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 phosphene in a quantity as high as 20 parts per billion (PPB) in Venus’ atmosphere.

Phosphene (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 phosphene, 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 phosphene 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 – phosphene. 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
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