(This is my first question here and I apologize and can edit if it's out of place or formed improperly or has other issues.)

The September 20, 2020 news of phosphine found in the clouds on Venus has generated a large amount of cautious excitement from the scientific community. In general the discovery finds a statistically significant amount of phosphine (PH3) in the upper atmosphere of Venus, that cannot be explained with abiotic chemical pathways that are known to occur on Venus. A tentative suggestion is that there may be a biological pathway within the clouds of Venus that creates the amount of phosphine observed. Thus, the hypothesis is that Venusian "life" may be responsible for the creation of phosphine. Accordingly, there is an unexplained phenomenon and there is a suggestion that it is reasonable to have an appeal to alien life to explain the observed phenomenon.

The 1968 book Chariots of the Gods? does not explain how ancient civilizations created megaprojects and instead hypothesizes that aliens could be responsible for some ancient megaprojects (such as the Nazca lines/Egyptian pyramids/etc.) Accordingly there is an "unexplained" phenomenon and there is an appeal to alien life to explain the observed phenomenon.

I find it easy to dismiss Chariots of the Gods as pseudoscientific claptrap, not least because the assumption that ancient civilizations were incapable of owning their own history and artifacts marginally racist and bordering on repulsive.

I also find it easy to dismiss the book because the appeal to aliens as gods seems like a replacement of one curiosity (how could ancient civilizations build all their megaprojects?) with another, harder to disprove hypothesis (could be aliens!) However why does that second dismissal not carry over to phosphine on Venus? "How could there be phosphine on Venus? Could be aliens!"

Why do I feel more comfortable pondering whether there is a biological pathway for the creation of phosphine on Venus than wondering about ancient aliens building Mayan temples?

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    Probably because instead of merely pondering the authors went through the trouble of ruling out all known non-biological pathways to producing phosphine in requisite quantities given conditions on Venus. So, at present, it is the only hypothetical explanation available, see Is phosphine evidence of life on Venus? on Skeptics SE. – Conifold Sep 16 '20 at 18:55
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    There is no theoretically possible (!) natural chemical reaction which could in any way produce the amount of phosphine other than biological processes (of very simple organisms). On the other hand, it is a possible, but rather far-fetched explanation to use extraterrestrial intelligence (complex, space-faring, sapient life) as explanation for things that could just as well be made by humans, even though with almost inconceivable effort. One is an inference by elimination, the other one an improbable hypothesis among others. I don't see how they are on par in any way. – Philip Klöcking Sep 16 '20 at 18:56

I can't say why you personally find one more believable than the other. I think the discrepancy is the nature of each proposition. Ancient aliens theorists just assert that ancient people couldn't have built the things they did. They never say why it is impossible, and they ignore evidence to the contrary. We have found the quarries where the stones used in the pyramids were carved, we have found the causeways they were moved along, we understand or at least have theories about every step of the process. We can explain every step of the construction process without aliens.

On the other hand, the paper detailing the phosphine on Venus goes into detail about why each abiotic explanation doesn't work. Further, none of the scientists involved are saying it's definitely life. They only say that it is possible, and they don't currently have any other explanations. Contrast this with what ancient aliens theorists say, where a carving that vaguely looks like a guy in a space helmet is conclusive proof aliens built everything.


The phosphine, as far as I understand it, not only doesn't come from known abiotic sources (it might still come from unknown ones yet to be proposed), but also rapidly decomposes (in the order of a thousand years or something). So the proposition is not only that alien microbes produced the phosphine, but that they have to still be there. We can and probably will at some point literally go and check to see if they are there.

That is less like ancient aliens and more like claiming these aliens not only were here at some point, but are literally here today, at a place we can go and check. I don't think anyone has proposed where we need to go look for them though. Also, of course proposing nearby alien microbes is less extreme than intelligent aliens.

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