If you think acid rain here on Earth is nasty stuff, consider poor Venus, forever shrouded in a hothouse of clouds made primarily of sulphuric acid. It’s around 460°C at the surface (that’s ~840°F, or about twice as hot as the oven in your kitchen ever gets). There are no spacecraft around Venus, and no rovers on its surface, which would melt them within minutes. It’s the brightest object in the night sky, thanks in part to those reflective clouds, and in part due to its proximity to the sun. It’s the last place you’d expect to find signs of life.
Jane Greaves, an astronomer at Cardiff University in Wales, had read scientific papers positing that, if you were an alien astronomer looking at Earth, phosphine could be a biosignature for our planet. She aimed a ground-based telescope in Hawaii at Venus to observe the planet for just a few hours, not expecting to find anything.
Whereupon Venus says, “Hold my beer.”
What she and her associates found was traces of a gas in the Venusian atmosphere called phosphine. On Earth, phosphine — a toxic gas — is produced by microorganisms. She found the spectrographic signature of phosphine reflected in the light from the clouds of Venus. Observations from another telescope, in Chile, captured the same mark. Soon, Greaves was in touch with Sousa-Silva, a molecular astrophysicist at MIT, who has spent her career studying phosphine. As crazy as it might sound, the most plausible explanation for the presence of phosphene is life.
The only way to know for sure is to send a spacecraft to Venus and test the atmosphere for the presence of bacterial life. The upper atmosphere of the planet is a lot cooler than the surface is, closer to what we experience on Earth. It’s in the upper atmosphere where the phosphene was discovered, and it shouldn’t be there. The atmosphere of Venus is so acidic that phosphene is destroyed pretty quickly, so for the gas to be detected in any quantity at all, something must be making more of it on a continuous basis.
Phosphene is pretty rare in our star system. It comes from swamps and marshlands, and the guts of some animals. Saturn and Jupiter make it in their storm systems, under violent conditions that exist nowhere else. So how could Venus be making the stuff?
Sousa-Silva and the other researchers decided to try to find out. They mimicked similar processes on Venus using computer simulations. They tried atmospheric lightning, and meteor strikes. and even geological activity, even though Venus doesn’t have plate tectonics, because they couldn’t think of anything else that could produce enough energy to force phosphine into existence.
What they got from the simulations was phosphine in such tiny amounts that they wouldn’t have been detectable from Earth. That’s how they ended up giving the possibility of life serious consideration. Still, it’s just as likely that it’s just some new chemical process they haven’t seen before. “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.”
Before Venus became the solar system’s EZ-Bake Oven, it was mostly covered with oceans for billions of years, as habitable as the seas of Earth. If there were any life forms in those seas or on the surface, they would have been forced to adapt as the atmosphere gained carbon dioxide and its water evaporated into space, escaping into the skies. If there is life on Venus, 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.