Possible marker of life seen on the Venus

An international team of astronomers today announced the invention of a rare molecule -- phosphine -- within the clouds of Venus. On Earth, this gas is merely made industrially or by microbes that thrive in oxygen-free environments. Astronomers have speculated for many years that top clouds on Venus could offer a home for microbes -- floating freed from the scorching surface but wanting to tolerate very high acidity. The detection of phosphine could point to such extra-terrestrial 'aerial' life.

"When we got the first hints of phosphine in Venus's spectrum, it was a shock!," says team leader Jane Greaves of Cardiff University in the UK, who first spotted signs of phosphine in observations from the James Clerk Maxwell Telescope (JCMT), operated by the East Asian Observatory, in Hawai'i. Confirming their discovery required using 45 antennas of the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, a more sensitive telescope during which the ecu Southern Observatory (ESO) is a partner. Both facilities observed Venus at a wavelength of about 1 millimetre, for much longer than the human eye can see -- only telescopes at high altitude can detect it effectively.

The international team, which incorporates researchers from the united kingdom , US and Japan, estimates that phosphine exists in Venus's clouds at alittle concentration, only about twenty molecules in every billion. Following their observations, they ran calculations to ascertain whether these amounts could come from natural non-biological processes on the earth . Some ideas included sunlight, minerals blown upwards from the surface, volcanoes, or lightning, but none of those could make anywhere near enough of it. These non-biological sources were found to form at the most one ten thousandth of the quantity of phosphine that the telescopes saw.

To create the observed quantity of phosphine (which consists of hydrogen and phosphorus) on Venus, terrestrial organisms would only got to work on about 10% of their maximum productivity, consistent with the team. Earth bacteria are known to form phosphine: they take up phosphate from minerals or biological material, add hydrogen, and ultimately expel phosphine. Any organisms on Venus will probably be very different to their Earth cousins, but they too might be the source of phosphine within the atmosphere.

While this detection of phosphine in the clouds of Venus has come as a surprise, the researchers are sure that it will be observed. "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," says team member Anita Richards of the UK ALMA Regional Centre and the University of Manchester. "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," adds Greaves, who led the study published today in Nature Astronomy. 

Another team member, Clara Sousa Silva of the Massachusetts Institute of Technology in the US, investigated phosphine as a "biosignature" gas of non-oxygen-using life on planets around other stars, since normal chemistry produces too 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 5% of acid in their environment -- but the clouds of Venus are almost entirely made of acid."

The team claims that their discovery is important because they can rule out several possible ways to produce phosphine, but they understand that verifying the existence of "life" requires a lot more work. Although the high Venus clouds have temperatures of up to a pleasant 30 degrees Celsius, they are extremely acidic — about 90 per cent sulphuric acid — which presents big problems for any microbes seeking to live there.

Leonardo Testi, ESO astronomer and ALMA European Operations Chief, who did not participate in the new report, 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."

More observations of Venus and of rocky planets beyond our Solar System, including the upcoming Extremely Large Telescope of ESO, may help to gather clues as to how phosphine may originate in them and contribute to the search for signs of life beyond Earth.

The analysis was presented in the paper "Phosphine Gas in the Cloud Decks of Venus" to appear in Nature Astronomy.

The team consists of Jane S. Greaves (School of Physics & Astronomy, Cardiff University, UK [Cardiff]), Anita M. S. Richards (Jodrell Bank Centre for Astrophysics, The University of Manchester, UK), William Bains (Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, USA [MIT]), Paul Rimmer (Department of Earth Sciences and Cavendish Astrophysics, University of Cambridge and MRC Laboratory of biology , Cambridge, UK), Hideo Sagawa (Department of Astrophysics and Atmospheric Science, Kyoto Sangyo University, Japan), David L. Clements (Department of Physics, Imperial College London, UK [Imperial]), Sara Seager (MIT), Janusz J. Petkowski (MIT), Clara Sousa-Silva (MIT), Sukrit Ranjan (MIT), Emily Drabek-Maunder (Cardiff and Royal Observatory Greenwich, London, UK), Helen J. Fraser (School of Physical Sciences, The Open University , Milton Keynes, UK), Annabel Cartwright (Cardiff), Ingo Mueller-Wodarg (Imperial), Zhuchang Zhan (MIT), Per Friberg (EAO/JCMT), Iain Coulson (EAO/JCMT), E'lisa Lee (EAO/JCMT) and Jim Hoge (EAO/JCMT).

An accompanying paper by some of team members, titled "The Venusian Lower Atmosphere Haze as a Depot for Desiccated Microbial Life: A Proposed Life Cycle for Persistence of the Venusian Aerial Biosphere," Released in August 2020 in Astrobiology. A further related study by some of the same authors, "Phosphine as a Biosignature Gas in Exoplanet Atmospheres," was published in Astrobiology in January 2020.