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Outline

A planet which is heated tidally by its Brown Dwarf parent and permanently dark needs to have native alien life which has [somewhat similar biochemistry to Earth] and also be [permanently habitable] by Humans without [extensive life support] and/or [extensively genetically engineered] bodies.

This planet has very active tectonics and volcanism, a result of this tidal heating, a combination of this volcanism and the biochemical nature of this planet's ecosystem should be be capable of producing an atmosphere composed of 22% oxygen, 50-70% nitrogen, 0.02-5% CO2, ~7% water vapor, + [leftover] + trace.

If this atmosphere is not achievable, the atmosphere needs to be otherwise [permanently habitable] to humans, these humans could have physiological adaptations to adapt to this atmosphere through non-[extensive genetic engineering].

Volcanism on this planet produces similar gases and particulates in similar concentrations to those on Earth (just use Earth numbers);

The principal components of volcanic gases are water vapor (H2O), carbon dioxide (CO2), sulfur either as sulfur dioxide (SO2) (high-temperature volcanic gases) or hydrogen sulfide (H2S) (low-temperature volcanic gases), nitrogen, argon, helium, neon, methane, carbon monoxide and hydrogen. Other compounds detected in volcanic gases are oxygen (meteoric), hydrogen chloride, hydrogen fluoride, hydrogen bromide, nitrogen oxide (NOx), sulfur hexafluoride, carbonyl sulfide, and organic compounds. Exotic trace compounds include mercury, halocarbons (including CFCs), and halogen oxide radicals. (From wikipedia: volcanic gas)

Most members of Trophic level 0 (producers) on this planet produce energy through the chemical equation:

12H$_2$S + 6CO$_2$ → C$_6$H$_{12}$O$_6$ + 6H$_2$O + 12S

However other methods do exist; such as those relating to elemental Sulfur, ferrous Iron, etc. (see Chemotroph) Photosynthesis is not a viable way to produce energy on this planet.

Through some [reasonable] system, (either other processes which these trophic level 0 lifeforms engage in or through related related lifeforms, etc.) this combination of geology and ecosystem needs to produce the atmosphere outlined above, or at least an atmosphere habitable by Humans.

Note that really, the main goal of this is to create an ecosystem which can produce oxygen through some chemical process involving life, so that this planet can have Earth-like quantities of oxygen in its atmosphere

Note that CO2 can have a lot of variance in its percentage of the atmosphere, greenhouse heating can be countered simply by moving the planet farther away from the brown dwarf, though this only works while the planet is still being heated tidally (CO2 likely wouldn't be produced as much from volcanic gases because less tidal heating means less active volcanoes); this is why I've let the range of allowed CO2 concentrations be between 0.02% and 5%)

Definitions

somewhat similar biochemistry to Earth:

Humans can gain energy through carbohydrates from alien flora and fauna, and alien life lacks substances which are particularly dangerous to Humans. Note that this does not necessitate things like having nutrients, proteins, and lipids which are necessary for Human life.

permanently habitable:

Humans can live on this world for an excess of 15 thousand years

extensive life support:

Life support apparatuses which either contain

entirely different atmospheres inside the apparatus than outside, or

are built to keep hazardous gases and particulates from contacting all parts of the Human body

(Eg. this does not include things like gas-masks)

extensively genetically engineered:

Different from Earth-humans to an extent that

radically changes the human appearance to an extent where we would not recognise them as human,

extensively changes the physiology of humans (adding organs or systems, changing the basic function of organs or systems, significantly changing the human skeletal structure)

Note that this does not include things like adding some kind of filter to the Human respiratory system which gets rid of dangerous gases or particulates, adapting human skin to be resistant to corrosive atmospheric gases or particulates, etc.

Reasonable

The system needs to have some reason to exist; for example, water electrolysis is an obvious way to produce oxygen from the products of H$_2$S chemosynthesis, however the lifeforms on this planet need to have a reason (and way) to engage in water electrolysis.

