The tantalizing possibility that life exists in the clouds of Venus is once again causing a stir amongst planetary scientists this week. Researchers out of the Massachusetts Institute of Technology, Cardiff University, and the University of Cambridge have proposed that some longstanding ‘anomalies’ in the composition of Venus’ atmosphere might be explained by the presence of ammonia. But ammonia itself would be a strange compound to discover there, unless some unknown process – such as biological life – was actively producing it. Perhaps more intriguingly, ammonia can remove the acidity from Venus’ hostile cloud-tops, suggesting that an airborne, ammonia-producing microbe might have evolved the ability to turn its hostile surroundings into something habitable.

Venus, a planet known for its inhospitable surface temperatures and pressures, along with sulfuric acid rain (that boils off before it can even reach the surface) and a runaway greenhouse effect, seems an unlikely place to look for life. Almost certainly, the planet’s surface is barren of anything we might recognize as ‘alive’. But it just might be possible that something could have evolved to live amongst the Venusian clouds, where pressures and temperatures are more manageable. Over the years, observers have noticed that Venus’ atmosphere contained elements it shouldn’t: most recently in 2020, when the discovery of phosphine suggested a biological microorganism might be actively producing the gas, which would quickly degrade without an ongoing source. Follow-up studies tempered the excitement about phosphine somewhat, suggesting there was not as much of it as originally thought, but its origin nevertheless remains an open question.

This latest study, which will appear in the Proceedings of the National Academy of Sciences, has nothing to do with phosphine, yet once again suggests the possibility of Venusian microbes floating in the planet’s atmosphere.

As Sara Seager, one of the paper’s co-authors, explains, “The phosphine detection ended up becoming incredibly controversial, but phosphine was like a gateway, and there’s been this resurgence in people studying Venus.” Those researchers began combing through past data, turning up more anomalies, and creating models that might explain them.

Artist’s conception of an aerial biosphere in the cloud layers of Venus’ atmosphere. Figure credit: J. Petkowska/MIT.

These anomalous chemicals include nonspherical particles (sulfuric acid usually produces round droplets), oxygen, and ammonia (NH3), a gas that should be nearly impossible to produce from the components available on Venus.

“Ammonia shouldn’t be on Venus,” Seager explained. “It has hydrogen attached to it, and there’s very little hydrogen around. Any gas that doesn’t belong in the context of its environment is automatically suspicious for being made by life.”

Abiotic processes don’t seem capable of producing ammonia on Venus, and yet it’s pretty clear that it’s there. Both American and Soviet missions to Venus have detected it (like Venera 8 and Pioneer), confirming its presence with at least some degree of certainty.

What’s more, the presence of ammonia in the clouds of Venus suggests that life there, if it exists, has evolved to change its environment and make it more livable. Ammonia would neutralize sulfuric acid, turning Venus’ nasty, inhospitable atmosphere into a cozier home for the organism producing it. “No life that we know of could survive in the Venus droplets,” Seager suggests, “but…maybe some life is there, and is modifying its environment so that it is livable.”

There are other possibilities, of course. Perhaps dust and minerals from the surface might be thrown into Venus’ atmosphere, creating many of the anomalies observed. But there is little evidence that there are enough eruptions, meteorite impacts, or other similar events to explain the anomalies completely. For the moment, no hypothesis seems as feasible as the biological one.

Of course, this is just a hypothesis, and will require in-situ testing to prove. If there really is something alive on Venus that has evolved to change its environment, we’re going to have to go there to find out.

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