Rare Molecule: Life on Venus?

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GIs there life on Venus? The press releases that circulated on Monday at least seemed to suggest so. Astronomers working with Jane Greaves from Cardiff University had observed the trivalent hydrogen phosphide PH₃, also called phosphine, in the cloud layer of our neighboring planet, which could be interpreted as an indication of the existence of alien life. The scientific work itself, which appeared in the journal “Nature Astronomy”, on the other hand, reads much more cautiously: “We emphasize that the detection of phosphine does not provide robust evidence for life, only for unusual and unexplained chemistry”, it says at the end of the study. And yet: the authors do not want to rule out life as the origin of the chemical anomaly, even if this is not the most likely explanation.

The astronomers had discovered the signature of the molecule PH₃ in observations with the James Clerk Maxwell Telescope (JCMT) in Hawaii and the Alma Observatory in the Chilean Atacama Desert. The recorded absorption line in the millimeter range of the electromagnetic spectrum corresponds to the deepest rotational transition of PH₃ and, according to the astronomers, cannot have been caused by any other chemical species. Using chemical models, the thickness of the line can be used to determine the frequency of the molecule. According to this, there are around 20 phosphine molecules for every billion molecules in the atmosphere – a tiny figure that would, however, be larger than the one here on earth. Most of the gas here is produced by us humans; for example, the hydrogen phosphide is used in the agricultural industry. However, it has also been detected in the digestive systems of animals and as a product of certain bacterial cultures.

This is how artists imagine the hydrogen phosphides in the atmosphere of Venus.


This is how artists imagine the hydrogen phosphides in the atmosphere of Venus.
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Image: Reuters

That one could use PH₃ to prove life on strange planets, was suggested only recently by some of those astronomerswho now claim to have found the molecule in Venus’ atmosphere. It is not one of the classic biomarkers discussed for this purpose, such as coexisting oxygen and methane. However, PH₃ is a molecule that is highly reactive, so it is quickly chemically converted. Therefore, if it is observed, there must be some reliable production mechanism. Inside stars or gas giants, the molecule is formed at high temperatures and then transported from there to the outside into the atmospheres of the heavenly bodies, where it has already been detected. In environments where the temperatures are not high enough for this chemical process, one has to look for an alternative source: and these could be biological processes.

One problem with the use of this phosphorus hydrogen as a biosignature is that there are still many questions about phosphorus chemistry. Even the biological production of phosphine on earth is not yet fully understood. The fact that astronomers cannot unambiguously explain their discovery of Venus using known non-biological chemical processes is therefore by no means particularly surprising, especially since it is hardly known how the chemical reactions known from earthly laboratories are influenced by the droplets of the Venus clouds . The discovery could just as easily be explained by the fact that there are chemical reactions in the Venusian atmosphere that we have not yet known in this form – this is what astronomers mean when they write of “unusual and unexplained chemistry”.

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