This is definitely the big news of the week – scientists have detected phosphine gas in the clouds of Venus. This is a big deal because phosphine gas is a potential marker for life. This adds Venus to the list of worlds in our solar system that are candidate hosts of life, along with Mars, Europa, Enceladus and others. Europa and Enceladus are moons with an icy shell and definitely liquid water underneath. The presence of liquid water is what makes them intriguing candidates for potential life. Mars is currently dry and desolate, but in the past was warmer and wetter. Life could have evolved on Mars, and we may find the fossil evidence of such life. Or, unlikely but possible, life could have barely clung to some ecosystems in the Martian soil.

But Venus was not a serious contender for life, and least not after we sent probes there. Prior to the first probe in 1962 scientists and science-fiction writers fantasized about life on Venus. It is our nearest neighbor, almost the same size as Earth, and all those clouds might contain water vapor. Perhaps Venus was a jungle planet. But now we have sent multiple probes to map the planet, and Soviet probes even landed on Venus (surviving for only a short period of time). Here is NASA’s summary of the planet:

Venus has a thick, toxic atmosphere filled with carbon dioxide and it’s perpetually shrouded in thick, yellowish clouds of mostly sulfuric acid that trap heat, causing a runaway greenhouse effect. It’s the hottest planet in our solar system, even though Mercury is closer to the Sun. Venus has crushing air pressure at its surface – more than 90 times that of Earth – similar to the pressure you’d encounter a mile below the ocean on Earth.

Crushing heat, gravity, and sulfuric acid do not make for a hospitable world. However, hope for life on Venus was never completely abandoned. Optimists pointed out that in the upper atmosphere of Venus there is a sweet spot where the temperatures are warm and comfortable for organic reactions and the pressure would be less. Sure, there would still be an acidic atmosphere, but there are extremophiles on earth that thrive in high acidity (acidophiles). I don’t think this was considered a high probability, more of a footnote on the quest for life in our solar system, but Venus could not be completely ruled out as a host for life.

This brings us to the current study – why is the presence of phosphine so exciting? As the authors of the study point out, because it shouldn’t be there. Phosphorous should react with oxygen quickly, but phosphine is PH3. The chemists modeled the possible chemistry in the upper atmosphere of Venus and they could find no situation in which there is a steady-state of phosphine. This means that the phosphine must be being steadily replenished – but where is the phosphine coming from? Again, the chemists tried to model any possible abiotic (not from life) source of phosphine, and there simply isn’t one.

Now both of these conclusions are essentially arguments from ignorance. This does not make them invalid, but it is a huge qualifier on the conclusions of the study. There is no known chemistry to maintain a steady state of phosphine, and there is no known abiotic source. As the authors themselves point out – there could be an abiotic source currently unknown to science. That would solve the mystery, and could be an interesting discovery in itself – new chemistry. Either way, scientists have discovered something new here. But this is not the same thing as knowing that the phosphine comes from life, only inferring that it might come from life because we currently don’t know of any other source.

That phosphine can be produce by living organisms is already well established based upon life on Earth. PH3 in the atmosphere of Earth comes from life. This is why the scientists were looking for it on Venus – specifically because it is a good biomarker for life. Also, PH3 has a clear spectral signature, and that it how it was detected here:

Single-line millimetre-waveband spectral detections (quality up to ~15σ) from the JCMT and ALMA telescopes have no other plausible identification. Atmospheric PH₃ at ~20 ppb abundance is inferred.

So, we can be pretty sure PH3 is in the atmosphere of Venus, and that it shouldn’t be there unless there are organisms making the stuff. What about elsewhere in the solar system? The small rocky worlds do not have PH3 (again, other than from life on Earth). The chemical cannot exist in an oxidizing environment, unless it is produced in reducing environment under intense pressure, like deep inside gas giants. PH3 can then filter up in small amounts into the upper atmosphere of gas giants, but there is no way for it to get into the atmosphere of rocky worlds, even if there is some being made deep under ground.

The authors calculated the likely survival time of PH3 in the Venusian atmosphere, and then using the upper limit of their estimates calculated what the production rate would need to be to account for 20 ppb. They then calculated whether or not photochemical reactions could account for that production rate – and it couldn’t, by any known reaction. There simply is not enough hydrogen in the atmosphere, there is too much oxygen, and there is not enough energy to drive the necessary reactions. So something unkown is going on.

They acknowledge there could be unknown photochemical or geochemical reactions. Specifically, not much is known about the droplets in Venus’s atmosphere of what chemistry might be going on inside of them. But biochemical reactions are also on the list. There is a reason that the scientists were looking for PH3 in the first place – it is a good biosignature for life.

What comes next is unknown. This finding is a great incentive to fund more probes to Venus, specifically to explore these unanswered questions. NASA is already planning the next probe to Venus to study its atmosphere – DAVINCI+, scheduled for possible launch in 2026. This is still in the early planning phase, and I wonder how much this new finding will influence the mission. Hopefully it will speed up the timetable.