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ISAS, JAXA, Akatsuki / Processing: Meli thev
Some have said it smells like garlic, although most descriptions opt for something far more disgusting, like rotten fish. It is explosive and can spontaneously ignite in air at room temperature. If it enters the lungs, depending on the dose, it will cause irritation and difficulty in breathing, or it will depress the central nervous system and kill you.
For years, phosphine was unknown to most people. If you weren’t working in semiconductor manufacturing, fumigating rats, or didn’t know the science behind chemical weapons, you wouldn’t have heard of it. Until the beginning of this year, that is, when a team of scientists found clues to it in the atmosphere of Venus and speculated that it could be created by a living being.
Phosphine’s bizarre journey from a chemical weapon to a potential sign of life is mainly thanks to the work of Clara Sousa-Silva, a quantum astrochemist at the Massachusetts Institute of Technology, who has spent the last decade studying the molecule. “Phosphine is a horrible molecule, it’s disgusting in every way,” he says. “It’s almost immoral, if a molecule can be.”
In September 2020, this repulsive 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 gotten there, leaving life as one of the few remaining options. But the problem is that we know very little about how this molecule is produced by life on Earth.
Phosphine is a simple molecule composed of a phosphorus attached to three hydrogens, but it is difficult to create. Phosphorus and hydrogen are not atoms that get along, it’s not a natural partnership. In planets like Earth, rocky planets with atmospheres, phosphine cannot be created spontaneously. There must be something else, which puts energy to separate oxygen and phosphorus, which make mating much happier and more stable. In some cases this extra energy comes from external sources, such as lightning.
Traces of phosphine found on Earth are found in places like swamps, soils and muds, where anaerobic decay is occurring, when bacteria break down dead things in the absence of oxygen. These are known as shadow ecosystems, hiding from the oxygen-loving life that we know much more about. It has also been found in Antarctic soil, where it is thought that the phosphorus compounds in penguin poop could be reduced – by taking away oxygen using enzymes or electrons – to create phosphine, again from bacteria.
Little is known about why phosphine is created in these ecosystems. It is unlikely to be a waste product, because it takes energy to make it. It could be a defense mechanism, making the most of the gas’s toxicity. Another option is that it could be a signaling element, similar to the way some trees produce a molecule called isoprene as a means of communicating with each other. “The life that does this is willing to sacrifice energy in this because it is worth it to produce a signal that is not ambiguous as to whether they have released, say, water,” Sousa-Silva says.
There is a third, more controversial reason why bacteria could produce phosphine on Earth, which is that it is part of a barter economy. Bacteria could exchange 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 could play a role in its overall cycle, which is not fully understood.
The ways in which phosphine is produced on Earth have not been researched enough to let us know which of these is correct. “I’d like to know why this whole stinking life is going to great lengths to produce this molecule,” says Sousa-Silva. “I’d like to know, but we don’t know.”
The reason we don’t know is simple. When it comes to the types of life on Earth, humans have disproportionately studied oxygen-breathing, aerobic ones, choosing them over anaerobic ones. Does this make sense. “Those are the ones we use and also the ones that smell good to us,” Sousa-Silva says. “So, it’s not surprising, but it’s still disappointing.”
Phosphine was first discovered in swamps in the 1980s, but it wasn’t until much later in 2006 when research on how penguin poop increases phosphine levels was first published. in the Antarctic soil. Scientists have begun studying penguin poop because rising global temperatures mean the poop is more likely to thaw, releasing bacteria into the soil. “The penguin population and its microbiome under global climate change can lead to altered biogeochemistry in the Antarctic,” says Huansheng Cao, of Duke Kunshan University in China, who has studied the impact of penguins on phosphates in soil.
The rest of the phosphine on Earth was created in the laboratory, usually by heating the phosphoric acid to 200 degrees Celsius. Unlike bacteria, why humans produce phosphine is entirely clear. Its properties make it useful in semiconductors, where it is used as a dopant to change the conductivity of the material and kills. Phosphine was used as a chemical warfare agent during World War I and is now classified as an agent of terrorism.
