How could a toxic gas be a sign of life of Venus?

Scientists recently announced that they had found possible signs of life in the clouds of Venus. We probably should have suspected as much all along.

Venus is a natural place to look for life beyond Earth. It is Earth’s twin — almost the same size and structure — and closer to us than Mars, the current favorite of astronomers looking for life elsewhere in the solar system. Venus is also closer to the Sun, which provides the warmth necessary for life as we know it. In the past, a few scientists have suggested that Venus was a source of primordial life that was later seeded on Earth. That theory, lithopanspermia, never gained popularity because current conditions on Venus seemed very inhospitable to life. The high concentration of carbon dioxide in the atmosphere of Venus ensures that the planet has a runaway greenhouse effect that makes its surface incredibly hot, way hotter than your oven, which kills off microorganisms. And the clouds in its atmosphere are acidic. So scientists turned their attention elsewhere, to Mars and the moons of Saturn. So far, no definitive signs of life have been found on any of them.

But the latest announcement about Venus is a tantalizing one. Astronomers haven’t actually seen life on Venus. Instead, they have observed evidence of a gas called phosphine in the planet’s clouds. What could phosphine have to do with extraterrestrial life?

Phosphine is a highly toxic gas formed when one atom of phosphorus combines with three atoms of hydrogen. Giant planets such as Jupiter have a lot of hydrogen in them and in their atmospheres, and are known to produce phosphine. But on Venus and Earth, there is very little hydrogen in the atmosphere. So, the thinking goes, any phosphine detected is likely associated with life, because someone or something would have had to make it.

Humans produce phosphine industrially for use in computer chips, in flame retardants, and sometimes as a fumigant for rodents. It occasionally ends up as a byproduct in clandestine meth labs. The chemical is also made by microbes through a process that is not well understood, says Derek Lowe, a chemist who has studied phosphine. Typically, it is found in anaerobic (oxygen-free) environments where there is decaying organic matter. In the 1990s, two Swiss researchers found that phosphine was being produced by microbes in sewage, but when the sewage was irradiated, killing the microorganisms, the phosphine could no longer be detected. So phosphine seems to be indicative of life — a biomarker.

Venus’s atmosphere is mostly carbon dioxide, and any phosphine would chemically combine with that gas quickly. So the phosphine detected on Venus — at a concentration of about 20 parts per billion — is somehow being constantly replenished.

MIT astronomer Sara Seager and colleagues are hypothesizing that microbes in the clouds of Venus could be producing the telltale phosphine. These microbes would live high above the scorching temperatures of Venus’s surface, and they would have the ability to exist in a very acidic environment. Hardy microbes thrive in extreme conditions near geothermal vents on Earth, so this is plausible.

It’s also possible that some unknown chemical reaction is creating the tantalizing phosphine on Venus. Chemists often detect things that they can’t understand. Clara Souza-Silva, an astrobiologist who was involved in the detection of phosphine on Venus, points out that it took a long time for scientists to figure out why truffles smell the way they do.

Could lightning or volcanic activity be causing the phosphine above Venus? Both are possible. But Seager and colleagues don’t have any such explanation yet for the amounts of phosphine that they’ve observed — hence their conclusion that the phosphine could have a microbial origin.

The astronomer and science popularizer Carl Sagan, who himself had worked out a scenario of life on Venus, liked to say, “Extraordinary claims require extraordinary evidence.” That standard, according to many astronomers not associated with the published papers, has yet to be met. More follow-up observations are needed. “Atmospheric chemistry on Venus is a complicated problem,” says Cornell astronomy professor Jonathan Lunine.

To scientists like Lunine and Lowe, the detection of phosphine on Venus is reminiscent of the detection of methane on Mars. Methane can be a biomarker, but it, too, can be produced by volcanic activity. Methane in the Martian atmosphere was first detected in 2004 and then more definitively by NASA’s Curiosity rover a decade later. Yet we still don’t know what’s producing it.

Saswato R. Das is a science and technology writer.

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