Well, as a chemist – one who does amateur astronomy on the side, yet – it’s obligatory that I write about the phosphine on Venus paper that came out yesterday. This one’s embargo was spectacularly leaky, so everyone who’s really into this stuff had various kinds of advance warning, but the news certainly has made a splash.
So what’s phosphine? It’s a simple compound, formula PH3, and if you’ve worked with it you’re likely either doing some rather funky phosphorus chemistry, or you’re in the electronics fabrication industry (where it’s used to make semiconductors). Alternatively, you could be fumigating rodents out of farm buildings; it’s good for that, too. Phosphine has a reputation as being extremely foul-smelling and flammable, but that’s largely due to it often having some diphosphane (P2H4) in it, depending on how it’s prepared. But that just makes pure phosphine an extremely toxic gas without as much foul odor to warn you, so it’s a mixed improvement.
There are small and variable amounts of phosphine in Earth’s atmosphere, and its origins have been the subject of much argument. In our oxygenated world, phosphorus just doesn’t show up in a reduced form like this without a good reason. The overwhelming majority of the phosphorus on Earth is in the form of phosphate (as oxidized as phosphorus can get), and taking that all the way back down to phosphine is an energetically costly process. One possibility is that it’s formed during lightning strikes – plenty of electrons available there for any reduction you might care to run. But it’s also become obvious since the late 1980s that phosphine can be produced in anaerobic environments like sewage sludge, with no lightning to be had. Somehow, some hard-core microorganisms (as yet unidentified) seem to come up with enough electrons to do the job enzymatically. Here’s a recent paper that looks at adding various potential nutrients for that purpose to such sludge, and they found that glucose is particularly effective at increasing phosphine production. And it’s already been reported that chemical sterilization of the sludge shuts phosphine production off entirely.
The new paper generating all the headlines reports detection of phosphine in the upper cloud decks of Venus, where there really shouldn’t be any (for the same reasons as on Earth). Everything else is oxidized: the carbon on Venus is in the form of carbon dioxide (which makes up 96% of the atmosphere and gives the place the most extreme greenhouse effect possible), and the sulfur is in the form of sulfuric acid (plenty of that, too), so the phosphorus there should be phosphoric acid/phosphate, not phosphine. The authors calculate that lightning and volcanoes could only generate enough of the gas through known processes to come several orders of magnitude short of what’s actually observed, and there aren’t any other abiotic processes (photochemistry, etc.) that are hypothesized to be able produce the gas. The paper has detailed estimates and models for phosphine production and decomposition by such routes, and none of them, all the way to delivery of reduced phosphorus minerals by meteorites, seem to get anywhere close to producing the amount that’s observed. So we’re left with either some unknown inorganic route to it, and nothing particularly plausible comes to mind. . .or we have that apparent microbial production that we’re seeing on Earth.
Now, microbes on Venus seem rather unlikely at first, because Venus is basically Hell in the sky. That was confirmed, most thoroughly, by the Soviet Venera landers in the 1970s and 1980s. That program was a major accomplishment. Venera 3 in 1966 made it to Venus’s surface (the first human object to impact another planet), but its instruments had failed on arrival in the atmosphere. Venera 4 (1967) was thought at first to have possibly survived a landing (although its communications had also ceased on the way down), but the US Mariner 5 spacecraft was arriving at nearly the same time and measured the atmospheric pressure as being at least 50x that of Earth, far beyond what the Venera probe had been built to withstand. The Soviet engineers armored up, and Venera 7 in 1970 was the first time humans landed a working probe onto the surface of another planet. But it only survived 23 minutes down there, because the pressure was indeed 91 atmospheres and the ambient temperature was 464 C (867 F), hot enough to melt lead, tin, zinc, and various alloys. And to get to this tourist trap you have to drop through kilometers of thick sulfuric-acid-laced clouds. Venera 13 (1981) set the existing record, surviving for just over two hours on the surface.
So why would anyone imagine microbial life? Well, there’s that thick cloud deck. And it’s become apparent that microbes can be isolated from the cloud droplets here on Earth. Harold Morowitz and Carl Sagan raised the possibility in 1967 of similar organisms in the temperate regions of Venus’s atmosphere, and it’s never been something to rule out. If phosphine really is a biological signature, its presence in Venus’s atmosphere is very suggestive. The authors are careful not to call it definitive evidence for life, however.
This idea has two main places it can break down: as mentioned, there could be abiotic routes to phosphine that we don’t know about. The chemistry of Venus’s atmosphere is known to some extent, but no one would claim that we have the full picture. The surface of the planet is covered in huge ancient basaltic lava flows, but the present day activity is still mysterious. It appears that Venus is somewhat volcanically active (sudden spikes global in sulfur dioxide and other such evidence), but we don’t know much more, and we certainly don’t know about the composition of hypothetical current eruptions. That said, it would have to be something pretty odd to produce the phosphine that’s been seen.
And that’s the other place this hypothesis can break down: perhaps the identification of phosphine is itself erroneous? The authors consider this at length, to their credit. The spectral band that they’re using is a rotational transition in the millimeter wave range, and the two radio telescope facilities that were used to collection data were the James Clerk Maxwell telescope on Mauna Kea and the Atacama Large Millimeter Array (ALMA) in Chile, both excellent instruments and among the best that radio astronomy has to offer. There is a nearby absorption band from sulfur dioxide, however, but the authors calculate that it could only be responsible for the data if it were twice as hot as the measured temperatures in the upper cloud decks. But that does leave an outside chance that there is something messed-up with our knowledge of sulfur dioxide in the Venusian atmosphere that led to a false call for phosphine.
The paper suggests a number of follow-up experiments to try to nail all this down, but none of them are easy (see “Strategies to confirm PH3” in the Supplementary Material). The ALMA facility could be used to search for another particular phosphine band, but that would take several days of solid data collection, which is a real challenge. There’s also a possibility for infrared observations, but ground-based ones will be complicated by absorption bands in the Earth’s atmosphere. Someone will surely try that one, though. I think eventually we’ll see proposals for aerosol sampling probes, perhaps even with a return-to-Earth component (no doubt various teams are running the numbers on this right now!)
Now for a final bit of speculation: if all of this does hang together – if there is phosphine on Venus, if we have detected it correctly, and if it is evidence for Venusian microorganisms – what does that tell us? Well, one thing you might wonder is if we brought it there ourselves, via the various probes that have entered the Venusian atmosphere. I find that unlikely, considering the conditions of travel, the conditions of atmospheric entry, and the unlikelihood of common bacterial contaminants being adapted to survive in the Venusian cloud conditions even if they did arrive in viable condition. But it’s not completely impossible.
The biggest question is what this would mean for extraterrestrial life in general. Like the arguments about methane on Mars (which have a similar profile of possible biogenic signature versus arguments about detection and abiotic sources), these things could mean that microbial life gets underway pretty readily. That would be a huge thing to prove, because it’s absolutely certain at this point that there are a huge variety of planets around a huge variety of stars. It’s likewise certain that the simple molecules of life-as-we-know-it are abundant in the universe, all the way to amino acids, carbohydrates, and nucleotides in carbon-containing meteorites and (for some cases) in interstellar clouds.
What we just flatly don’t know (and still wouldn’t know) is how easy or likely the transition to multicellular life might be, and how often any of the resulting complex organisms manage to come up with the ability to use technology to investigate their surroundings. Like we’re doing now. We’re left with some very long lines to try to connect some very important dots. . .