An international team of astronomers have discovered a rare molecule – phosphine – in the clouds of Venus that could indicate the presence of life.
On Earth, phosphine is only made industrially, or by microbes that thrive in oxygen-free environments.
The phosphine is there. How it got there is another question. Dr Dave Clements
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. The new discovery, led by Cardiff University and including Imperial College London researchers, is described today in a paper in Nature Astronomy.
The team first used the James Clerk Maxwell Telescope (JCMT) in Hawaii to detect the phosphine, and were then awarded time to follow up their discovery with 45 antennae of the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile.
Lead researcher Professor Jane Greaves of Cardiff University said: “This was an experiment made out of pure curiosity, really – 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!”
Co-author Dr Dave Clements, from the Department of Physics at Imperial, says it is tricky to observe Venus due to its brightness. He said: “We expected that the signal would go away as we processed the data, but it didn’t. The follow-up observations with ALMA only strengthened the conclusion that this was a real and good detection – the phosphine is there. How it got there is another question.”
No known natural cause
Once they had confirmed the detection, the researchers teamed up with Dr Hideo Sagawa of Kyoto Sangyo University, who used his models of the Venusian atmosphere to interpret the data, finding that phosphine is present but scarce – only about twenty molecules in every billion.
Even so, analysis of all the possible known natural sources could not produce as much phosphine as observed. Massachusetts Institute of Technology 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 these sources were found to only 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 percent of their maximum productivity, according to calculations by Dr Paul Rimmer of Cambridge University.
Earth bacterium can absorb phosphate minerals, add hydrogen, and ultimately expel phosphine gas. It costs them energy to do this, so why they do it is not clear. The phosphine could be just a waste product, but other scientists have suggested purposes like warding off rival bacteria. On Earth, phosphine can be found in pond slime, the intestinal fauna of badgers, and penguin guano.
A different form of life
The team believes 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. Although the high clouds of Venus have temperatures around a warm 50 degrees centigrade, they are incredibly acidic – around 90 percent sulphuric acid – posing major issues for microbes to survive there.
If we gather enough evidence in the future to show it is there, the most pressing question becomes: how similar is it to life on Earth? Does it also use DNA, which would suggest a common origin for life on Earth and Venus, or is it something completely different? Dr Dave Clements
Any microbes on Venus would likely be very different to their Earth cousins to survive in hyper-acidic conditions.
Another team-member, Dr Clara Sousa Silva from MIT, had been thinking about searching for phosphine as a ‘biosignature’ gas of non-oxygen-using life on planets around other stars, because normal chemistry makes so little of it.
She comments: “Finding phosphine on Venus was an unexpected bonus! The discovery raises many questions, such as how any organisms could survive. On Earth, some microbes can cope with up to about five percent of acid in their environment – but the clouds of Venus are almost entirely made of acid.” Professor Sara Seager and Dr Janusz Petkowski, also both at MIT, are investigating how microbes could shield themselves inside droplets.
Imperial’s Dr Clements said: “It would be great if this phosphine detection means we have detected life, but at the moment it’s not a smoking gun – it’s just the hint of an indication of a possibility.
“But if we gather enough evidence in the future to show it is there, the most pressing question becomes: how similar is it to life on Earth? Does it also use DNA, which would suggest a common origin for life on Earth and Venus, or is it something completely different?”
The team are now eagerly 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. New space missions could also travel to our neighbouring planet and sample the clouds in situ to further search for signs of life.
'Phosphine gas in the cloud decks of Venus' by Jane S. Greaves et al. is published in Nature Astronomy.
Top image: Image of Venus, observed in the 365nm waveband by the Venus Ultraviolet Imager (UVI) on board the Akatsuki probe. The observations were made on 6 May 2016, when the spacecraft saw the whole planet illuminated. Credit: J. Greaves / Cardiff University
Animation credit: ESO / M. Kornmesser / L. Calçada & NASA / JPL / Caltech
Based on a press release by the Royal Astronomical Society.
Article text (excluding photos or graphics) © Imperial College London.
Photos and graphics subject to third party copyright used with permission or © Imperial College London.
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