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Let us assume we have a rocky planet by the name of Pyros orbiting a single star, the Star of Random. It has an atmosphere consisting of 77% methane, 21% oxygen, 1% water vapor, and 1% other gases (argon, CO2, NH3?), all at a surface pressure of 101kPa (one Earth atmosphere). Temperatures at the surface are sufficient for liquid water to exist despite Pyros' sun being dimmer and/or further away, as methane is an effective greenhouse gas. Furthermore, the atmosphere composition is apparently stable -- it's too rich to sustain combustion on a wide scale.

What adaptations would 1) photosynthetic and 2) chemoheterotrophic life have on this planet compared to similar categories of Earth organisms? Furthermore, how far could life develop on such a planet? Assume that the nitrogen cycle is primarily subterranean, supplemented by small amounts of atmospheric NH3.

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  • $\begingroup$ worldbuilding.stackexchange.com/questions/45248/… is veryy closely related. $\endgroup$ – user25818 Jan 31 '18 at 23:49
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    $\begingroup$ Life will develop explosively. $\endgroup$ – ShadoCat Feb 1 '18 at 0:00
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    $\begingroup$ Downvoted for lack of knowledge of basic chemistry. Assuming, for the sake of discussion, that such an atmosphere could be created, the first spark would cause it to ignite. See e.g. internal combustion engines running on natural gas. $\endgroup$ – jamesqf Feb 1 '18 at 4:00
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    $\begingroup$ If you're going to magic an atmosphere like that and keep it stable, then you can magic life developing any damn way you want :-) $\endgroup$ – Kilisi Feb 1 '18 at 9:15
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    $\begingroup$ Methane does not have a particular odor. But it is common in a reducing environment. (It is reduced carbon.) Reduced sulphur has a very strong odor. If conditions promote reduced carbon, reduced sulphur is likely - and that is the source of the smell. Thus, swamps smell from sulfur compounds - and have methane - and both derive from reducing environment (anaerobic). The NH3 in the question would have an odor, but not the methane. $\endgroup$ – DPT Feb 1 '18 at 20:59
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Methane in an oxic atmosphere and photons (star) will quickly oxidize.

Your atmosphere is unstable.

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  • $\begingroup$ So you're saying the ternary flammability charts are taken in complete darkness? Or are you saying that the high energy radiation from the star would be the culprit? $\endgroup$ – Shalvenay Jan 31 '18 at 23:49
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    $\begingroup$ @Shalvenay I don't know how the ternary flammability charts are generated. Methane has a half-life in our atmosphere of about 12 years. parliament.nz/resource/0000214523 There was a period on early Earth when methane was abundant in air and CO2 was replete in the ocean. Photosynthetic organisms evolved and made O2 from the dissolved CO2 in the water. As that O2 escaped into the air, it oxidized the methane into more CO2 which dissolved back into the ocean (a feedback loop). There were other ramifications of O2 - the evolution of the atmosphere is an amazing topic. $\endgroup$ – DPT Jan 31 '18 at 23:55
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    $\begingroup$ @Shalvenay Flammability is irrelevant. The molecules will react without producing flame, slowly on a human timescale but blazingly fast on a geologic one. $\endgroup$ – Logan R. Kearsley Feb 1 '18 at 0:15
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Turning our 'suspension of disbelief' fields to max, I'm going to assume that a large measure of handwavium is stabilising this atmosphere and have a bit of fun.

How would this atmosphere affect photosynthesis? It would render it completely irrelevant.

With a few changes to known biologies, you've just created a 'soup in a can' of atmosphere. Assuming you have the ability to metabolise hydrocarbons directly (that's what methane is after all) you take your CH4 (methane), 2x O2 and you create water and carbon dioxide while at the same time producing energy. You've got a breathable atmosphere that can give you energy at the same time.

The reason for this is the same as the reason that the atmosphere is really unstable; biological energy production takes advantage of the same exothermic reactions (in controlled and measured amounts) that make these chemicals want to go boom in the first place. This is why fertiliser is so useful as an ingredient for explosives as well. The chemical reactions that release so much energy at once are not that dissimilar to those that happen in organic bodies constantly on very small scales.

Your problem here is that eventually, WITHOUT photosynthesis, your life forms will convert all the O2, and a reasonable proportion of the methane into water and CO2, and you need photosynthesis after all.

Such a planet (again assuming a handwavium approach to keeping this atmosphere stable in the first place) will probably develop simple life forms that will 'eat' the atmosphere, converting it to a mix more in line with early Earth, and generating water at the same time. That life will eventually die out, and the next wave would start with plants that use heat to generate the endothermic reactions of photosynthesis. Heat and light are basically the same thing if you take eyes out of the equation so this second generation of life would likely be similar to what we would expect to see on Earth, except that it may well still be designed to consume methane instead of carbohydrates. That means that the animals that eventually predate the plants may well ALSO have their metabolisms designed to consume methane, meaning that they predate the plants for nutrition more than energy, which they can get from the atmosphere.

I would stress that this is all speculative, and is based on the existence of an impossible situation in the first place. That said, it could be a very different world because food as a concept may be designed around trace elements rather than energy generation, meaning it could be a very small amount of predation that is limited to plant life (no carnivores) so as to maximise mineral nutrition by consuming plants that leech it from the ground very effectively.

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