Astronomers find possible sign of life in clouds above Venus
Could there be extra-terrestial life in the clouds above Venus?
An international team of scientists, including Cambridge astrochemist Paul Rimmer, have announced the discovery of a rare molecule in the planet’s atmosphere called phosphine that could indicate the presence of microbes.
Venus has a scorching surface of 465 degrees Celsius. But for decades astronomers have speculated that its high clouds - which are at a much more pleasant 30C - could harbour microbial life capable of tolerating extreme acidity.
Phosphine could be a signature of this life, for on Earth te gas is only made industrially or by microbes that thrive in oxygen-free environments.
Team leader Jane Greaves, of Cardiff University, first spotted the signs of phosphine in observations from the James Clerk Maxwell Telescope (JCMT), operated by the East Asian Observatory, in Hawaii.
“When we got the first hints of phosphine in Venus’s spectrum, it was a shock,” she said.
The observation was confirmed using a more sensitive telescope - namely the 45 antennas of the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile.
The facilities observed Venus at wavelengths of about one millimetre, much longer than visible to the eye. Such wavelengths require telescopes at high altitudes.
Anita Richards, of the UK ALMA Regional Centre and the University of Manchester, added: “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.”
Prof Greaves added: “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 international research team from the UK, US and Japan has estimated that phosphone exists in the clouds of Venus at small concentrations of about 20 molecules per billion.
They then ran calculations to explore whether these amounts could come from natural non-biological processes on the planet - such as from sunlight, minerals blown upwards from the surface, volcanoes or lightning.
But these were found to make at most one ten thousandth of the amount of phosphine detected.
However, terrestrial organisms would only to work at about 10 per cent of the maximum productivity to create the observed quantity of the gas, which consists of hydrogen and phosphorus.
Paul Rimmer, a post-doc in the University of Cambridge’s Department of Earth Sciences, with affiliations at Cavendish Astrophysics and the MRC Laboratory of Molecular Biology , brought to bear his expertise in the geochemical and astronomical contexts for prebiotic chemistry.
“This discovery brings us right to the shores of the unknown,” he said. “Phosphine is very hard to make in the oxygen-rich, hydrogen-poor clouds of Venus and fairly easy to destroy. The presence of life is the only known explanation for the amount of phosphine inferred by observations.
“Both of these facts lie at the edge of our knowledge: the observations could be caused by an unknown molecule, or could be caused by chemistry we’re not aware of. Ultimately, the only way to find out what's really happening is to send a mission into the clouds of Venus to take a sample of the droplets and look at them to see what's inside.”
On Earth, bacteria take up phosphate from minerals or biological material, add hydrogen, and expel phosphine.
While organisms on Venus are likely to be very different to those on Earth, they could be the source of the atmospheric phosphine.
But they would have to be hardy souls, for the planet’s high clouds are made of about 90 per cent sulphuric acid.
Clara Sousa Silva, a member of the team from the Massachusetts Institute of Technology in the US, has probed how phosphine can be a “biosignature” gas of life that does not use oxygen on planets around other stars, since normal chemistry makes so little of it.
“Finding phosphine on Venus was an unexpected bonus,” she said. “The discovery raises many questions, such as how any organisms could survive. On Earth, some microbes can cope with up to about five per cent of acid in their environment — but the clouds of Venus are almost entirely made of acid.”
While an exciting finding, confirming the presence of life requires a lot more work.
ESO astronomer and ALMA European operations manager Leonardo Testi, who was not involved in the study, said: “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.”
To gather clues of how phosphine originates, more observations of Venus and of rocky planets outside our Solar System are needed, including with ESO’s forthcoming Extremely Large Telescope.
Professor Emma Bunce, president of the Royal Astronomical Society, said: “A key question in science is whether life exists beyond Earth, and the discovery by Professor Jane Greaves and her team is a key step forward in that quest. I’m particularly delighted to see UK scientists leading such an important breakthrough – something that makes a strong case for a return space mission to Venus.”
The study was published on Monday in Nature Astronomy.
Q&A with Dr Paul Rimmer, of the University of Cambridge’s Department of Earth Sciences
What part of the work were you involved in?
I mostly worked on the photochemistry of phosphine (PH3). I figured out how PH3 is destroyed in the atmosphere and how long it can survive in the clouds, which provides the test a mechanism must pass in order to explain the data. I helped test some of the abiotic mechanisms, specifically the photochemistry and meteoritic delivery.
Was the discovery a surprise to you?
I wasn't there when Jane discovered PH3. When she told me about it, and when she and Anita found the signal in the ALMA data, I was surprised, mostly because it just shouldn't be there, based on what I knew at the time about the photochemistry. It should be hard to make and easy to destroy.
What's next for the work?
My primary work is on origins of life and atmospheric chemistry. My next Venus work will be looking at the sulphur cycle, to see whether that can tell us anything more about what the clouds are really like.
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