The story so far
Early last fall, a research team led by Professor Jane Greaves (Cardiff University) announced the discovery of phosphine with fanfare around the world. The team’s findings were based on data from two telescopes: the James Clerk Maxwell Telescope (JCMT) and the Atacama Large Millimeter Array (ALMA), both of which suggested the presence of phosphene in an amount as high as 20 parts per billion (PPB) in the atmosphere of Venus.
Phosphene (PH3) is not a very stable gas and tends to decay quickly, which means that in order for it to exist on Venus (or Earth for that matter) there has to be a continuous process for it. replenish. On gas giants like Jupiter, the high heat and pressure created by the planet’s enormous gravity sink can easily produce phosphene, but such conditions do not exist on smaller rocky worlds. Here on Earth, microbes and industrial processes can create it, just like volcanoes.
On Venus, the amount of phosphine detected seemed to suggest that geological processes such as volcanoes were not sufficient to be the source of the gas. Greaves and his team took care to rule out, as best they could, any known geological and chemical processes before dramatically claiming that it could be a sign of extraterrestrial life. As far as they could tell, biology was the only known process that matched the data.
Of course, the claim drew close scrutiny, and within months several attempts were made to duplicate the result. As often happens, these additional studies complicated the picture. Some researchers have 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 further challenged the data.
The follow-up studies finally seemed to settle on the position that yes, phosphine is indeed present on Venus, but in amounts much lower than those suggested by the initial study: closer to 1-5 ppb, not 20 ppb. These smaller quantities opened the door to an alternative to the biological hypothesis: the Venusian volcanoes.
Explosive volcanic phosphine
Even with the new lower phosphine levels (1-5 ppb), it would still take an extraordinary volcanic event to recreate what was observed in the atmosphere of Venus. Simple lava flows would not push the phosphene high enough to match the observations. It would take a powerful eruption to push the material about 70 km above the planet’s surface. Ngoc Truong and Jonathan Lunine, researchers who wrote 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.
The process works like this: Magma deep within the planet is rich in a substance called phosphide. When blown into the air by an eruption, phosphide can mix with sulfuric acid, which is common in the atmosphere of Venus. The reaction between these two substances produces – you guessed it – phosphene. As Lunine says: “Phosphine does not tell us about the biology of Venus. He tells us about geology. Science points to a planet that has explosive volcanism active today or in the very recent past. “
Lunine and Truong make compelling arguments. But here’s the catch with the volcanism hypothesis. We don’t even know if Venus is volcanically active (that was in the past, but now? We just don’t know). Despite our proximity to Earth, we know surprisingly little about the surface of Venus. Its clouds hamper observations in the wavelengths of visible light, and landers sent to the surface of the planet do not survive more than a few hours in a hostile environment. Orbiters like Magellan (launched in 1989) have mapped the planet using radar, but reliable information on the planet’s geology is surprisingly hard to come by.
NASA’s Pioneer Venus mission in 1979 found sulfur dioxide in the atmosphere that could indicate volcanism, and Magellan observed some geological features that could signify recent volcanic activity, but none of this is conclusive. For the moment, the notion of active volcanoes on Venus is just as speculative as the notion of microbial life. Both theories are working hard to make sense of the evidence as best they can, but neither can be proven: yet.
If we are able to solve this riddle and learn the source of phosphine in the atmosphere of Venus, we will have learned a lot about Venus in the process, regardless of the answer. If microbial life is the source of phosphene, the implications are obviously a game-changer. If the source of the phosphene is eruptive volcanism, we will have learned something new about the geology of a planet long shrouded in mystery.
Three new missions are planned 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 search for phosphine, but all are intended to give a more complete picture of the planetary system. One of the main priorities of these missions is to provide a map of the surface of Venus at a much higher resolution than what Magellan was able to do. These three missions could help solve the mystery of phosphine, but, as usual in planetary science, they are likely to raise as many questions as they answer.
Where do I place my bets? It’s a tough call. Venus is a hostile place – volcanism there seems more plausible than life – but the Universe is a strange place, and extremophilic microbes have been found in inhospitable habitats here on Earth. If anything is alive in the clouds of Venus it would be a surprise, but it would not be beyond the realm of possibility. Only time will tell, and the real answer could be something quite different. Greaves and Lunine both admit that the source of the phosphine might end up being a third option: There might be some unknown chemistry in Venus’ atmosphere that we haven’t yet discovered.
Anyway, I can’t wait to find out.