Today, there are rumors about an upcoming announcement of the discovery of (non-conclusive) evidence for life on Venus, apparently they detected some phosphorus compound that is not produced by any known abiotic chemistry, but is produced by terrestrial bacteria.

Of course, the more parsimonious explanation is that there is some abiotic chemistry that produces this phosphorus compound, and we just don't know of it.

It occurs to me that this is not the first time this happened: the Viking landers' labeled release experiment on Mars also gave a positive result, and as I understand it, some of the people involved maintain to this day that it actually detected life. Again, the standard explanation involves a hypothetical abiotic process.

One might tell a similar tale about the fossilized bacteria in meteorites from Mars.

Now, I do not doubt that in most or all cases the abiotic explanation is correct. But there seems to be a pattern here. How often has this happened? How many more examples are there of claimed signs of life in space being dismissed as being caused by poorly understood abiotic processes?

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    $\begingroup$ You've certainly listed the first ones that came to mind. Interesting question! $\endgroup$ Commented Sep 14, 2020 at 17:30
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    $\begingroup$ In the case of Venusian phosphine, the mystery is they've tried abiotic models such as volcanic emission and can't account for all of what is seen. Thus The Question persists. $\endgroup$ Commented Sep 14, 2020 at 19:50
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    $\begingroup$ The one thing that works against a biotic source of the phosphine in the atmosphere of Venus is the amount of sulfuric acid in the atmosphere. The clouds are composed of 75 to 95 percent sulfuric acid, "which is catastrophic for the cellular structures that make up living organisms on Earth". Phosphine gas in the cloud decks of Venus $\endgroup$
    – Fred
    Commented Sep 14, 2020 at 20:30
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    $\begingroup$ Carl Sagan was one of the first to suggest microbes may be able to live in the clouds of Venus. He also came up with the quote "extraordinary claims require extraordinary evidence" $\endgroup$ Commented Sep 15, 2020 at 13:55
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    $\begingroup$ This could be extended to the work of paleontologists & Terran artifacts as well. Granted we know life exists here :-) but just when and what types appeared at what point in history are semi-open questions. $\endgroup$ Commented Sep 15, 2020 at 14:18

1 Answer 1


Excellent question! One thing to consider: How do you define a "sign of life"?

The reason I'm answering a question with another question is because it gets at the heart of the issue that scientists have to deal with in making a determination of whether or not the balance of evidence indicates that life has been discovered off of Earth. The scientists (rightly, I think) use an EXTREMELY high bar for the kind of evidence that would justify announcing that "We are not alone" - this is because such an announcement, if valid, will likely cause significant impact on our species and how we think about and approach the universe. You want to be really careful before you cause that big of an impact. You want to be very certain you are correct.

So, what, then, is a sign of life? In addition to the ones you mentioned, there are also occasional detections of very regular repeating signals from other star systems - is that a pulsar, or a communication? There are stars we've found that have frequent and almost regular dimming that is unexpected based on our solar formation models - evidence of a civilization working on a Dyson sphere, or the result of a planetary collision leaving an unusual debris field? In the future, we may use JWST to detect free oxygen in the atmospheres of exoplanets (though I haven't heard of any such detections to date) - on our world, free oxygen in the atmosphere only happened because cyanobacteria released it from the water of our oceans (killing nearly all extant life in the process) - would finding free oxygen be a life sign on that exoplanet, or could we attribute it to some other process? Oumuamua was cigar shaped, and rotating at a rate where that shape would fly apart if it was your standard rubble pile asteroid, indicating an internal structure that had some tensile strength - a metallic frame, from an alien spacecraft perhaps, or just a somewhat metallic asteroid made by an unknown geological process? There's a rock on Mars that reaaaalllly looks like a face - ancient design, or random erosion? There are crop circles and UFO sightings that show up all over the world - human pranksters, secret military tech, or alien visitors?

You get the idea. The trouble the scientists have is that our sample size for planets that have life is 1. In fact, our total sample size for planets explored in even moderate detail is 1; even on Earth, our exploration of the oceans is pretty weak, and that's 71% of the planet. On Mars we've got a few data points from a few specific locations. On Venus, we've got a few hours, total, of lander data from all landers combined, and no rovers. We spent some time on the Moon, and brought back samples, and we've got a couple asteroid samples, but those are the only places other than Earth we actually have samples from (other than meteorites - we can trace some of those back to Mars and elsewhere, but they aren't expected to be particularly representative given the energetic processes required to liberate them from local gravity).

So the truth is that we don't really know what we are looking for. We know what life looks like on one planet, and what civilization looks like along one developmental trajectory. We can only speculate about everywhere else based on things we've seen here, so there are a bunch of things we know are life-signs here, but we don't know if they are life signs anywhere else.

As such, here's the answer to your question: How often do we dismiss potential signs of life as abiotic? All the time. Constantly. Why? Because for any thing we can actually find or detect with our current level of access to other worlds, there are almost always a variety of possible explanations with different probabilities of being right. And, for an announcement of the magnitude of "We found extraterrestrial life" to be made and accepted, the probability it is right needs to be 100%. We would need to know, for certain, that there was no other explanation at all that made any sense. So far, no potential life signs have given us that level of certainty.

Incidentally, I went to the Mars Society convention last year, and they had a panel discussion about whether there was life on Mars. 4 out of 4 experts said they thought that either "yes, there is life now" or "yes, there was life, but it may now be extinct." The Viking lander data was a key point for all of them - yes, there are abiotic explanations, but these folks contended that the possibility of an abiotic explanation doesn't change the fact that the science got a positive reading for life using an approach that would have been pretty definitive on Earth-based geochemistry, and the abiotic rationale does not preclude Martian life that has simply adapted to Martian geochemistry. But, maybe a bigger point for all of them is that there was liquid water on the Martian surface for like a billion years, and every place we find liquid water, so far, we basically always find life. Does that make liquid water a life sign? Or does that make liquid water a co-variable that winks at us and nudges us and says "probably you should look over here" but confirms nothing? Scientists will tell you it's the latter, and they are right. But the winking and nudging is still real too.

  • $\begingroup$ Welcome to Space Stack Exchange! That's an interesting and well written overview of the many challenges in the search for conclusive evidence of extraterrestrial life. However the way this site works is that specific questions are asked and the answers should as much as possible stick to just answering that specific question. The question here is specifically about abiotic or possibly abiotic processes that have been observed elsewhere in the universe. $\endgroup$ Commented Jul 7 at 20:30
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    $\begingroup$ Thank you for this - Would you be open to taking a second look at the original question? I interpret it as asking how often we dismiss potential life signs as abiotic - i.e. not caused by life. I do answer this question specifically - the answer is "constantly". To my eyes the questioner doesn't want to learn about abiotic processes, they want to learn about life signs that we dismiss. $\endgroup$ Commented Jul 7 at 20:35
  • $\begingroup$ JonathanHuffman - I think you are right about that to some extent, because the “there seems to be a pattern here” part of the question hints at an underlying theory that possibly there is a reason why some discoveries have in some ways been written off as being explainable. So yes you have given some of the reasons why that might be. But it seems for this question they are primarily looking for additional data that might support this theory, by trying to find out if there are other examples of discoveries that have been relegated to probably being caused by some yet unknown abiotic processes. $\endgroup$ Commented Jul 7 at 21:15

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