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The lifeforms don't have to use hydrogen sulfide as a solvent. Maybe they've got an internal water-ammonia eutectic mixture or something; anyway, it doesn't matter. The point is, I want an ocean of cryogenic hydrogen sulfide, and I want some kind of creatures living in/by it.

Thought one: maybe it could be a carbon planet, because water is unstable on carbon planets, and there wouldn't be oxygen to burn the hydrogen sulfide, so that would let a small amount of hydrogen sulfide collect in seas without being totally overwhelmed by water.

Except... carbon has a higher affinity for hydrogen than sulfur does, so hydrogen sulfide isn't actually geologically stable on carbon worlds, either.

Thought two: maybe its a really small world, like Io, such that the lightest volatile chemical it could hold on to during formation was hydrogen sulfide.

Except, if there are lifeforms there (and no oxygen, because hydrogen sulfide, and no atmospheric hydrogen, because small world that can't hold onto it), then it will be energetically favorable for heterotrophs to strip the hydrogen off of sulfur to produce methane--and both methane and H2S are easily photolyzed (because no ozone layer, because no oxygen), so hydrogen will be lost, carbonaceous biomatter and H2S will be converted into methane and solid sulfur, more hydrogen will be lost, and the sea will dry up.

Thought three: maybe it's just a really cold, otherwise Earthlike world, where water is frozen out.

Except, water and H2S form a relatively high-temperature gas clathrate, so water ice would just trap all of the H2S, unless there's a truly enormous quantity of it.

So: is there a plausible way to get a sea (it doesn't have to be a huge ocean, just definitely "sea" sized) of hydrogen sulfide, which is geologically stable, on a world with lifeforms of some sort?

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I think the lifeforms are the key to this, the sea isn't a purely geologically derived feature but a biological one. Earth didn't always have oxygen, our current atmosphere is the result of a massive pollution event caused by the toxic effluent of the first photosynthetic organisms to evolve here. The sea of hydrogen sulfide is similarly the byproduct of an organism which has no use for it. I suggest that it will be relatively shallow and there will be mats of algae/bacteria on the seabed using the sunlight that filters down to chew sediment and produce a number of wastes including but especially H2S. If it's cold enough to have a sea of H2S at all then the sediments can easily include rock hard frozen water as one potential source of hydrogen. The H2S isn't geochemically stable over any extended time frame but is constantly being renewed, just like our atmospheric oxygen load.

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    $\begingroup$ That, of course, raises the question of why anything would produce that much H2S as a waste product, given that splitting water to produce it is extremely energetically expensive. $\endgroup$ Feb 15 at 7:03
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    $\begingroup$ @LoganR.Kearsley At such low temperatures you're probably talking about a flux rate in the hundreds or thousands of moles per annum range, maybe even less. Cyanobacteria produced trillions of moles of oxygen a year for millions of years during the GOE and split two moles of water for every one of them, at least for hydrogen sulfide you're at 1:1 and if you have a photosynthetic pathway that uses a different portion of the EM spectrum you can have a cold world that receives quite a lot of usable energy. $\endgroup$
    – Ash
    Feb 15 at 7:20
  • $\begingroup$ "Why would any organism produce oxygen as a waste product? Splitting water to produce it is extremely energetically expensive, and it's toxic to every living thing except those damned cyanobacteria." Yet here we are, breathing the stuff, a billion years later... $\endgroup$
    – Zeiss Ikon
    Feb 15 at 16:40
  • $\begingroup$ @ZeissIkon Because they needed the alternative hydrogen source after early geochemical supplies of H2S were exhausted. If H2S is around, you won't produce oxygen, because it's cheaper and safer to produce sulfur. Producing H2S as an autotrophic waste product would be like producing water as an autotrophic waste product, not oxygen. $\endgroup$ Feb 15 at 16:46
  • $\begingroup$ You might make H2S if you were using sulfur to desaturating environmental alkanes - making ethene or acetylene from ethane for example. Then your unsaturated carbon molecules store your energy and you can regenerate it with hydrogen stripped from H2S. $\endgroup$
    – Willk
    Feb 15 at 17:34
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Underground sea.

enter image description here

https://www.deviantart.com/josheiten/art/Bio-Cave-356339930

Your sea is deep under the surface. Atmospheric pressure at that depth is high, and it is warm. The heavy H2S is well below the topside world of light and oxygen and nitrogen. Solid rocks and metal in between keep it from mixing. Explorers need to bring their own oxygen supply.

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