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I am currently working on world-building a planet, and I wanted to try to diverge from Earth's atmosphere a little bit, but not extremely. I am planning on still having it be a oxygen-nitrogen rich atmosphere, but I would like to maybe add another substance into the mix to make things a little more interesting. I have thought about ammonia, hydrogen sulfide or maybe hydrogen chloride being present in the atmosphere but I'm curious on what other chemicals might be possible for my ecosystem.

I'm planning on the atmosphere consisting of: 80% N2, 17% O2, 1% CO2, 1% X, and the rest being random other inert gases. X represents the new chemical I want to add in. I would love to get some ideas for other chemicals to put into the atmosphere and how they would affect the environment and possible life! Thank you!

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  • $\begingroup$ Um... So your question is what are some interesting things I could add to the atmosphere? By the way, that 1% CO2 (10,000 ppm) is already pretty interesting considering we are sitting at round-about 400 ppm. Going to be some thriving plants. $\endgroup$ – puppetsock Aug 19 at 21:18
  • $\begingroup$ No water vapor? On Earth, water vapor provides most of the greenhouse effect which keeps us cozy and warm. Earth's atmosphere contains about 0.4% water vapor (1% at sea level) and only 0.04% carbon dioxide -- ten times less carbon dioxide than water vapor. (And 1% argon, but I suppose this goes into "random inert gases".) At 1% carbon dioxide (25 times more than on Earth) and normal atmospheric pressure you will already see physiological effects. $\endgroup$ – AlexP Aug 19 at 21:35
  • $\begingroup$ Please note this community is not about fishing for ideas, but about solving precise worldbuilding problems. $\endgroup$ – L.Dutch Aug 20 at 2:36
  • $\begingroup$ @AlexP Argon does not go into random inert gases, it is the product of the radioactive decay which keeps the planet warm. $\endgroup$ – TheDyingOfLight Aug 20 at 11:07
  • $\begingroup$ Percentages are meaningless unless you give us the total atmospheres pressure or the partial pressures. $\endgroup$ – TheDyingOfLight Aug 20 at 11:08
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Some interesting inert gases

  • Neon - to have pretty red-orange lightning
  • Helium-3 - infinite energy if there's civilization that develops nuclear fusion.
  • Heavy gases like fluorocarbons or sulfur hexafluoride - while inert, can settle in terrain depressions and cause choking hazard, making exploration interesting. Can be "mined" for use as refrigeration coolant.

Other interesting idea: Ammonia. It would need some process which replenishes it (may be some exotic biology that produces it by photosynthesis instead of oxygen) but with combination with elevated CO2 it would have fantastical fertilizing effect. Like, desert after the rain would bloom not in days but in a hour.

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Perhaps, G-23 Paxilon Hydrochlorate...

Seriously though, sulfur dioxide might be a good choice. It would give a really great mouth of hell vibe along with insane acid rain.

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Hydrogen sulfide is an extremely dangerous gas: it is enough that it represents 0.002-0.005% of the volume of the air to endanger the life of those who come into contact with it. Currently, carbon dioxide represents approximately 0.04 % of the volume of the Earth's atmosphere, if hydrogen sulfide represented 1 %, it would be 25 times the volume of carbon dioxide! This would bring terrible consequences for the ecosystem on our planet, as you can imagine. Another danger is the fact that hydrogen sulfide is flammable, therefore, a high concentration of this gas in an atmosphere with some oxidant (oxygen, chlorine, fluorine, etc.) is not a good idea. In addition, when dissolved in water, it forms a weak acid, and assuming your planet has bodies of water on its surface and clouds of water in its sky, like in our planet, that would make them acidic. But this would be worse if that 1 % now corresponded to hydrogen chloride, since it forms a strong acid in aqueous solution. Hydrogen fluoride is neither an option: it is also very corrosive.

Ammonia has an acidity comparable to that of carbon dioxide, but dissolves in water to form a strongly alkaline solution (ammonium hydroxide). Imagine a lake contaminated with ammonium hydroxide, in the worst case, its pH would be greater than 10, which means that the only living beings that would be able to survive in it, would be alkaliphiles.

All these compounds would be dangerous for life on Earth if they represented 1 % of the atmospheric composition. But if your planet is inhabited by extraterrestrial life forms, perhaps they would have evolved to interact with them without problems. If you want to incorporate into the atmosphere of your planet a certain compound that interacts with native life, it will be very different from Earth's, especially if you choose any of these compounds.

You can consult this list of molecules detected in interstellar space so far, you could choose any of them to fill in that missing space in the atmospheric composition of your planet. Carbon dioxide represented a more significant fraction of the Earth's primitive atmosphere, but the plants evolved to use it metabolically and ended up reducing it, something similar could happen on your planet, just make sure that the compound is abundant in interstellar space, while simpler better.

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