Artist’s impression of Venus and the phosphine (PH₃) detected in the atmosphere.
Credit: ESO/M. Kornmesser/L. Calçada & NASA/JPL/Caltech

“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 Hawaiʻi. Confirming their discovery required the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, a more sensitive telescope. The reason why she was so shocked is that phosphine can be produced by microbes on the Earth ^[1] , although the research team does not think that they found life on Venus.

How do you find life on a planet from quite far away? One way is to study its atmosphere and find a biomarker that can be evidence of the presence of living forms. If a molecule in the atmosphere is mainly produced by living organisms and the contribution from abiotic origins is negligibly small, it can be a good biomarker.

The international team led by Greaves, including Hideo Sagawa at Kyoto Sangyo University, studied the signal of phosphine in the radio spectra and found that the amount of the molecule is about 20 parts per billion in the atmospheric molecules. This is quite a small amount, but enough to astonish the researchers. This is because researchers have supposed that most of the phosphorus, if it existed in the first place, would bind with oxygen atoms because the Venusian atmosphere has a huge amount of oxygen atoms, although most of them are in the form of carbon dioxide (CO₂).

ALMA image of Venus, superimposed with spectra of phosphine observed with ALMA (in white) and JCMT (in grey). As molecules of phosphine float in the high clouds of Venus, they absorb some of the millimeter waves that are produced at lower altitudes. When observing the planet in the millimeter wavelength range, astronomers can pick up this phosphine absorption signature in their data as a dip in the light from the planet.
Credit: ALMA (ESO/NAOJ/NRAO), Greaves et al. & JCMT (East Asian Observatory)

The team carefully examined the possible origins of the phosphine: production by chemical reaction in the atmosphere driven by strong sunlight or lightning, supply from volcanic activity, and delivery by meteorites. The team found that all of these known processes failed to produce the observed amount of phosphine. The amount of phosphine molecules produced by those processes is 10,000 times smaller than the amount detected with the radio telescopes.

The researchers supposed that phosphine is produced by unknown photochemistry or geochemistry, but they also considered the possibility of biological origin. On Earth, some microbes produce and egest phosphine. If similar living organisms were in the Venusian atmosphere, they could produce the detected amount of phosphine.

“Although we concluded that known chemical processes cannot produce enough phosphine, there remains the possibility that some hitherto unknown abiotic process exists on Venus,” says Sagawa. “We have a lot of homework to do before reaching an exotic conclusion, including re-observation of Venus to verify the present result itself.”

Venus is Earth’s twin, in terms of size. However, the atmospheres of the two planets are quite different. Venus has a very think atmosphere and the devastating greenhouse effect raises the surface temperature as high as 460 degrees Celsius. Some researchers argue that the upper atmosphere is much milder and possibly habitable, but the extremely dry and deadly acidic atmosphere would make it difficult for a life form similar to the ones on Earth to survive on Venus.

Further observations with large telescopes on the Earth, including ALMA, and ultimately on-site observations and a sample return of the Venusian atmosphere by space probes will provide crucial information to understand the mysterious origin of the phosphine.

Paper and the research team
These observation results were presented in J. S. Greaves et al. “Phosphine Gas in the Cloud Decks of Venus” in Nature Astronomy published on September 14, 2020.

The research team members are:
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 Molecular Biology, Cambridge, UK), Hideo Sagawa (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 (East Asian Observatory [EAO] /JCMT), Iain Coulson (EAO/JCMT), E’lisa Lee (EAO/JCMT), and Jim Hoge (EAO/JCMT).

This research was supported by JSPS KAKENHI (No. 16H02231), STFC (grant ST/N000838/1), Radionet/MARCUs, Heising-Simons Foundation, the Change Happens Foundation, and the Simons Foundation (495062).

The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of the European Southern Observatory (ESO), the U.S. National Science Foundation (NSF) and the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Republic of Chile. ALMA is funded by ESO on behalf of its Member States, by NSF in cooperation with the National Research Council of Canada (NRC) and the Ministry of Science and Technology (MOST) in Taiwan and by NINS in cooperation with the Academia Sinica (AS) in Taiwan and the Korea Astronomy and Space Science Institute (KASI).
ALMA construction and operations are led by ESO on behalf of its Member States; by the National Radio Astronomy Observatory (NRAO), managed by Associated Universities, Inc. (AUI), on behalf of North America; and by the National Astronomical Observatory of Japan (NAOJ) on behalf of East Asia. The Joint ALMA Observatory (JAO) provides the unified leadership and management of the construction, commissioning and operation of ALMA.

With a diameter of 15m (50 feet) the James Clerk Maxwell Telescope (JCMT) is the largest single dish astronomical telescope in the world designed specifically to operate in the submillimetre wavelength region of the electromagnetic spectrum. The JCMT is used to study our Solar System, interstellar and circumstellar dust and gas, evolved stars, and distant galaxies. It is situated in the science reserve of Maunakea, Hawaiʻi, at an altitude of 4092m (13 425 feet). The JCMT is operated by the East Asian Observatory on behalf of NAOJ; ASIAA; KASI; CAMS as well as the National Key R&D Program of China. Additional funding support is provided by the STFC and participating universities in the UK and Canada.