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This news article came out about evidence tied to the possibility of life on Venus. The crux of the evidence is the detection of phosphine. The claim is that we only know of two ways to make this compound. Artificially with chemistry, or by anaerobic bacteria.

Recently, phosphine has been examined the same way* and found to be a good biomarker for the same reasons. On Earth, phosphine (PH3) is only made by humans artificially, or by anaerobic bacteria, generally in rotting corpses. Finding it in an alien atmosphere at relatively high levels would be a decent indicator (though not proof) of biological processes.

While Dr. Plait does a good job of putting adequate skepticism in his article, more "popular" media outlets may not be as cautious, or leave the caveats until much later in their article (such as this People Magazine article that doesn't mention any caveats until much later in the article which many people may not read). Even Popular Mechanics seems to be less cautions than other outlets. The CNN headline outright says "signifies life has been detected" in the headline.

So, are the two methods of producing this chemical overstated? Or is this actually some compelling evidence?

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    The article you quote from specifically states in its headline "may have found evidence of life on Venus", and the quote itself calls it "a decent indicator (though not proof) of biological processes". I don't think the article is making the claim that phosphene is evidence for life on Mars, and certainly not that it's "compelling" evidence. – F1Krazy Sep 14 at 16:10
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    @LarianLeQuella Maybe you should link that one then, because the reputable sites, including the primary source (ie the paper) answer your question clearly. The paper states that we know of know alternative methods, but there may be as yet undiscovered geochemical processes which produce detectable levels of phosphine. – spacetyper Sep 15 at 0:37
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    In the coming months, 1000ds of scientists will research will do labwork and astronomy work to confirm the finding and find evidence of other biosignatures that can be detected, so we don't know much from a single study, but we will know a lot in a year's time!!! – aliential Sep 15 at 14:13
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    Headlines are always sensational, since we live in a "clickbait" environment. In all the interviews I've heard so far, the scientists have been very careful to walk it back. They're not even certain yet whether they really detected phosphine. – Barmar Sep 15 at 16:31
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    @ChrisH Venus was close to us, about 2 months space flight away, in June 2020, and it will only be close again in January 2022, that gives Elon Musk about 11 months to devise and launch a space mission :) – aliential Sep 15 at 19:02
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TL;DR

The 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.

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. It's a good 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.

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    +1 but with a request. Could you perhaps add in a discussion of other times people have thought that only life could produce something, only to have it later discovered that a natural process was responsible? I think that would give added weight to the answer for non-skeptical people being directed here. – Larian LeQuella Sep 14 at 16:55
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    @LarianLeQuella That's a bit tricky, because we haven't really had any other detections like that. I've discussed methane variations on Mars that could also be easily linked to abiotic processes, even though 1) nobody thought they were conclusive evidence of life and 2) it's still unclear what's causing them. – HDE 226868 Sep 14 at 17:13
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    +1 for The Bad Astronomer being your primary source. He's always really cautious (and comprehensive) about these things. – jdunlop Sep 14 at 23:37
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    So, basically the same sensationalist headlines striking again. I guess soon there will be even more extreme ones, like "what scientists don't want you to know about life on Venus". I remember a Nature article about how vitamin C can slightly alleviate one of the many harmful side effects of chemotherapy, but that article was reported in the media as "Scientists finally proved what doctors tried keeping hidden from us: vitamin C instantly cures all forms of cancer!" – vsz Sep 15 at 5:28
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    @npst Sorry, I should have been clearer - I mean that while those pathways can and may produce phosphine on Venus, they've been "ruled out [as the source of the unexplainably high concentrations observed]". In other words, they can produce some of the phosphine, but nowhere near all of it. – HDE 226868 Sep 18 at 13:34
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Any strong claims made by news outlets are journalistic rather than scientific.

The linked article is pretty cautious, and reflects the researchers' position rather well. Other stories have gone a little further: Sky News, for example Signs of alien life detected on Venus -- Microbes unlike any life on Earth could be thriving high in the clouds of Venus, according to a new discovery by astronomers.

The journal paper on which that article was based states: Even if confirmed, we emphasize that the detection of PH3 is not robust evidence for life, only for anomalous and unexplained chemistry. (this isn't the only caveat, but the most quotable one).

The press release from the lead author's institution* has the headline Hints of life on Venus - which by the standards of a press release (even from a university) is suitably careful.


* and mine - I know her though we don't work together; I had a couple of hours warning an announcement was coming, but no idea of the content.

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