OF EARTH’S TWO planetary neighbours, Mars and Venus, it is Venus which shines brighter in the sky, comes closer in space, and is more similar in size and physical structure—almost Earth’s twin. But over the past 60 years it has been to Mars that science has paid the most attention. There are currently six operational spacecraft in orbit around it and two more on its surface, one of them an extremely sophisticated rover; more are on their way. Venus is observed by a single small satellite. Yet following a new discovery made with telescopes on Earth, it is Venus which arguably now looks more likely to harbour the thing that planetary science has come to care about more than anything else: life.

The telescopes—the James Clerk Maxwell Telescope (JCMT) in Hawaii and ALMA in Chile—work not in visible light, but with sub-millimetre- and millimetre-wave radiation, which lies in between infrared light and radio waves. The hot depths of Venus’s atmosphere give off a fair bit of radiation at these wavelengths. The molecules in the cooler air above them absorb some of it as it passes out into space; the specific wavelengths absorbed depend on the molecules doing the absorbing. As a team of scientists from various institutions has now reported in Nature Astronomy, one of the chemicals thus revealed appears to be PH3, or phosphine, a molecule composed of phosphorous and hydrogen.

This is a striking anomaly. In an atmosphere composed mostly of carbon dioxide, like Venus’s, phosphine should only be able to survive briefly before chemistry destroys it. So for it to be present persistently—the observers reckon it makes up perhaps 20 parts per billion of the atmosphere—something must be producing it at the same rate as atmospheric chemistry gets rid of it. But what?

On Earth, where there are on average a few parts per trillion of phosphine in the atmosphere, its presence seems to be almost entirely due to chemists (among other things, it is a potentially deadly by-product of badly run meth labs) and microbes. That means it has strong potential as what astrobiologists, who look for life and the things that make it possible in extraterrestrial contexts, call a “biomarker”.

This would be tantalising enough in itself. What makes it yet more fascinating is that for decades a small band of scientists has been suggesting that Venus might be able to support microbial life. The discovery in the 1960s that the surface of Venus was far hotter than the ovens used to sterilise surgical instruments seemed to rule out any chance of life there; the challenge which the surface’s extreme conditions pose both to life and to human technology did a lot to swing attention out towards Mars. But a few scientists wondered if there might be life above the searing surface. The water droplets in Earth’s clouds contain living bacteria; though Venus’s clouds are incredibly acidic, might they too be inhabited by some sort of super-hardy bug? The detection of phosphine is thus a potential sign of life in a place people have previously imagined as habitable. In itself, that context does not make a biological source more likely. But it does make the possibility yet more intriguing.

From the farthest planets to the nearest

The idea of detecting life through an otherwise inexplicable anomaly in a planet’s spectrum dates back to the 1960s, when it was given voice by James Lovelock, a British chemist and inventor. It came into its own when astronomers started discovering planets around other stars, or exoplanets. Most of these planets are in inhospitably unearthlike orbits, but some sit within what astronomers call the “habitable zone”—the zone in which, under various conditions, the surface might support liquid water. Astrobiologists like Sara Seager of MIT started putting real effort into working out what anomalous gases might be visible once they got telescopes good enough to analyse the spectra of such planets’ atmospheres.

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A few years ago some of the scientists who work with Dr Seager and in her team started to get interested in phosphine. Though it is not clear how microbes make it, or something which decomposes into it, its association with life is pretty clear (among other things, penguin guano seems rich in the stuff). There seem to be no appreciable mechanisms for making it abiotically either in the depths of the Earth or through the “photochemical” reactions driven by sunlight which create other short-lived gases in the atmosphere. And it has some nice distinct spectral lines which should be eventually observable in the infrared light from some sorts of exoplanet.

In 2017 Janet Greaves of the University of Cardiff, thinking along similar lines, decided to see if the JCMT could be used to detect longer-wavelength phosphine lines in the atmosphere of Venus—not an exoplanet, but much easier to study. She and her colleagues saw something in the appropriate part of the spectrum; but the signal was weak. Dr Greaves decided to pursue the idea further using ALMA, an array of 50 antennae in the Atacama Desert which is much more powerful than the JCMT. Those observations, made last year, provided a significantly better signal-to-noise ratio.

