For those living under a rock, earlier this month astronomers announced the detection of unexpectedly-high concentrations of phosphine gas in the atmosphere of Venus. Phosphine is a rather simple chemical, and does not persist in the environment for very long. Venus, in particular, is not a hospitable environment for the compound: the planet lacks a magnetic field, allowing ultraviolet radiation from space to break down phosphine into even simpler molecules. To see the gas in such concentrations indicates that some process is replenishing the supply. The scientists involved attempted to estimate which geochemical processes could produce phosphine as the necessary rates, and found no plausible combination fits the bill.
However, phosphine does sometimes form from biological processes, which might be an explanation. The upper atmosphere of Venus is relatively temperate—you still wouldn’t want to vacation there, but compared to many other Solar System environments, it’s practically built to support life. Could there be microbes lurking in the Venusian clouds?
I mean, probably. Astrobiologists have speculated about this possibility for a long time. The environment is, while not ideal, at least on paper conducive to microbial life. So, it these findings do turn out to be indicative of life on (or rather, above) Venus, how much should they worry us?
The answer, of course, is: it depends.
First off, why do need to be worried at all? The answer is the Fermi Paradox. The simplest explanation for why we don’t see life elsewhere in the universe is that the primordial spark leading from chemistry to biology—abiogensis—is rare. We don’t see aliens because there aren’t very many of them, and none happen to be close by. That’s a more comforting explanation than many of the alternatives.
If there’s other life in our Solar System, though, that changes the calculus.
If the development on life on planets and moons is a common phenomena, that means that living organisms have had many more chances to evolve past an early filter, or many more chances for intelligence to get past a late filter.
Either possibility is concerning. If there isn’t an early filter, then the silent skies imply something or other is preventing civilizations from lasting very long and spreading very far. This most likely means very bad things for the future of our own species. The discovery of an early filter would be far preferable from a civilizational-longevity standpoint, and rare abiogenesis is one of the earliest and strongest filters there is.
However, the existence of life on Venus does not necessarily imply separate abiogenesis. It’s possible that the microbes came from Earth and merely adapted to existence in the Venusian atmosphere—or, for that matter, that life originated on Venus, and only later adapted to survive on Earth. Scientists have long hypothesized that certain kinds of microbes might be able to survive in deep space for extended periods of time inside rocks blasted free from the surface of a planet by large-scale meteorite impacts. These same rocks might then become meteorites themselves, and if any such microbes survived, they could seed the new world they found themselves upon.
Some planetary scientists will even argue that we have evidence of this already. The Martian meteorite ALH 84001 is famous for microscopic structures which look plausibly similar to microorganisms. The debate is still ongoing, but mass transfer between planets via meteorite is a plausible mechanism.
Crystalline structures reminiscent of microorganisms in Martian meteorite ALH84001.
Source: Wikimedia Commons
However, this hardly proves what’s called the panspermia hypothesis, and certainly does not prove that it could work in this case. However, panspermia is a relevant factor to consider, and will, in the end, affect how seriously we should take Venusian microbes if we do, in fact, find them.
If we found microbes, and they turned out to be related to those on Earth, this is would reduce our concerns on the grounds of abiogenesis. Regardless of which planet they originated from, it would tell us that the initial development of life occurred only once. This is good news from the perspective of an early filter preference, but would be tempered by demonstrating that microbes can survive in space for long periods of time. Could such microbes survive long enough, and in sufficient numbers, to make the trip between star systems, and seed exoplanets with life?
Once again, we don’t know, but astronomers and astrobiologists have speculated about the possibility for a long time. Confirming interplanetary panspermia would raise the probability estimate for interstellar panspermia, again weakening the early filter likelihood. Now, to be clear, there are other early filter possibilities, but rare abiogenesis is by far the most preferable.
The most important thing to remember, though, is that the anomalous Venusian phosphine does not constitute proof of alien life. It is reasonably strong evidence—frankly, stronger evidence than I expected to encounter this year. However, there may still be a geochemical process or set of geochemical processes which we aren’t accounting for that explains the data without life. It’s also possible that the scientists made mistakes or were simply working from incomplete or erroneous data. Astrogeology is by no means a mature field, which makes drawing confident conclusions about the interiors and atmospheres of other planets a risky proposition. Unless we actually send a probe to scoop up some of the Venusian atmosphere and find that it does, in fact, contain alien microbes, we won’t be able to confirm the hypothesis with certainty.
The effect on our Drake estimate, then will depend on several numbers. How likely do we think that the phosphine results are real? How likely do we think that, if the results are real, they are explained by alien life? If it is alien life, how much does that affect our estimate of the prevalence of life on other worlds (whether through abiogenesis or panspermia) Personally, I’d put the odds that the phosphine results are true at around 95%, the odds that the phosphine is caused by microbes between five and 20%, and if there turn out to be microbes, increase the prevalence of life on planets by perhaps a factor from two to ten. But I’m not an astrobiologist, I’ve just read a book or two.
All told, I’m going to be a bit more worried, but won’t dramatically update my position on the implied threat from alien life and the lack of evidence for extraterrestrial civilizations until scientists have had more time to study the question. If anything, the biggest conclusion we should take from this finding in the necessity for additional missions to Earth’s Twin.
One of the proposals for the next round of Discovery missions is a Venusian atmospheric probe, and phosphine may very well catapult it over the selection threshold. Personally I was rooting for the two non-Venus missions, though I supposed I could accept the Io mission getting bumped. But I still want that Triton flyby probe.
Nathaniel M. Routh 