“It kills very quickly in several ways, so you can bounce back from one of the ways it kills you before being killed by the other,” says Sousa-Silva. But all of these killing mechanisms are unique to oxygen-loving life, because it interacts with oxygen. This is why these shadow ecosystems are quite happy to produce phosphine, because they are not dependent on oxygen.
Ten years ago, the only thing that was known about other planets was its presence on Jupiter and Saturn, where it had been seen in the upper atmosphere. Because it can only be produced spontaneously under the much lower conditions on these planets, its presence was a sign of violent storms dredging the gas to the surface.
These gas giant detections were possible because they were in large quantities and close together. Astronomers had only a vague idea of what the spectral signature of the phosphine would be, the impact it produces on the light that passes through it by absorbing certain wavelengths. But this information was needed to find phosphine on any other planet using spectroscopy, a technique by which astronomers study the light of a star as a planet passes in front of it. Or even to find it on our planet.
“If it had been produced, for example, by some terrible organization that is happy to produce it as an agent of war, as we have done in the past and are still doing, we could not remotely detect it,” Sousa-Silva said. “I was really angry.” As a technologically advanced species, he says, we should have the ability to detect most molecules, at least the ones we use to kill each other.
Sousa-Silva spent four years classifying her molecular footprint during her PhD in work that paved the way for the possibility of seeing gas on Venus. Each gas absorbs light of specific wavelengths, corresponding to a difference in the energy states of the electrons within the molecule. These are called absorption lines, because when a full rainbow of light passes through the molecule, only these specific wavelengths can be absorbed. When they are, they leave a missing black line in the spectrum. For phosphine, Sousa-Silva calculated 16.8 billion possible absorption lines.
It was during the time he spent working on the molecular footprint of phosphine that he began to think of it as a potential sign of life. “I’ve started to get all these clues that phosphine is a potentially very good sign for life,” he says. He began to see phosphine as a hint of sacrifice, and sacrifice is something that only life does. “It felt so romantic and tragic,” he says.
The natural progression then, after cataloging the potential phosphine absorption spectra, was to examine ways in which it could be a sign of life or a biological signature. This was a long job that started in 2016 and took until January of this year to be published. When he started, it seemed like a silly thing to do and he had to deal with rejected grant proposals. “Nobody cared,” he says. “Nobody knew about phosphine and the few people who knew about phosphine only knew it as this horrible blemish in human development.”
Nobody cared until a few months ago, when phosphine was potentially seen on Venus. Of the 16.8 billion lines classified by Sousa-Silva, only one has been seen on our planetary neighbor, which is partly why many are skeptical of the detection. “You can see how important it is to get at least one more out of that $ 16.8 billion,” he says.
As for Venus, it’s close enough that we don’t have to rely on just looking at the light. “The only way to be really sure is to get to Venus and take those measurements there,” says Paul Byrne, an associate professor of planetary science at North Carolina State University. Byrne and other astronomers remain skeptical of phosphine and its role as a biological signature, although it is confirmed to be present. Criticism from other scientists prompted the authors of the original paper to reanalyze their data, later discovering that the average phosphine levels on Venus are seven times lower than previous estimates.
If phosphine is a sign of life, it is the kind of life that would look very different from what we know on Earth. Learning more about the strange behavior of the bacteria on Earth that produce it would help build a picture of what potential alien life might be like. But for now, interest in funding this type of research remains low.
In September 2020, Sousa-Silva, who was an author of the paper announcing the detection of phosphine on Venus, saw the world begin to take notice of the molecule that had devoted a decade to research. When he began his phosphine research, it seemed like an unwise career move. “It could still prove to be a very reckless career move,” he says. “But since it is possible that it is present on Venus and given that there is a very small chance that it is there, which is a sign of life, it seems that the bet has really gone in my favor.”
Clara Sousa-Silva is one of the speakers of WIRED Live, the inspirational festival that brings the WIRED brand to life. Speakers include DeepMind co-founder Demis Hassabis, NCA CEO 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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