Home >Opinion >Views >Deck the clouds with the phosphine gas

Two scientific pursuits excite me more than others: studying ant behaviour and the search for life elsewhere in this universe. So I was thrilled to read that a team of astronomers has detected signs of life on another planet. In this strange time when lives by the thousand of lives every day are being snuffed out by a virus, we find there may be life on Venus.

Exciting? But allow me to explore some other signs of possible life out there that we might have detected.

In late 2017, there was ‘Oumuamua, which zoomed in and out of our solar system. (See my Cigar from outer space, 5 January 2018). Of course many objects zip through space, but this one was about 250m long and only 35m wide. In a universe filled with generally rounded artefacts, this one was like a cigar. What did that mean?

Many astronomers speculated that it might be a spaceship. For if you wanted a vehicle to explore other worlds, you might choose to build it in that shape. Had some technologically advanced civilization come to the same conclusion and shaped ‘Oumuamua that way?

So as ‘Oumuamua sped away from us, we started listening for radio signals from it. For you’d expect a craft like that to emit signals, from its machinery or for communications or both. After all, our own spacecraft — Voyager, Pioneer, Cassini and more—have all communicated with Earth as they roamed our system, sending out radio signals.

Only, we found no signals from ‘Oumuamua. Perhaps it was no more than an odd-shaped rock.

Then there’s KIC 8462852, or Tabby’s Star, about 1,500 light-years away. In around 2011, its brightness started varying in a bizarre way (See my Have we found a structure?, 23 October 2015). Once it dimmed by as much as 22%. Various explanations were offered, of which one was particularly interesting: a “Dyson sphere" around the star.

Originally conceived by the thinker and mathematician Freeman Dyson, this is what he called “an artificial biosphere which completely surrounds (a) star." He imagined that a civilization might attempt to build such a sphere in order to gather and use all the energy from its star, rather than just a tiny fraction of it like we do with our Sun. Certainly this would have to be a civilization in possession of technology and knowledge far greater than our own.

So was Tabby’s Star dimming because someone was building a Dyson sphere around it? Was this a sign of life there? It’s possible, but over the last few years, astronomers have offered other, more plausible explanations. For example, perhaps an “exomoon" is disintegrating as it orbits the star. The resulting debris could obscure our view of it. Whatever the explanations, it is unlikely that Tabby’s dimming is a sign of life.

In both these cases, the signs we sought to confirm were of sentient, intelligent life. That is, of course, what the well-known Search for Extraterrestrial Intelligence (SETI) is all about. Patterns in radio signals, evidence of structures, an effort to communicate—any of those would fit that particular bill.

Yet think of this: life doesn’t necessarily have to be technologically advanced, nor offer us signs that it is intelligent. After all, if some extraterrestrial civilization checked out our planet millions of years before the appearance of homo sapiens, they would have found trees and early animals, water and bacteria and carbon dioxide—nothing there we would term intelligent as we understand that word today. And yet even those millions of years ago, the Earth was teeming with life. In fact, if life-forms in that civilization also produced carbon dioxide, they would undoubtedly take the presence of that gas as a sign that there’s life on Earth.

So just what do we mean by life? Or really, what is a sign of life?

Water is one. For life, as we denizens of Earth know it, at any rate, water is fundamental. So when we find evidence of water on other bodies in space, it intrigues us, because it suggests that life is possible there. For example, we have images of what looks like flowing water — or more accurately, the trails flowing water would leave behind — on Mars. Jupiter’s moon Europa, astronomers believe, has a huge subsurface ocean that contains up to three times as much water as the Earth does (see Giant steps are what you take walking to the moon, 11 July 2019). How do we know this? Among other things, the Hubble Space Telescope once photographed what look like enormous plumes of water vapour erupting off Europa’s surface. (Aside: spare a thought for photographing such water vapour, even enormous eruptions, with a telescope that’s 700 million km away).

Evidence of water is a major reason there’s so much interest in Mars among astronomers, and also why NASA is planning to launch a mission to Europa in 2024.

But we don’t need to detect water. There are other possible signs, and scientists refer to them as “biosignatures". For example, take my mention of carbon dioxide above. To that other civilization, it would have been a biosignature: a substance that amounts to scientific evidence of life as that civilization knows it. Similarly, there are biosignatures we might find on other celestial bodies.

And that’s where Venus — remember Venus? This column began with Venus — comes in. Because what the team of astronomers found on Venus is the presence, in its cloud cover, of a gas called phosphine. Made of phosphorus and hydrogen atoms, phosphine is explosive and tends to smell of garlic or rotting fish. It is also poisonous: it can cause vomiting, diarrhoea, pulmonary problems, muscle pain and more. Not a pleasant gas to have wafting around you, certainly, and that’s why it has been used as a chemical weapon in wars. It even made an appearance in the hit series Breaking Bad, killing off two bad guys.

But here’s why phosphine is a biosignature: it occurs naturally when organic matter decomposes. It is produced by bacteria that don’t need oxygen (anaerobic bacteria). It is found in swamps and in animal excreta—in relatively large amounts, for instance, in the air above penguin colonies. We know of no other way, here on Earth, that phosphine is naturally produced. As the team’s paper (“Phosphine gas in the cloud decks of Venus", Nature Astronomy, 14 September 2020) explains, “Trace (phosphine) in Earth’s atmosphere is uniquely associated with anthropogenic activity or microbial presence — life produces this highly reducing gas even in an overall oxidizing environment." Or, in the simpler language one member of the team used: “As far as we can tell, only life can make phosphine."

(Second aside: spare a thought for detecting phosphine in a cloud with a telescope that’s 150 million km away. How that happened is a story, perhaps, for another column).

So, if there’s phosphine in the clouds of Venus, that’s pretty Venus-shattering news, for we have long assumed that Venus, with its severely acidic atmosphere and fierce surface temperatures, is utterly inhospitable to life. With this new discovery, we’ll have to revisit, at least, that assumption. What produced that phosphine? Answering that may even force us to revisit our ideas of life itself.

Just that much is exciting enough. Though if someone found evidence of ants on Venus, that’ll really make my day.

Once a computer scientist, Dilip D’Souza now lives in Mumbai and writes for his dinners. His Twitter handle is @DeathEndsFun

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