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!

  • $\begingroup$ 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
    Commented Aug 19, 2019 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
    Commented Aug 19, 2019 at 21:35
  • $\begingroup$ Please note this community is not about fishing for ideas, but about solving precise worldbuilding problems. $\endgroup$
    – L.Dutch
    Commented Aug 20, 2019 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. $\endgroup$ Commented Aug 20, 2019 at 11:07
  • $\begingroup$ Percentages are meaningless unless you give us the total atmospheres pressure or the partial pressures. $\endgroup$ Commented Aug 20, 2019 at 11:08

2 Answers 2


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.


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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