Astronomers using the James Clerk Maxwell Telescope (JCMT) and the Atacama Large Millimeter/submillimeter Array (ALMA) have detected phosphine (PH3) gas in temperate but hyperacidic cloud decks of Venus. In Earth’s atmosphere, this gas is uniquely associated with anthropogenic activity or microbial presence.
A composite image of Venus as seen by JAXA’s Akatsuki spacecraft. Image credit: Institute of Space and Astronautical Science / Japan Aerospace Exploration Agency.
Studying the atmospheres of rocky planets gives clues to how they interact with surfaces and subsurfaces, and whether any atmospheric compounds could reflect the presence of life.
Characterizing the atmospheres of extrasolar planets is extremely challenging, especially for rare compounds. The Solar System thus offers important testbeds for exploring planetary geology, climate and habitability, via both sampling and remote monitoring.
An ideal biosignature gas would be unambiguous. Living organisms should be its sole source, and it should have intrinsically strong, precisely characterized spectral transitions unblended with contaminant lines — criteria that are not usually all achievable.
It was recently proposed that any phosphine gas detected in the atmosphere of a rocky planet is a promising sign of extraterrestrial life.
“We found that JCMT and ALMA 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,” said lead author Professor Jane Greaves, a researcher in the School of Physics and Astronomy and the Institute of Astronomy at Cardiff University.
“This phosphine signal is perfectly positioned where others have conjectured the area could be habitable,” said co-author Dr. Janusz Petkowski, a scientist in the Department of Earth, Atmospheric, and Planetary Sciences at MIT.
The researchers then used a model of the Venusian atmosphere to interpret the data.
They found that phosphine on Venus is a minor gas, existing at a concentration of about 20 out of every billion molecules in the atmosphere.
They used computer models to explore all the possible chemical and physical pathways not associated with life, that could produce phosphine in Venus’ harsh environment.
The authors considered various scenarios that could produce phosphine, such as lightning, volcanic or meteoritic delivery.
They then modeled how phosphine produced through these mechanisms could accumulate in the Venusian clouds.
In every scenario they considered, the phosphine produced would only amount to a tiny fraction of what the new observations suggest is present on Venus’ clouds.
Phosphine molecules were detected in the Venusian high clouds in data from the James Clerk Maxwell Telescope and the Atacama Large Millimeter/submillimeter Array. Image credit: ESO / M. Kornmesser / L. Calçada / NASA / JPL / Caltech.
“We really went through all possible pathways that could produce phosphine on a rocky planet,” Dr. Petkowski said.
“If this is not life, then our understanding of rocky planets is severely lacking.”
If there is indeed life in the Venusian clouds, the team believes it to be an aerial form, existing only in Venus’ temperate cloud deck — between 48 and 60 km (30-37.3 miles) above the surface — far above the boiling, volcanic surface.
“A long time ago, Venus is thought to have oceans, and was probably habitable like Earth,” said co-author Dr. Clara Sousa-Silva, also from the Department of Earth, Atmospheric, and Planetary Sciences at MIT.
“As Venus became less hospitable, life would have had to adapt, and they could now be in this narrow envelope of the atmosphere where they can still survive.”
“This could show that even a planet at the edge of the habitable zone could have an atmosphere with a local aerial habitable envelope.”
The team is now awaiting more telescope time to establish whether the phosphine is in a relatively temperate part of the clouds and to look for other gases associated with life.
“Technically, biomolecules have been found in Venus’ atmosphere before, but these molecules are also associated with a thousand things other than life,” Dr. Sousa-Silva said.
“The reason phosphine is special is, without life it is very difficult to make phosphine on rocky planets. Earth has been the only terrestrial planet where we have found phosphine, because there is life here. Until now.”
The team’s paper was published in the journal Nature Astronomy.
J.S. Greaves et al. Phosphine gas in the cloud decks of Venus. Nat Astron, published online September 14, 2020; doi: 10.1038/s41550-020-1174-4