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If microbes on Venus produce phosphine, wouldn't there also be other gases produced by microbes since earth has many kinds of bacteria and the fast reproduction rate of bacteria make genetic mutations likely?

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    $\begingroup$ I think you're confusing the idea of something being a possible indicator of life with something being proof of life. Two completely different standards. $\endgroup$ Commented May 31, 2023 at 11:23
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    $\begingroup$ It's a hypothesis. There's no proof that PH3 is originated from a biological source. The paper you mentioned explained that clearly. Again, there is no proof that there is any microbial entities on Venus. $\endgroup$ Commented May 31, 2023 at 16:21
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    $\begingroup$ That article is arrant garbage. You may as well try and say that there are tigers on Jupiter, or there are hedgehogs living on Mars. Anyways, as pointed out by @NilayGhosh in the comments, phosphine can be produced by inorganic sources as well. $\endgroup$
    – Alastor
    Commented Jun 1, 2023 at 16:50
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    $\begingroup$ I'd love to upvote a question similar to this but the "is to be proof" is pulled from thin air. The premise of your question is that it is considered scientific proof and that's not a thing, so -1 for a faulty premise. Certainly your linked article in no way says "microbes on Venus produce phosphine"! $\endgroup$
    – uhoh
    Commented Jun 2, 2023 at 5:30

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Sure, if it is something reactive like oxygen or fluorine

Basically, if you find anything in the atmosphere that is way too reactive to exist for a long period of time, like oxygen, fluorine or chlorine, then there are only two is only one logical explanation:

  • Some moronic alien dumped the gas onto a planet(pun intended)
  • The planet has life on it.

In fact, we don't even need an hypothetical for this. This example already lies on Earth.

Earth was basically a slightly upscaled version of Venus 3 point something billion years ago. Its atmosphere comprised of hydrogen, methane, ammonia and water vapor. And the only lifeforms were either methanogens or sulfur bacteria. There were photosynthesizers, but those were mainly purple sulfur bacteria, releasing stinky fart gas hydrogen sulfide, a foul-smelling gas, instead of oxygen.

Then things changed. Some bacteria (most likely stinky sulfur photosynthesizer) got a mutation from UV rays or something like that, and the next thing we know, the bacteria becomes green and now uses carbon dioxide and water and sunlight to produce glucose and oxygen and what not. We know this happened as we have banded-iron formations on Earth, which happened when oxygen attacked the dissolved iron present in the ocean into rust (hydrated ferric oxide). Rust, having a lower solubility than water, precipitated out of the water and settled out to the bottom of the ocean, where it got crushed into hard red rocky formations that we call banded-iron formation.

In the early days, oxygen was consumed up by the reducing atmosphere of baby Earth. However, as the cyanobacteria (as they are called that) started multiplying exponentially, they started producing more oxygen faster than it could be consumed by the reducing atmosphere. Some of them even started fixing nitrogen from the atmosphere (maybe ammonia) and turning them into nitrates.

The result: Earth's atmosphere turned from a reducing one, to that of a oxidising one. This killed off the methanogen and other pre-cyanobacteria lifeforms, like the sulphur bacteria. Now, methanogens still exist, but only in oxygen-deprived areas like cow bowels, animal dung, swamps, wetlands. And today, you can only find purple sulphur bacteria in stagnant water bodies, hot springs or microbial mats.

Similarly, although we wouldn't likely identify hydrogen sulfide as a lifeform(as hydrogen sulfide is also formed inorganically as well), we would very likely find out if there is life on a planet, if there was a reactive gas that shouldn't exist on that planet, like oxygen or fluorine. The reason we know this can happen: It already has happened.

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  • $\begingroup$ Not all oxygen implies life. Oxygen in Ganymede's atmosphere is generally deemed to result from ultraviolet radiation acting on the water ice at the moon's surface. $\endgroup$ Commented Jul 3, 2023 at 10:59
  • $\begingroup$ Yeah, but I said, Oxygen in "unusually high" quantities. Ganymede has a tenuous atmosphere, which means that oxygen can easily, if briefly, form due to ultraviolet light $\endgroup$
    – Alastor
    Commented Jul 3, 2023 at 13:31
  • $\begingroup$ Your claim that there is only one logical explanation has no useful evidence to support it, as we only have a sample size of 1 planet with life and none with significant amount of reactive gases. We have loads of theories, but all based on observation. $\endgroup$
    – Rory Alsop
    Commented Oct 30, 2023 at 9:09

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