The researchers managed to produce phosphine in these scenarios in tiny amounts, not enough to be detected from Earth. Which is how Sousa-Silva and the team found themselves seriously considering the explanation that scientists keep at the very bottom of the list because it’s usually the least likely. As the saying goes, extraordinary claims require extraordinary evidence. “I’m skeptical,” Sousa-Silva said. “I hope that the whole scientific community is just as skeptical, and I invite them to come and prove me wrong, because we’re at the end of our expertise.”
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Several scientists who specialize in Venus, and who were not involved in the new research, say the findings are compelling. Like Sousa-Silva, they are skeptical. Some point out that, after more observations, the signature of phosphine could turn out to be the trademark of another molecule. Astronomers are the first to doubt data that hint at life, but this time, they seemed willing to entertain the possibility. “This discovery is now putting Venus into the realm of a perhaps inhabited world,” says Martha Gilmore, a planetary geologist at Wesleyan University who has proposed a robotic mission to study Venus in depth.
The idea that life might reside in Venusian clouds has been floating around in the astronomy community for decades. Carl Sagan, the astronomer who popularized the “extraordinary claims” mantra, explored the concept in a 1967 paper** with the biophysicist Harold Morowitz. Before Venus became a planet-size furnace, it was a watery world, covered in oceans that flowed for billions of years, as habitable as the seas of Earth. As the atmosphere swelled with heat-trapping gases and water evaporated into space, life-forms on the surface, forced to adapt, could have escaped into the skies. If life indeed resides in Venus’s atmosphere, it might be the last remnant of a wrecked biosphere.
Sousa-Silva daydreams often about what such aerial life-forms might be like. “It’s fascinating to imagine what kind of complexity could arise if you’re not scared of sulfuric acid,” she said. Venusian life-forms would have a more difficult existence if they resembled earthly microorganisms, Sousa-Silva says, because they would have to work hard to extract the very little water vapor in the atmosphere to survive.
Recognizable or not, any Venusian life would probably smell bad. Phosphine is so toxic that it has been used as a chemical agent in warfare and by terrorist groups. “We’ve evolved to think that toxic things smell bad,” Sousa-Silva said, adding that any Venusian beings might be hardwired to feel the same way. Venusian life “would smell disgusting [to us], but we would be repulsive to them,” she said.
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The presence of phosphine on Venus and its mystery origins mean scientists around the world must reexamine what they think they know about the second planet from the sun. The discovery also bolsters the argument for sending new missions to Venus, such as orbiters to map the cloud tops and balloon probes to fall through the atmosphere. “We have to go back to that atmosphere and figure out, what could this mean?” Jim Garvin, the chief scientist at NASA’s Goddard Space Flight Center, who leads a proposed mission to Venus, told me.