Humans

Whenever I mention the word Human, it doesn't mean Earth-human, I mean whatever kind of genetically engineered adaptations are necessary to adapt to this environment

Leftover

This just means any other necessary byproducts

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    $\begingroup$ My only problem with this setup is this: H$_2$S is very poisonous. If your enviornment has sufficient free hydrogen sulfide to be the primary 'food' of chemotrophs, then it seems like there will be enough free hydrogen sulfide in the atmosphere to kill any humans. This would seem to violate your 'no extensive genetic engineering' mandate. $\endgroup$
    – kingledion
    Commented Dec 24, 2017 at 4:56
  • $\begingroup$ That was one of my worries, I was thinking that there was some way to adapt skin and eyes to be less or un-affected by H2S and adding some kind of filter to the respiratory system (or non-extensive life support filter)? Such changes don't constitute changing the basic function of either system (Eg. skin protects, respiratory system breathes), though I'm of course not sure what kind of changes those would be - hence the question.In the case that there is no way, I might try slightly lifting some parts of the genetic engineering constraint? $\endgroup$
    – PrimarySecondary
    Commented Dec 24, 2017 at 5:11
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    $\begingroup$ My answer here may interest you: worldbuilding.stackexchange.com/questions/96261/… $\endgroup$
    – Dubukay
    Commented Dec 24, 2017 at 8:02
  • $\begingroup$ That's one deep rabbit hole, looks like there's no simple chemosynthetic reaction that directly produces oxygen... I suppose I could always shift over to thermosynthesis as you outlined in your post. Thermosynthesis wouldn't necessarily need heat-difference, right? It would just need a specific temperature (Say 20C) and then it would start doing its thing wouldn't it? I think this type of thing would work, especially on a tidally heated world like how I mentioned. $\endgroup$
    – PrimarySecondary
    Commented Dec 24, 2017 at 10:36
  • $\begingroup$ @PrimarySecondary There wouldn't necessarily need to be a difference locally (i.e., across the extent of the organism), but you need one somewhere--otherwise, no work (useful energy) can be extracted. And if the gradient is elsewhere, part of the conversion process will be binding atoms into molecules that are more stable at the elsewhere-temperature than they are at the organism's operating temperature, followed by physical transport of those molecules to the organism... which is exactly what chemosynthesis is. So if you want thermosynthesis to mean anything, you do need a gradient. $\endgroup$
    – Logan R. Kearsley
    Commented Dec 24, 2017 at 13:21

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Stratospheric electrodissociation of water.

Water can be split to hydrogen and oxygen by non-biological methods. On earth this is mostly photodissociation because we have a star which is gushing radiant energy.

If I recall correctly, this is the rogue planet you asked about here as regards a light source. Life on a rogue planet

This planet is analogous to Jupityer's moon Io. Interactions between the moon and its planet could create energetic auroras.

https://en.wikipedia.org/wiki/Aurora

Auroras are produced when the magnetosphere is sufficiently disturbed by the solar wind that the trajectories of charged particles in both solar wind and magnetospheric plasma, mainly in the form of electrons and protons, precipitate them into the upper atmosphere (thermosphere/exosphere) due to Earth's magnetic field, where their energy is lost.

If there is energy enough to produce light there is enough energy to do abiotic water hydrolysis. Electrochemistry happens in our upper atmosphere too but discussion of this is dominated by oxygen and ozone chemistry. I imagine it must be difficult to distinguish oxygen caused by electrodissociaton of stratospheric water versus oxygen that came up from the surface.

In any case: a plausible method for oxygen on your planet is that the energetic auroras serve double duty. Water coming up from the warm ocean surface (and water volcanoes! don't forget them!) would put water up into the stratosphere where there are enormous electrical charges. Oxygen and ozone rain down from the surface.

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  • $\begingroup$ The photodissociation works with UV part of the spectrum. The same process then produces ozone from oxygen, which blocks UV... So it cannot really produce significant concentration of oxygen on the surface, I think, just an ozone layer that protects people from ionizing radiation. $\endgroup$
    – Ville Niemi
    Commented Dec 24, 2017 at 16:06
  • $\begingroup$ As I understand, this would be more of a long-term oxygen production method - slow build up over perhaps a few billion years - but as soon as oxygen becomes present, I imagine that life on this world will have begun using it to some extent. For example I remember looking at a reaction which used H2S, O2 and something to produce H2 and something else; H2 would be useful to organisms like this perhaps as a lifting gas; being higher in the atmosphere probably means more H2S. Additionally, some chemosynthetic creatures use O2; hydrothermal vent bacteria use CO2 + 4H2S + O2 -> CH20 + 4S + 3H2O $\endgroup$
    – PrimarySecondary
    Commented Dec 25, 2017 at 11:15
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    $\begingroup$ I think you might have to lose the H2 to space to make this work or it it will recombine with the O2 to reform water. You want to minimize stuff that will naturally react away the O2 - early earth had a lot of stuff like that (reduced nitrogen, iron, carbon) which is why photodissociation did not create an O2 atmosphere and we had to wait for photosynthesis to do it. But if your moon does not have a lot of oxidizable the trickle of O2 from electrodissociation can build up. $\endgroup$
    – Willk
    Commented Dec 26, 2017 at 2:13

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