Mini-Throughput: Life on Venus Edition
Maat Mons, a massive volcano on Venus, as seen by the Magellan mission.
A few weeks ago, I mentioned the possibility of life being detected not in the usual suspected locations but in the clouds of Venus. While Venus’s surface would be incredibly inhospitable to life — a runaway greenhouse effect has produced a choking carbon-dioxide atmosphere with nearly 100 times Earth’s pressure and a temperature of nearly 900 degrees — there could be life in the more temperate cloud regions, where temperatures and pressures are similar to those of Earth.
Well, this week the internets exploded with reports of a possible detection of life on Venus. Well … kinda. Phil Plait goes in depth on the discovery. What the scientists actually detected was a chemical called phosphine. While we see this chemical in other planets of the Solar System, the atmospheres of rocky planets like Venus aren’t quite hot enough to create it. Venus doesn’t have anywhere close to the geological activity that would be needed to produce phosphine. And there are very few other chemical pathways by which phosphine can be manufactured. Moreover, phosphine decays quite rapidly so it must be constantly refreshed to be detected at this level.
There is one way phosphine can be created, however, that has not yet been ruled out: microbial life.
Now the authors of the papers are appropriately cautious, noting that life is just one possible explanation. It is also possible that there is a geological or chemical process going on that we don’t understand. In particular, the paper I noted above talked about how winds might waft microbial life back up into the clouds to keep the biosphere going. A similar process could be feeding unexpected chemicals into those clouds. And even the detection of phosphine is based on a single spectral line. We’d like to see more detections.
Still, the result is intriguing at minimum, it means that we still have a lot to learn about our own Solar System. At most, it means that we have found a biosignature that was kind of unexpected.
— Candace Gray (@ARedheadOfVenus) September 14, 2020
My bet is on a chemical or geological process. But until we’ve explored all the possibilities, we won’t know for sure.
I’ve said before that I think we will eventually find evidence of microbial life in our solar system — if not actual microbes then the fossils of them. And while that’s not as exciting as little green men, such a finding would hint that life is common in our universe — at least the beginnings of it. And that would be a big step toward figuring out that we are, indeed, not alone out here.
We need to do an atmosphere scoop.Report
I have a lengthy list of things that I would rather see the government spend R&D money on before they do an atmospheric scope for Venus.Report
There is a lengthy list of things that need more R&D before NASA can even try to do an atmosphere scoop (i.e. in and out of the upper atmosphere). We could do an atmosphere dive, like Juno, but I’m not sure how much data we can get from that before the probe is spent.Report
I would rate the theory as slightly less plausible than finding a plesiosaur in Loch Ness, which is what came to mind when I watched the BBC “The Sky at Night” episode on the discovery.
Among the problems are:
No known biological processes produce phosphine. There is anecdotal evidence that biological processes might produce phosphine, but nobody knows what such mechanisms might be.
Earth’s atmospheric phosphine doesn’t seem particularly associated with biological life. It can be detected at ground level, so one assumption is that some kind of organic decay must produce it, but radon gas likewise bubbles up through the subsoil and nobody thinks that’s biological.
Phosphine concentration in the Earth’s atmosphere various by about a factor of 100,000. You can find it over the arctic tropospheric concentrations that are almost a thousand times higher than you might find over a swamp. If phosphine did have a short atmospheric half life, this would indicate no correlation to biological processes. (See figure 1 of this January paper on phosphine detection in “Astrobiology”.
Phosphine can be produced by lightning. The Journal of Atmospheric Environment published a 2004 paper titled “Phosphine and methylphosphine production by simulated lightning—a study for the volatile phosphorus cycle and
cloud formation in the earth atmosphere” (Link is https colon //epic.awi.de/id/eprint/11998/1/Gli2004a.pdf – I’m avoiding the spam filter)
From the abstract:
The atmosphere of Venus includes H2O, HCl, and HF, and it rains H2SO4. Most of the atmospheric phosphorus is phosphorous anhydride, P4O6, at 2 ppm below 25 km, which reacts with sulfuric acid to form phosphoric acid (H3PO4) and sulfur dioxide (SO2) in the lower cloud decks. As it sinks lower, it should form P4O10.
One reaction with phosphine is:
sulfuric acid + phosphine –> water + phosphoric acid + sulfur dioxide
4H2SO4 + PH3 –> 4H2O + H3PO4 + 4SO2
And as the above paper shows, lightning can produce phosphine in an atmosphere, and all the atmospheric ingredients are present for the above reaction to run in either direction, along with intense Venusian lightning storms.
One question nobody has investigated is whether phosphine could also be produced above lightning storms by sprites and blue jets.
So my problem with the theory claiming evidence of life on Venus is that phosphine isn’t known to be evidence of life, but it is known to be evidence of lightning. And it also is the equilibrium form of phosphorus at temperatures above 800 K (as on Jupiter and Saturn), while Venus is already at 737 K.Report
Nat Geo: Venus might not have much phosphine
Three teams have gone over the phosphine study and can’t find evidence of it in other data sets, or in the original data set that made the first team think they’d found abundant phosphine. It seems that they might have gone too far with the polynomial curve fitting of very noisy data.
Here’s one of the papers.
<a=https://arxiv.org/pdf/2010.07817.pdfA stringent upper limit of the PH3 abundance at the cloud top of Venus (PDF)
So, it looks like the Venusian phosphine excitement was premature.Report