ISAS, JAXA, Akatsuki / Processing: Meli thev

Some have said it smells like garlic, although most descriptions opt for something much more disgusting, such as rotten fish. It’s explosive and can ignite spontaneously in the air at room temperature. If it gets into your lungs, depending on the dose, it will either cause irritation and trouble breathing, or depress your central nervous system and kill you.
For years, phosphine was unknown to most people. If you didn’t work in semiconductor manufacturing, fumigating rats or know the science behind chemical weapons, you wouldn’t have heard of it. Until earlier this year, that is, when a team of scientists found hints of it in the atmosphere of Venus and speculated it could be created by a living thing.

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Phosphine’s bizarre journey from chemical weapon to a potential sign of life is mostly thanks to the work of Clara Sousa-Silva, a quantum astrochemist at the Massachusetts Institute of Technology, who has spent the past decade studying the molecule. “Phosphine is a horrific molecule, it’s foul in every way,” she says. “It’s almost immoral, if a molecule can be.”
In September 2020, this foul molecule was found floating in the clouds of Venus. In the study, the researchers were able to rule out many of the potential ways it could have got there, leaving life as one of a few remaining options. But the problem is, we know very little about how this molecule is produced by life on Earth.
Phosphine is a simple molecule, made up of one phosphorus attached to three hydrogens, but it’s hard to create. Phosphorus and hydrogen are not atoms that get along well, it is not a natural partnership. In planets like Earth, rocky planets with atmospheres, phosphine cannot be created spontaneously. Something else needs to be there, putting energy in to prise apart oxygen and phosphorus, which make for a much happier and stable pairing. In some cases this extra energy comes from outside sources, like lightning strikes.
The trace amounts of phosphine found on Earth are in places like swamps, soils and sludges, where anaerobic decay is happening, when bacteria break down dead things in the absence of oxygen. These are known as shadow ecosystems, hiding away from the oxygen loving life we know much more about. It’s also been found in Antarctic soil, where it is thought phosphorous compounds in penguin poo could be reduced – taking the oxygen away using enzymes or electrons – to create phosphine, again by bacteria.

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Little is known about why phosphine is created in these ecosystems. It’s unlikely to be a waste product, because it requires energy to make it. It might be a defence mechanism, making the most of the gas’s toxicity. Another option is it might be a signalling thing, similar to the way some trees produce a molecule called isoprene as a way of communicating with each other. “The life that does this is willing to sacrifice energy into this because it’s worth it to produce a signal that is unambiguous compared to if they released say, water,” says Sousa-Silva.
There’s a third, more controversial, reason bacteria might produce phosphine on Earth, which is that it’s part of a barter economy. The bacteria could be exchanging phosphorus for other nutrients, with different living things in their ecosystem. Phosphorus is a necessary element for life, and the phosphine produced by these bacteria might play some role in its global cycle, which is not fully understood.
The ways phosphine is produced on Earth have just not been studied enough for us to know which of these is correct. “I would love to know why all this smelly life is going out of their way to produce this molecule,” says Sousa-Silva. “I would love to know, but we don’t.”
The reason we don’t know is simple. When it comes to the types of life on Earth, humans have disproportionately studied the oxygen-breathing, aerobic ones, choosing them over the anaerobic ones. This makes sense. “Those are the ones we use, and also those are the ones that smell nice to us,” says Sousa-Silva. “So, it isn’t surprising, but it is still disappointing.”

