So #phosphine/phosphane/PH3 was found in the atmosphere of Venus and people are speculating whether this implies the existence of microbial life in Venus's atmosphere. What's the chemistry behind this? I'll try to explain in a way that's also understandable for non-chemists 1/13

One Earth, the most common phosphorus (P) compounds contain oxygen, like phosphate, which is present in many minerals and also seawater and fresh water. P compounds without oxygen are pretty reactive in an oxygen atmosphere. 2/13

Elementary P and also PH3 have the hazard warnings flammable, toxic, corrosive, and environmental hazard. Dangerous stuff for most Earthlings! 3/13

How do we synthesize PH3? Under oxygen-free conditions, you can put elementary P (or certain other P-containing compounds) and molecular hydrogen (H2) together. Above a certain temperature (>800 K), the chemical equilibrium favors the formation of PH3. 4/13

This happens, for example, in the atmospheres of Jupiter and Saturn, where PH3 was also detected. There's more than enough H2 and at a certain altitude, the temperature is high enough. 5/13

BUT: On Earth, we can also find traces of PH3 in certain oxygen-poor environments where it is produced by anaerobic bacteria from environmental phosphorus at reasonable temperatures. How do they do it? 6/13

Nobody knows yet, but this is very interesting from a biochemical point of view: Remember that PH3 is very flammable in an oxygen atmosphere? The reaction, leading to phosphoric acid, releases a lot of energy. The reverse reaction *requires* this energy to proceed. 7/13

The generation of PH3 requires either a lot of energy or high temperatures in most (all?) cases we know of, except for that bacterial metabolism. Those bacteria seem to have found another way. 8/13

There are also other photochemical/atmospheric-chemical pathways, but none favor the production of PH3 under the environmental conditions in Venus's atmosphere. It is simply not hot enough. 9/13

So: All non-bio chemical pathways we know that generate PH3 don't really work under the conditions in Venus's atmosphere. The only one we know that is left is the metabolism of anaerobic bacteria. But does that mean that we found an indication for microbial life on Venus? 10/13

All science is based on experience and it's hard to imagine something we have never experienced. There might be chemical pathways leading to PH3 that we just don't know yet. But this is a great opportunity for atmospheric chemistry to explore possible alternative pathways! 11/13

It is also a great opportunity for biochemistry to investigate the bacterial metabolism in more detail and figure out the reaction mechanism. This and a thorough investigation of Venus's atmosphere will allow us to determine whether the PH3 was produced by bacteria or not. 12/13

All in all, this is an exciting discovery, not only for astronomy but also for chemistry! I am looking forward to all the upcoming research inspired by this 13/13

Sources/further reading:
"Phosphine gas in the cloud decks of Venus": https://www.nature.com/articles/s41550-020-1174-4
"Phosphine as a Biosignature Gas in Exoplanet Atmospheres": https://www.liebertpub.com/doi/10.1089/ast.2018.1954
https://arxiv.org/abs/1910.05224