杏吧原创

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

The planet鈥檚 atmosphere is聽mainly made up of carbon dioxide聽and its surface is too hot for liquid water. at聽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聽to 3 billion years, but didn鈥檛 account for the planet鈥檚 lack of plate tectonics. 鈥淲e鈥檝e never seen evidence of plate tectonics on Venus,鈥� says H枚ning. The new calculations including this detail suggest this viable period was during Venus鈥檚 first 900 million years. That is still long enough for life to聽evolve聽鈥� though it probably isn鈥檛聽sufficient time for complex life to develop in the way it did on聽Earth, says H枚ning.

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

Using their model, he and his team found that any such water would have reacted with the CO鈧� released into the atmosphere by volcanic eruptions. This would have produced carbonic acid, which would have dissolved silicate rock, helping to capture CO鈧� in rocks as carbonates. Unlike on Earth, which has plate tectonics, these carbonates wouldn鈥檛 have been recycled back into the planet鈥檚 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 CO鈧� through the cracks in the surface. This would have set off a strong greenhouse effect, leading even more of Venus鈥檚 rocks to release CO鈧� and resulting in the environment we see on Venus today.

鈥淣ine hundred million years is聽less than a third of some of the more optimistic estimates for how聽long Venus may have been habitable,鈥� 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聽map the planet鈥檚 surface and聽analyse its atmosphere to determine whether Venus really did once have water. 鈥淭he longer it聽was [habitable], the better the chances that these missions will聽find evidence of those past聽wet聽environments.鈥�

But Helmut Lammer at the Austrian Academy of Sciences in Graz says he is still sceptical that Venus was ever habitable. 鈥淭he atmosphere on Earth for the first billion years wasn鈥檛 stable due to extreme UV radiation from the sun,鈥� 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鈥檚聽model is accurate.

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