After the first observations, Dr Greaves’ team and Dr Seager’s heard about each other and pooled their resources. Dr Seager’s team has worked out that any microbial metabolism producing phosphine would probably work best in a very acidic environment—rather like that of Venus, where much of the cloud deck is almost pure sulphuric acid. In the early 2000s some scientists suggested that curious features in the way that the clouds absorb ultraviolet light might be down to microbes making some sort of pigment or other compound to protect themselves from it—perhaps pure sulphur, which could be made through photosynthesis. Dr Seager, Dr Greaves and some of their colleagues developed a new model for how such life might function, with cells reproducing in the cloud droplets and turning into desiccated spores as the droplets fall towards the surface; rising winds then bring some of these spores back up to the clouds, where they get absorbed into—or possibly catalyse the creation of—new droplets in which to reproduce once again. This hypothesis is being published in the journal Astrobiology.

This speculation is fascinating, but also of a sort which might raise alarm bells. The team did not look at the whole spectrum dispassionately to see what was there; it specifically sought out a feature that could be explained by phosphine, a molecule in which at least some of the scientists were already invested, and found what they were looking for. What is more, as they say in their paper, “we emphasise that the detection of PH3 is not robust evidence for life, only for anomalous and unexplained chemistry.” Two things need to happen before things get truly exciting. Other teams need to make their own observations, ideally at other wavelengths. And a really thorough search for ways of making phosphine without biology under the conditions seen on and above Venus needs to draw a blank.

Mars retreats

On the first of these two requirements, the history of methane on Mars provides a cautionary tale. In 2004 scientists using three Earth-based telescopes and a spacecraft orbiting Mars all thought they had detected what appeared to be the spectral signature of methane in the planet’s atmosphere. It was a classic Lovelock anomaly. Chemical models insist that methane does not last all that long in the Martian atmosphere, so these observations suggested there had to be a continuous source of the gas. And on Earth most, though not all, methane is produced by microbes. What was more, there was an increasingly widespread belief that, although there is now only a smidgen of water on the surface of Mars, there might be plenty more below it, perhaps in deep aquifers. On the Earth microbes—including microbes that produce methane—are found many kilometres below the surface. Maybe Mars had a similar “deep biosphere”?

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Maybe. But if so, there is currently no persuasive evidence that it is producing methane. In 2018 the European Space Agency’s ExoMars Orbiter started to look at trace gases in Mars’s atmosphere with much more sensitive instruments than had been used before. It has seen no evidence of methane at anything like the level previously claimed, which makes it hard to credit the earlier observations. It is true that NASA’s Curiosity rover has detected methane more recently; but with ExoMars coming up empty, many see that as the way to bet.

This tale of woe makes it very clear that looking through the Earth’s thick atmosphere for signs of a tiny amount of gas in the atmosphere of another planet is an exacting and error-prone undertaking. Hence the need for observations of phosphine over Venus from other groups using other instruments. At the same time, though, the chain of reasoning which made a deep Martian biosphere plausible applies, mutatis mutandis, to theories about life above Venus, too.

Mars appears always to have been a pretty cold, dry place. But in the distant past, when it had a thicker atmosphere, it clearly had running and standing water at its surface, at least sporadically; Curiosity is currently studying mudstones laid down in an ancient lake. As Mars lost its atmosphere its surface became ever more arid and frigid. That put evolutionary pressure on any microbes previously living in those surface waters to migrate deeper and deeper into the still warm and moist subsurface.

