Earth’s sister planet, Venus, has not been considered a priority when looking for extraterrestrial life. Its surface temperature of around 450 ° C is believed to be hostile to even the most resistant microorganisms, and its dense, sulfurous and acidic atmosphere has kept the surface almost completely free of visiting spacecraft.
We have only been able to take a brief look at its arid landscape from the two modules of russian landing that reached the surface of Venus in the 1980s. It is therefore not surprising that a report published in Nature Astronomy that the upper levels of Venus’s atmosphere contain a molecule that is a possible sign of life has supposed a kind of shock.
The molecule in question is PH₃ (phosphine). It is a highly reactive, flammable and extremely smelly toxic gas, which is found, among other places, in penguin dung and in the entrails of badgers and fish.
It is present in the Earth’s atmosphere only in small amounts, less than a few parts per trillion, because it is rapidly destroyed by the process of oxidation. However, the fact that this molecule is present in our oxidizing atmosphere is due to the fact that it is continuously produced by microbes. Therefore, it is proposed that phosphine in the atmosphere of a rocky planet is a solid signal for life.
It should not be stable in the atmosphere of a planet like Venus, where it would rust rapidly unless, as on Earth, there is a constant fresh supply. So why were the study authors looking for phosphine in such an unpromising environment? Are you sure you have found it?
Reading between the lines of the report, it appears that the team did not expect to find phosphine. In fact, they seemed to be actively seeking his absence. Venus was to supply the “base atmosphere” of a rocky planet, free of a phosphine biosignature. Scientists researching rocky exoplanets could compare the atmospheres of these bodies with those of Venus, to identify any possible biological signatures of phosphine.
So finding a global concentration of the molecule around 1,000 times that of Earth was somewhat surprising. In fact, he had the authors perform one of the most detailed forensic dissections of their own data that I have ever seen.
The first data set was obtained in June 2017 using the James Clerk Maxwell Telescope (JCMT) in Hawaii and unequivocally indicated the presence of phosphine, so a second data set was recorded, using a different instrument in a different telescope.
These observations were taken in March 2019, at higher spectral resolution, using the Atacama Large Millimeter Array(ALMA) in Chile. The two data sets were almost indistinguishable. Phosphine is present in the atmosphere of Venus, with an irregular distribution in the middle latitudes, decreasing towards the poles.
But where did it come from? The raw material for phosphine is phosphorus, an element with well-known chemistry that underpins many possible chemical reactions. Phosphorus in the atmosphere of Venus was measured by the Vega probes (from the former Soviet Union) and it was found to occur as the oxidized molecule P₄O₆.
In trying to explain the presence of phosphine, the astronomer Jane Greaves, from Cardiff University and his team, used Vega’s data and modeled almost 100 different chemical reactions in the atmosphere to see if they could recreate the phosphine they had found.
Despite doing it under varying conditions (pressure, temperature, reagent concentration), they found that none were viable. They even considered subsurface reactions, but Venus would have to have volcanic activity at least two hundred times that of Earth to produce enough phosphine in this way.
How about a meteor that carries the substance to Venus? They also considered it, but found that it would not allow the amounts of phosphine that the data indicates. Furthermore, there is no evidence of a large recent impact that may have increased atmospheric phosphorus concentrations. The team also considered whether reactions with lightning or the solar wind could create phosphine in the atmosphere, but found that only negligible amounts would be produced this way.
Where does that leave us then? Phosphine is present in the atmosphere of Venus at concentrations well above the level that can be explained by non-biological processes. Does that mean that there are microbes present in Venus’s atmosphere, navigating through the clouds in aerosol droplets, a microscale Venus fly trap?
Evidences of life? Only “anomalous and inexplicable chemistry”
The authors do not claim to have found evidence of life, only of “anomalous and inexplicable chemistry.” But, as Sherlock Holmes told Dr. Watson: “Once you eliminate the impossible, what remains, however improbable, must be the truth.”
The presence of methane as a biological signal in the atmosphere of Mars is still hotly debated. It may be that astrobiologists searching for life beyond Earth now have an added atmospheric biological signal about which to discuss.
The European Space Agency is considering currently a mission to Venus that would determine its geological and tectonic history, including the observation of possible volcanic gases. This would give a better idea of the compounds in the atmosphere of Venus. The new study should advance this mission.
Monica grady She works for the Open University and is the Chancellor of Liverpool Hope University and Principal Investigator at the Museum of Natural History. It has received funding from the STFC and the UK Space Agency.