Lack of water rules out life on Venus: study
The search for life on our nearest neighbour has so far proved fruitless, although a 2020 paper rekindled hopes for Venus when it claimed to have detected phosphine gas -- known to be produced by bacteria on Earth -- in the planet's clouds.
The authors have since called their own findings into question.
In 2017, microbiologist John Hallsworth discovered a terrestrial fungus that can survive at 58.5 percent relative humidity -- the driest conditions at which biological activity has ever been measured.
"We bent over backwards to argue that the most extreme, tolerant microbes on Earth could potentially have activity on Venus," said Hallsworth at a press conference.
But he said nothing could cope with the miniscule amount of water in the planet's atmosphere, which is equivalent to a relative humidity of 0.4 percent.
"It's more than 100 times too low. It's almost at the bottom of the scale, at an unbridgeable distance from what life requires to be active."
Jupiter 'more optimistic
Chris McKay, a NASA planetary scientist and co-author of the research published in Nature Astronomy, noted that the conclusions of the study were based on the limited direct observations available, and therefore incomplete.
"It's hard to imagine that the results will change as we do further exploration," McKay told reporters.
"The results were much more optimistic," said McKay.
"There is at least a layer in the clouds of Jupiter where the water requirements are met."
"To show that that layer is habitable we would have to go through all the requirements for life and show that they're all met," he said, adding that determining things like ultraviolet exposure and energy sources would require further exploration.
Search for life
Three more Venus missions are planned for sometime around 2030 and McKay feels certain they will confirm the measurements used for the study.
"There could've been a time when Venus was earth-like," McKay said.
"One of the missions will fly through the atmosphere and measure trace gases... which will tell us a lot about Venus's evolutionary history and will start to address questions like how much atmosphere did Venus have, where did it go, what happened?"
And the study's authors hope their method of determining water activity will be applied to planets beyond our solar system -- especially with the upcoming launch of the James Webb Space Telescope (JWST) later this year.
"The JWST will be able to determine atmospheric profiles of temperature, pressure and water abundance in exoplanet atmospheres," the study concludes.
"These will allow assessments of water activity in their atmospheres using our approach."