Categories: Astronomy

Volcanic Activity on Venus Could Explain Phosphine

Ever since the announcement last September that astronomers found evidence of phosphine in the clouds of Venus, the planet has been getting a lot of attention. It’s not surprising. Phosphine is a potential biosignature: On Earth, it is produced by microbial life. Might a similar biological process be taking place in the skies of our sister planet? It’s a tantalizing prospect, and is definitely worth examining closely, but it’s too early to be sure. Microbes aren’t the only way to get phosphine. A new paper published on July 12th in the Proceedings of the National Academy of Science suggests that volcanism might instead be to blame for the strange chemistry in the Venusian cloud tops.

The Story So Far

Early last fall, a research team led by Professor Jane Greaves (Cardiff University) announced the discovery of phosphine to worldwide fanfare. The team’s findings were based on data from two telescopes: the James Clerk Maxwell Telescope (JCMT) and Atacama Large Millimeter Array (ALMA), both of which suggested the presence of phosphine in a quantity as high as 20 parts per billion (PPB) in Venus’ atmosphere.

Phosphine (PH3) is not a very stable gas and tends to decay quickly, meaning that for it to exist on Venus (or on Earth for that matter), there must be an ongoing process replenishing it. On gas giants like Jupiter, the high heat and pressure created by the planet’s enormous gravity well can easily produce phosphine, but such conditions do not exist on smaller rocky worlds. Here on Earth, microbes and industrial processes can create it, and so can volcanos.

On Venus, the sheer amount of phosphine detected seemed to suggest that geological processes like volcanos were not sufficient to be the source of the gas. Greaves and her team were careful to rule out, as best they could, any known geological and chemical processes before making the dramatic claim that it could be a sign of alien life. As far as they could tell, biology was the only known process that fit the data.

Of course, the claim attracted intense scrutiny, and within a few months several attempts had been made to duplicate the result. As often happens, these additional studies complicated the picture. Some researchers suggested that what Greaves thought was phosphine might actually be sulfur dioxide (SO2) in a different layer of the atmosphere. The discovery of a software malfunction at ALMA brought the data further into question.

The follow-on studies eventually seemed to settle on the position that yes, phosphine is indeed present on Venus, but in much lower quantities than the initial study suggested: closer to 1-5ppb, not 20ppb. These lower quantities opened the door for an alternative to the biological hypothesis: Venusian volcanos.

Phosphine From Explosive Volcanism

Even with the new, lower phosphine levels (1-5ppb), it would still require an extraordinary volcanic event to recreate what has been observed in Venus’ atmosphere. Simple lava flows would not push phosphine high enough to match the observations. It would take a mighty eruption to push the material to its position about 70km above the planet’s surface. Ngoc Truong and Jonathan Lunine, researchers who authored a new paper examining the potential role of volcanism in phosphine production, compared the necessary event to the famously dramatic eruptions of Krakatau in Indonesia.

Maat Mons, a large volcanic structure on Venus. Taken by the Magellan Spacecraft. Image Credit: NASA/JPL.

The process works like this: magma deep within the planet is rich in a substance called phosphide. When blasted into the air by an eruption, the phosphide can mix with sulphuric acid, which is common in Venus’s atmosphere. The reaction between these two substances produces – you guessed it – phosphine. As Lunine puts it, “The phosphine is not telling us about the biology of Venus. It’s telling us about the geology. Science is pointing to a planet that has active explosive volcanism today or in the very recent past.”

Lunine and Truong make a compelling case. But here’s the catch with the volcanism hypothesis. We aren’t even sure if Venus is volcanically active (it was in the past, but now? We just don’t know). Despite being so close to Earth, we know surprisingly little about the surface of Venus. Its clouds hinder observations in visible light wavelengths, and landers sent to the planet’s surface don’t survive more than a few hours in the harsh environment. Orbiters like Magellan (launch in 1989) mapped the planet using radar, but reliable information about the planet’s geology is surprisingly hard to come by.

NASA’s Pioneer Venus mission in 1979 found sulfur dioxide in the atmosphere that might point to volcanism, and Magellan observed some geological features that could mean recent volcanic activity, but none of it is conclusive. For the moment, the notion of active volcanos on Venus is just as speculative as the notion of microbial life. Both theories work hard to make sense of the evidence as best they can, but neither can be proven: yet.

What’s Next?

If we are able to solve this puzzle and learn the source of the phosphine in Venus’ atmosphere, we will have learned a great deal about Venus in the process, regardless of the answer. If microbial life is the source of the phosphine, the implications are obviously game-changing. If the source of the phosphine is eruptive volcanism, we will have learned something new about the geology of a planet that has long been shrouded in mystery.

Venus’s cloudy skies, as seen by Mariner 10 in 1974. Image Credit: NASA/JPL-Caltech.

Three new missions are scheduled to visit Venus in the near future: two spacecraft from NASA and one from the European Space Agency (ESA). None of the missions are directly designed to look for phosphine, but all are intended to give a fuller picture of the planetary system. One of the key priorities for these missions is to provide a much higher resolution map of Venus’ surface than Magellan was able to make. These three missions could help solve the phosphine mystery, but, as usual in planetary science, they are likely to raise as many questions as they answer.

Where am I placing my bets? It’s a tough call. Venus is a hostile place – volcanism seems more plausible there than life – but the Universe is a strange place, and extremophile microbes have been found in inhospitable habitats here on Earth. If something is alive in the clouds of Venus, it would be a surprise, but it wouldn’t be beyond the realm of the possible. Only time will tell, and the real answer may end up being something else entirely. Both Greaves and Lunine admit that the source of the phosphine might end up being a third option: there might be some unknown chemistry going on in Venus’ atmosphere that we’ve yet to discover.

Whatever the case, I look forward to finding out.

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Scott Alan Johnston

Scott Alan Johnston is a writer and historian of science and technology. He has a PhD from McMaster University in Hamilton, Ontario. You can follow him on Twitter @ScottyJ_PhD

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