Venus might never have had the conditions necessary for water to exist on its surface, meaning the planet wouldn鈥檛 have been habitable as once thought.
Today, Venus is a hellish world, with temperatures on its surface hot enough to melt lead. Yet the presence of water vapour in its thick atmosphere, coupled with surface features known as tesserae that look like ancient continents, suggest it could once have supported oceans and maybe life billions of years ago. Three new Venus spacecraft, two from NASA and one from the European Space Agency (ESA), were recently selected for launch in the next decade, partly to investigate this possibility.
at the University of Geneva in Switzerland and his colleagues, however, present a less rosy picture. Their climate modelling suggests that the planet was never cool enough for the water vapour to condense out of its atmosphere and form liquid water on the ground. 鈥淰enus probably never formed oceans,鈥� says Turbet.
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Previous modelling of Venus presumed that the planet started with liquid water, which then evaporated as a runaway greenhouse effect took place. Turbet, however, says it is more likely that Venus started out as a 鈥渟team鈥� world, with its vapour already trapped in the atmosphere. In such a scenario, the formation of clouds on the cooled, night-time side of Venus would have trapped heat on the planet, preventing temperatures from dropping far enough for water to condense.
鈥淭hey will create some precipitation, but it will never reach the ground because it will get re-evaporated while it鈥檚 falling,鈥� says Turbet. 鈥淚t was too warm for the water vapour to condense.鈥�
If correct, the findings may suggest that the window for planets to become habitable is even more narrow than astronomers had thought. The researchers found that Earth was only able to condense water early in its history because the sun was about 25 per cent dimmer, seemingly solving a problem known as the faint young sun paradox where Earth was thought to have been too cold to support liquid water. Had it formed today, our planet might well have been a 鈥渟team Earth鈥�, like Venus.
at the NASA Goddard Institute for Space Studies in New York, who has conducted previous climate studies of Venus, says that while the results describe one of the possibilities for the evolution of Venus, they don鈥檛 close the case just yet.
鈥淚 do not think this ends the debate on whether Venus ever condensed water on its surface,鈥� he says. 鈥淎ll of these models have limitations. It is necessary for additional models to replicate these results.鈥�
To find out for certain, we will need to study the surface of Venus directly to look for evidence of water, something that NASA and the ESA鈥檚 upcoming missions plan to do. at the University of Oxford, deputy lead scientist on the that is set to launch in the 2030s, says the presence of granite-like rock that makes up Earth鈥檚 continents could be such evidence.
鈥淥ur current understanding is you can鈥檛 create those widespread rocks without large amounts of liquid water on the surface,鈥� he says. 鈥淚f we don鈥檛 discover it, that leaves the question somewhat open.鈥�
Nature
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