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Sunday, 3 October 2021

Life on Venus may have only been possible for its first billion years


Venus may have been habitable for close to a billion years – far less time than previously thought, but still long enough for life to evolve.

The planet’s atmosphere is mainly made up of carbon dioxide and its surface is too hot for liquid water. Dennis Höning at VU Amsterdam in the Netherlands and his colleagues modelled how Venus may have developed its atmosphere.

Previous research has indicated that it could have been habitable for 2 to 3 billion years, but didn’t account for the planet’s lack of plate tectonics. “We’ve never seen evidence of plate tectonics on Venus,” says Höning. The new calculations including this detail suggest this viable period was during Venus’s first 900 million years. That is still long enough for life to evolve – though it probably isn’t sufficient time for complex life to develop in the way it did on Earth, says Höning.

Key to this model is that when Venus was born 4.5 billion years ago, the sun would have been a lot dimmer. As such, it would have been easier to maintain liquid water on the planet’s surface, says Höning.

Using their model, he and his team found that any such water would have reacted with the CO₂ released into the atmosphere by volcanic eruptions. This would have produced carbonic acid, which would have dissolved silicate rock, helping to capture CO₂ in rocks as carbonates. Unlike on Earth, which has plate tectonics, these carbonates wouldn’t have been recycled back into the planet’s mantle and would have instead continued to build up.

The hotter these carbonates got as they were buried deeper by successive volcanic flows, the more unstable they would have become, releasing CO₂ through the cracks in the surface. This would have set off a strong greenhouse effect, leading even more of Venus’s rocks to release CO₂ and resulting in the environment we see on Venus today.

“Nine hundred million years is less than a third of some of the more optimistic estimates for how long Venus may have been habitable,” says Richard Ghail at Royal Holloway, University of London. He says this is important because there are three missions to Venus in the works that will map the planet’s surface and analyse its atmosphere to determine whether Venus really did once have water. “The longer it was [habitable], the better the chances that these missions will find evidence of those past wet environments.”

But Helmut Lammer at the Austrian Academy of Sciences in Graz says he is still sceptical that Venus was ever habitable. “The atmosphere on Earth for the first billion years wasn’t stable due to extreme UV radiation from the sun,” he says. He argues that shortly after it formed, Venus would have received more than double the UV radiation that Earth did, forcing the atmosphere to expand and eventually dissipate, making it less likely that the team’s model is accurate.

Reference: 

Early habitability and crustal decarbonation of a stagnant-lid Venus by Dennis Höning, Philipp Baumeister, John Lee Grenfell, Nicola Tosi, Michael J. Way arxiv.org/abs/2109.08756

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