Astrobiology
Recent Controversies
Phosphine and Venus
On September 14th 2020 a paper appeared in Nature Astronomy which described what is probably the most exciting scientific discovery of my career. I was privileged to be involved in the discovery and analysis of the presence of phosphine on Venus.
Why is this exciting? Phosphine is a gas that on Earth is solely made by living organisms. There are no sources of phosphine that are not associated with life. Finding phosphine somewhere is a very strong hint that there is life there.
But as Laplace said "The weight of evidence for an extraordinary claim must be proportioned to its strangeness." Carl Sagan popularised this idea in the phrase "Extraordinary claims require extraordinary evidence", and there is little more extraordinary than the idea that a planet whose surface is 430oC and whose atmosphere has clouds of concentrated sulfuric acid actually harbours life. So to even entertain the idea that phosphine could be made by life we had to rule out every possible other explanation. The work I and several others did was to rule out other sources - volcanoes, atmospheric gas reactions, dust reactions, meteorites, lightning, everything. We could not find a plausible non-biological source for the phosphine.
Does this mean that we can say there is life on Venus? No, for two reasons. Firtsly, we have one piece of evidence that there is phosphine there, and the presence of phosphine hints that there is life. We need more evidence that the phosphine really is there, and then separate, independent pieces of evidence that there is life there, before we consider life as more than one, speculative explanation out of many. We might, after all, have got the chemistry of Venus wrong, and something entirely non-living in the clouds is making phosphine. We may have got the numbers wrong, and very inefficient processes, ones that we thought could not possibly be responsible for the phosphine we see, actually are the source. There could be some completely unknown substances there that make phosphine, or look like phosphine.
The second reason is the environment. Concentrated sulfuric acid! That is a substance that chews up every biochemical apart from fat in minutes. How could you possibly have a biochemistry in such an environment? Well, we have some really out-there speculations on that, but in science speculation is the start, not the end. You speculate, think 'what if ...', and then go out and test. We need to go out and test.
Here are our papers on phosphine and Venus so far. I hope to add to the list.