TL;DR
The claimed detection of phosphine is not conclusive evidence of life on Venus; it simply indicates that there are unknown chemical pathways on the planet that are producing it. While these may be biological in nature, they also may not be. Therefore, the discovery might be evidence of life on Venus.
Recent work has cast serious doubt on all of this, however. Recently, Snellen et al. 2020 performed a reanalysis of the ALMA data used by Greaves et al. They found that by performing the same procedure involving a 12th-order polynomial on other spectral features near the claimed phosphine line at 267 GHz, they were able to produce five other false positives at statistically significant signal-to-noise ratios. The 267 GHz feature is then only found at an SNR of 2, which is not statistically significant. All of the astronomers I've talked to are now a bit skeptical of the initial claim.
Assuming Snellen et al. are correct, then there may indeed be no discovery at all. I'm going to keep the remainder of this answer, however, because ideally further follow-up observations can lend credence to the ALMA/JCMT results or cast even further doubts on the claim.
What the article claims
I should start by noting that the article doesn't make bold claims about life on Venus. As the author, Phil Plait, writes early in the post,
[L]et's not jump to conclusions. The scientists involved certainly haven't. They're careful to say that what they've found is consistent with the presence of life in the Venusian atmosphere, but they don't come right out and state that it is the product of bacterial belches. Which is prudent; it may yet be from some as-yet-unknown non-biological chemistry going on there.
In his Twitter summary of the article, Plait says
So please don't run around saying scientists have found life on Venus. They have found evidence of something that could have been produced by life, but also may not have. We don't know.
I've found that he tends to be rather cautious in his writing. Other articles, while largely responsibly written, had some not-so-conservative headlines:
None of the articles I've read today claim that the observations constitute a discovery of life, but some writers and more cautious than others. Plait is, as usual, one of those.
Therefore, we have to be careful about what the article is claiming - namely, that the phosphine may be evidence of life. That much is certainly true.
Ways to make phosphine
There are certainly abiotic processes that might be able to produce phosphine on Venus. While there may only be two known pathways to produce it on Earth, there are certainly other options for other environments. For example, we've known for half a century that phosphine exists on Jupiter (see Larson et al. 1977), where extreme conditions not found on Earth or Venus allow for its production and subsequent transport to the atmosphere through convection. Therefore, we don't totally understand phosphine, because it's difficult to replicate the possible pathways in labs on Earth.
The paper the group published (Greaves et al. 2020) lists quite a few known pathways that could lead to phosphine on the surface or atmosphere of Venus:
- Production by lightning (too low by 7 orders of magnitude)
- Production by meteorites (too low by 8 orders of magnitude)
- A large-scale impact (no such evidence exists)
- Subsurface chemical reactions (oxygen fugacity is way too high)
- Photochemical production (too low by 5 orders of magnitude)
- Chemical reactions in the atmosphere or surface ("too energetically costly")
A second, substantially longer, paper is undergoing peer review (Bains et al. 2020, listed in the first paper as reference #35). A preprint of it has now been posted on arXiv.
These rates take into account the fact that phosphine can also be destroyed by a number of mechanisms, some of which are discussed by Sousa-Silva et al. 2020:
- Reactions with O, H, and OH radicals
- Destruction by ultraviolet radiation, the dominant pathway in some environments, via the reaction PH3 + hν -> PH2 + H.
These destruction mechanisms are what imply that there must be some source continuously producing phosphine.
Here's the thing: The fact that we've exhausted all known production pathways for phosphine does not conclusively show that there is life on Venus, and that it is responsible for producing the gas. Rather, it indicates that there is some chemical process happening on Venus that we don't fully understand. It might be biotic and it might be abiotic. Indeed, Greaves et al. write in their conclusion that
Even if confirmed, we emphasize that the detection of PH3 is not robust evidence for life, only for anomalous and unexplained chemistry. . . . To further discriminate between unknown photochemical and/or geological processes as the source of Venusian PH3, or to determine whether there is life in the clouds of Venus, substantial modelling and experimentation will be important.
Could it be something else?
A final thing to keep in mind is that this question implicitly assumes that the spectral line is, in fact, phosphine - and I made the same assumption in my answer. Assuming that the methodology is sound (which may not be true - see the TL;DR), it seems like a decent assumption to make, for a couple of reasons, which Greaves et al. list:
- The same line was detected by two telescopes, JCMT and ALMA.
- Different data processing methods yield the same result.
- No other features appear to overlap in that range.
- There are no other reasonable lines that could be responsible (sulfur dioxide was considered and found to be only a minor contaminant).
There are other bands that could be searched for phosphine emission; Sousa-Silva et al. note that infrared wavelengths might be promising, as phosphine has strong emission in the 2.7-3.6, 4.0-4.8, and 7.8-11.5 micron bands. The group notes that a carbon-dioxide dominated atmosphere may complicate things, but certainly the transition Greaves et al. found, PH3(1->0), was observable.
Biosignatures are complicated
All of that said, a single detection of a biosignature doesn't necessarily mean that there's life. A good example of this is Martian methane, whose presence, levels and variation have been debated for decades. Folks may remember that a few years ago, Curiosity detected seasonal variations in methane (Webster et al. 2018). While the production mechanism was likely abiotic, it was noted that methane may be a biosignature. At the same time, methane detection on Mars has historically not been unambiguous, and there are many possible abiotic production pathways.
An even better example might be the case of carbon monoxide on Titan, discovered in the early 1980s (see Lutz et al. 1983). For a long time, it was unclear how it could have arisen abiotically (or, well, biotically). We didn't have a solution until the late 2000s, when it became clear that geysers on Enceladus could be providing the necessary oxygen atoms (see also Horst et al. 2008 for a discussion of the resulting chemistry).
I admit that the Martian methane case is not an excellent analog because there are plenty of other pathways capable of adequately producing it, whereas all known abiotic pathways of phosphine production on Venus may have been ruled out as the primary source, but still. Follow-up observations would be nice.
