Life on Venus? Scientists say discovery of phosphine gas in planet’s clouds may be ‘a possible sign of life’
Confirming the discovery required using 45 antennas of Atacama Large Millimeter/submillimeter Array in Chile, a more sensitive telescope in which the European Southern Observatory is a partner
An international team of astronomers has made an unexpected and intriguing discovery: they have detected a rare molecule – phosphine, a possible marker of life – in the atmosphere of Venus. On Earth, this gas is made industrially, or by microbes that thrive in oxygen-free environments. Hence, the discovery has sparked excitement about the possibility of the presence of life forms on Venus.
Astronomers have speculated for decades that high clouds on Venus could offer a home for microbes – floating free of the scorching surface but still needing to tolerate very high acidity. The detection of phosphine molecules, which consist of hydrogen and phosphorus, could point to this extra-terrestrial ‘aerial’ life, according to the study published in Nature Astronomy. “The amount of phosphine that has been detected is relatively large. On Earth, phosphine can result from natural processes such as lightning and volcanic activity, but only in small amounts. The only known processes that produce phosphine on Earth in similar quantities are biological in origin,” write authors.
The researchers, however, clarify that the finding does not imply there is definitely life on Venus. But what is exciting is that this is the first detection of a “possible sign of life” for which scientists have no plausible alternative explanation currently. The team explains that they could be missing some other method of producing phosphine in the required amounts, but it is a very exciting possibility that needs more investigation.
The scientists first used the James Clerk Maxwell Telescope (JCMT) in Hawaii to detect the phosphine. “This was an experiment made out of pure curiosity taking advantage of JCMT’s powerful technology and thinking about future instruments. I thought we’d just be able to rule out extreme scenarios, like the clouds being stuffed full of organisms. When we got the first hints of phosphine in Venus’ spectrum, it was a shock,” says Professor Jane Greaves of Cardiff University, UK, who led the study. Greaves first spotted signs of phosphine in observations from the JCMT, operated by the East Asian Observatory, in Hawaii. Confirming their discovery required using 45 antennas of the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, a more sensitive telescope in which the European Southern Observatory (ESO) is a partner. Both facilities observed Venus at a wavelength of about 1 millimeter, much longer than the human eye can see – only telescopes at high altitude can detect it effectively.
While the discovery of phosphine in Venus’s clouds came as a surprise, the researchers are confident in their detection. “To our great relief, the conditions were good at ALMA for follow-up observations while Venus was at a suitable angle to Earth. Processing the data was tricky, though, as ALMA isn't usually looking for very subtle effects in very bright objects like Venus,” explains team member Anita Richards of the UK ALMA Regional Centre and the University of Manchester. Professor Greaves adds, “In the end, we found that both observatories had seen the same thing – faint absorption at the right wavelength to be phosphine gas, where the molecules are backlit by the warmer clouds below.”
The team, which includes researchers from the UK, US and Japan, estimates that phosphine exists in Venus’ clouds at a small concentration, only about 20 molecules in every billion. Following their observations, they ran calculations to see whether these amounts could come from natural non-biological processes on the planet. Massachusetts Institute of Technology (MIT) scientist Dr William Bains led the work on assessing natural ways to make phosphine. Some ideas included sunlight, minerals blown upwards from the surface, volcanoes, or lightning, but none of these could make anywhere near enough of it. These non-biological sources were found to make at most “one ten-thousandth of the amount of phosphine that the telescopes saw”.
To create the observed quantity of phosphine on Venus, terrestrial organisms would only need to work at about 10% of their maximum productivity, according to calculations by Dr Paul Rimmer of Cambridge University. Earth bacteria are known to make phosphine: they take up phosphate from minerals or biological material, add hydrogen, and ultimately expel phosphine. Any microbes on Venus will likely be very different from their Earth cousins though, to survive in hyper-acidic conditions. “Processes such as lightning or volcanic activity can produce small amounts of phosphine on Earth. The molecule can also be delivered by meteorites, or generated by exotic processes such as interaction with the solar wind. However these only produce very small amounts, far less than would be necessary to explain the observations,” say experts.
The scientists believe their discovery is significant because they can rule out many alternative ways to make phosphine, but they acknowledge that confirming the presence of life needs a lot more work. While the high clouds of Venus have temperatures up to a pleasant 30 degrees Celsius, they are incredibly acidic – around 90% sulphuric acid – posing major issues for microbes to survive there. Professor Sara Seager and Dr Janusz Petkowski, both at MIT, are investigating how microbes could shield themselves inside droplets.
The research team is also awaiting more telescope time, for example, to establish whether the phosphine is in a relatively temperate part of the clouds, and to look for other gases associated with life. More observations of Venus and of rocky planets outside the Solar System, including with ESO’s forthcoming Extremely Large Telescope, may help gather clues on how phosphine can originate on them and contribute to the search for signs of life beyond Earth. Leonardo Testi, ESO astronomer and ALMA European Operations Manager, who did not participate in the new study, says: “The non-biological production of phosphine on Venus is excluded by our current understanding of phosphine chemistry in rocky planets' atmospheres. Confirming the existence of life on Venus's atmosphere would be a major breakthrough for astrobiology; thus, it is essential to follow-up on this exciting result with theoretical and observational studies to exclude the possibility that phosphine on rocky planets may also have a chemical origin different than on Earth.”