
Nature reports on the research as does Discover magazine, both going with the line that Venus’s oceans only lasted a few hundred million years. In a cloudless atmosphere that may well be true – 1-D models demonstrated years ago an ocean would’ve been lost in ~600 million years. But 1-D models don’t include clouds or any other global heat-flow features that might’ve mitigated the greenhouse effect at least partially. James Kasting’s modelling of Venus’s greenhouse indicates clouds could stabilize the ocean against loss to space even if it was closer to the Sun than Mercury. David Grinspoon & Mark Bullock suggested that clouds might’ve allowed a stable ocean for something more like 2 billion years or more, with Venus becoming dry only in the last billion years or so. Kasting’s earlier study was of a simple model atmosphere and requires full simulation by a Global Climate Model for more confidence to be placed in the result. Either that or radiometric dating of the granite on Venus. If it is relatively new, then Venus was probably wet until the last billion years or less.
Finally I add my speculation that due to the rapid build up of O2 on Venus from hydrogen decomposition, Life there might have developed oxygen tolerance and utilization before Life here. Imagine life in the Venusian clouds once the surface is near boiling. As the cloud layer rose and the surface got hotter, the bacteria became more likely to be wafted into space via electromagnetic fields and carried away towards Earth, and beyond, via the solar-wind nipping off pockets of gas and dust from Venus. Deposited in Earth’s oceans these newcomers might’ve found photosynthesis even easier on the cooler Earth and started a Great Transition here (borrowing an evocative phrase from Grinspoon & Bullock.)