The surface of Venus, too, has dried out over its history: but through heating, not cooling. For billions of years the Sun has been growing brighter, thus changing the boundaries of its habitable zone. In the case of Mars, this warming was not enough to offset the cooling effect of losing most of the atmosphere. On Venus, though, it prompted what atmospheric scientists call a “runaway greenhouse effect”, boiling away the seas which many scientists believe to have graced the planet’s youth. If there had been microbes in the surface waters of Venus before this catastrophe, evolution would have urged them not into the depths, as it did on Mars, but into the skies, where even today the temperature remains bearable and water remains liquid, though admittedly in droplets not oceans.

Should Mars have Venus envy?

This idea has been much further from the mainstream than that of subsurface life on Mars. One reason may be that, though the existence of Earth’s deep biosphere is quite widely appreciated, beyond some recherché microbiological circles the fact that there are also bacteria busily metabolising up in the sky is widely ignored. And to be fair, the high-biosphere analogy is not perfect. Though bacteria live in Earth’s cloud droplets there is as yet no evidence that they reproduce there. That may be because the experiment is hard to do, but it may also be because they have no particular need to do so; the Earth’s surface, and the creatures that roam across it, provide bacteria with all the locales for reproduction they could possibly want.

It is a beguiling story of life finding a way. But it remains very speculative. If the phosphine is indeed present as described, there needs to be a strenuous effort to find, or rule out, non-biological sources. The team behind the detection has done some of this; it argues convincingly that the phosphine cannot come up from volcanoes, drift down from comets, or be made in mid-air through photochemistry. But the chemistry that happens on surfaces can be very different to what happens in mid air, and Venus’s atmosphere, as well as offering extremes of temperature, pressure and acidity, has surfaces to spare, both in its cloud decks and in the hazes that float above and below them. Imaginative chemists should have a field day working through ever more abstruse possibilities—and may make some very interesting discoveries of their own on the way.

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Then there is the possibility of going to take a closer look. NASA has not launched a mission to Venus since the 1980s, though some of its spacecraft have swung past it on their way elsewhere. But two Venus missions have reached the final stage of the selection process for the next round of its “Discovery” program of small planetary missions. One, VERITAS, is an orbiter mainly intended to map the surface in more detail; the other, DAVINCI+, features a small chemistry lab that would descend through the atmosphere beneath a parachute. If it can be made capable of detecting phosphine at a few parts per billion, the case for sending it would become even stronger than it already is. The next mission to Venus, though, is not American but Indian: the Shukrayaan-1 orbiter is currently pencilled in for launch in 2023, which should be enough time to put on a phosphine-optimised instrument. Meanwhile, Dr Seager has secured a grant from Breakthrough Initiatives, a research programme funded by Yuri Milner, a Russian billionaire, to investigate the scientific case for life on Venus and the technical challenges of a potential exploratory mission.

If you can make it here…

Carl Sagan, who wrote a rather remarkable article about the possibility of balloon-like creatures in the clouds of Venus in the 1960s, is well remembered in astrobiological circles for the dictum that “extraordinary claims require extraordinary evidence”. Planetary observations are full of anomalies: you cannot invoke extraterrestrials willy nilly to explain them, creating what the astrobiologist David Grinspoon calls “aliens of the gaps”.

No one is yet claiming that there is life on Venus, and so the current evidence can get by simply by being intriguing, which it definitely is. But if it does turn out that the phosphine is biological, the first half of Sagan’s dictum will need re-examining, at least as far as the hunt for life is concerned.

Not that long ago scientists had pretty much given up on finding life anywhere in the solar system beyond Earth. Now astrobiologists are investigating the possibility of life on, in or above Saturn’s moon Titan, or in the ice covered ocean of one of the planet’s other moons, Enceladus. Jupiter’s moon Europa is also a possibility, as are various other bodies which may contain subsurface oceans. And there is always Mars.

If science finds life on or in any of those bodies, the idea that its presence is ipso facto extraordinary will take a knock. If they find it over hellish Venus as well, or instead, life will come to look yet less like an odd exception. Indeed, at the microbial level at least, life may turn out to be quite ordinary.

But that will make it no less wonderful. In some ways, it may make it more so.

This article appeared in the Science & technology section of the print edition under the headline “A sniff of life?”

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Via The Economist