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Phosphine was first discovered in swamps in the 1980s, but it wasn’t until much later, 2006, when the research into how penguin poo increases phosphine levels in Antarctic soil was first published. Scientists only began studying penguin poo because increasing global temperatures means the poo is becoming more likely to thaw, releasing bacteria into the soil. “The penguin population and its microbiome under global climate change may lead to an altered biogeochemistry in the Antarctic,” says Huansheng Cao, at Duke Kunshan University in China, who has studied the impact of penguins on phosphates in the soil.
The rest of the phosphine on Earth has been created in labs, usually by heating phosphoric acid to 200 degrees Celsius. Unlike bacteria, the reason humans make phosphine is entirely clear. Its properties make it useful in semiconductors, where it is used as a dopant to modify the material’s conductivity, and it kills. Phosphine was used as a chemical warfare agent during World War I, and is classed as a terrorism agent today.
“It kills very quickly in multiple ways, so you can recover from one of the ways it kills you before being killed by the other one,” says Sousa-Silva. But all of these killing mechanisms are exclusive to oxygen-loving life, because it interacts with oxygen. That’s why these shadow ecosystems are kind of happy to produce phosphine, because they don’t rely on oxygen.
Ten years ago, the only thing known about it on other planets was its presence on Jupiter and Saturn, where it had been seen in the upper atmosphere. Since it can only be produced spontaneously in the conditions much lower down in these planets, its presence was a sign of violent storms dredging the gas up to the surface.
These detections on the gas giants were possible because it was in large quantities and nearby. Astronomers only had a vague idea what phosphine’s spectral signature, the impact it makes on light that passes through it by absorbing certain wavelengths, would be. But this information was necessary to find phosphine on any other planet using spectroscopy, a technique by which astronomers study the light from a star when a planet passes in front of it. Or even to find it on our own planet.
“If it was being produced, for example, by some terrible organisation that’s happy to produce it as a warfare agent, as we have in the past and are still doing, we couldn’t detect it remotely,” Sousa-Silva said. “I was really enraged.” As a technologically advanced species, she says, we should have the ability to detect most molecules, at least the ones we use to kill one another.
Sousa-Silva spent four years categorising its molecular fingerprint during her PhD, in work that paved the way for potentially seeing the gas on Venus. Every gas absorbs light of specific wavelengths, corresponding to a difference in the energy states of electrons within the molecule. These are called absorption lines, because when an entire rainbow of light passes through the molecule, only these specific wavelengths can be absorbed. When they are, they leave a black line missing from the spectrum. For phosphine, Sousa-Silva calculated 16.8 billion possible absorption lines.
It was during the time she spent working on phosphine’s molecular fingerprint she started to think of it as a potential sign of life. “I started picking up on all these clues of phosphine being a potentially very good sign for life,” she says. She began to see phosphine as a hint of sacrifice, and sacrifice is something only life makes. “That just felt so romantic and tragic,” she says.
The natural progression then, after cataloguing phosphine’s potential absorption spectra was to look at the ways it could be a sign of life, or a biosignature. This was a long body of work which began in 2016 and took until January this year to be published. When she started out, it seemed like a silly thing to do, and she faced rejected grant proposals. “No one cared,” she says. “No one knew about phosphine and the few people who knew about phosphine knew it as just this horrific stain in human development.”
Nobody cared until a few months ago, when phosphine was potentially seen on Venus. Out of the 16.8 billion lines Sousa-Silva categorised, only one was seen on our planetary neighbour, which is partly why many are sceptical about the detection. “You can see how important it is to get at least one more out of those 16.8 billion,” she says.
As for Venus, it’s close enough we don’t have to rely just looking at the light. “The only way to really be sure is to get to Venus and make those measurements there,” says Paul Byrne, associate professor of planetary science at North Carolina State University. Byrne and other astronomers remain sceptical about phosphine and its role as a biosignature, even if it is confirmed to be there. Critiques from other scientists caused the authors of the original paper to reanalyse their data, subsequently finding that average phosphine levels on Venus are seven times lower than their previous estimates.
If phosphine is a sign of life, it’s the kind of life that would look very different to what we know about on Earth. Learning more about the strange behaviour of bacteria on Earth that produce it would help build a picture of what the potential alien life might look like. But for now, interest in funding this kind of research remains low.
In September 2020, Sousa-Silva, who was an author on the paper announcing the detection of phosphine on Venus, watched as the world started to take notice of the molecule she had dedicated a decade to researching. When she began her research into phosphine it seemed like an unwise career move. “It may still turn out to be a very unwise career move,” she says. “But given that it’s possible it’s present on Venus and given that there’s a very small possibility if it’s there, that it’s a sign of life, it does feel like the bet really turned out in my favour.”
Clara Sousa-Silva is one of the speakers at WIRED Live – the inspirational festival bringing the WIRED brand to life. Speakers include DeepMind co-founder Demis Hassabis, NCA Director General Lynne Owens, architectural prodigy Bjarke Ingels, climate activist Vanessa Nakate and founding member of Queen, Brian May CBE. Book your tickets here.
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