I'm working on a solar system where there is a planet with a red-ish atmosphere, due to it mainly being composed of CO2. I know CO2 doesn't react with many chemicals and is pretty dull, molecule-wise. But . . . I don't know how I'm going to compose the atmosphere if it's mostly CO2.

Requirements for the composition:

  • Mostly CO2
  • Oxygen must be included


Rounded percentage of each gas in this atmosphere? (just the main few greater than or equal to 1%)

Other info:

  • The planet near Earth-size
  • Star is similar to our sun, just a bit larger
  • The planet has a lot of copper and iron near the surface, the rest of the composition of the soil is somewhere between Mars and Earth
  • The other terrestrial planets around the main planet cause the main planet to have a spin and year much like Earth's
  • The life forms have evolved to be able to use a smaller amount of oxygen to survive or to go without oxygen entirely
  • This is set in the future, so humans have figured out how to make space suits that suit climates like this
  • The humans that have been to this planet are visitors, but not living here
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    $\begingroup$ Please remember one and only one question per post. Otherwise, a well-constructed question! Thanks! $\endgroup$
    – JBH
    Nov 2 at 0:00
  • $\begingroup$ "with a red-ish atmosphere, due to it mainly being composed of CO2": how would CO2 make the atmosphere red? "humans have figured out how to make space suits that suit climates like this": climates like what? $\endgroup$ Nov 2 at 0:10
  • $\begingroup$ @ChristopherJamesHuff Part of the climate is that atmosphere itself, this is what I'm asking about. A lot of climate is based on the atmosphere. Otherwise, the soil is mentioned in the question itself. $\endgroup$ Nov 2 at 0:12
  • $\begingroup$ @ChristopherJamesHuff Have you read the worldbuilding question on orange atmospheres? I used that as the basis of my atmosphere. Correct me if I'm wrong about using CO2 though :) $\endgroup$ Nov 2 at 0:15
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    $\begingroup$ Mars has a reddish atmosphere not due to CO2, but because of atmospheric dust. This question has few constraints other than carbon dioxide and oxygen, but nothing to say how much of either should be present. What's to say that one set of gas percentages is any better than another? $\endgroup$
    – Monty Wild
    Nov 2 at 0:40

1 Answer 1


Not sure about other mammals but Earth humans will not tolerate this atmosphere. CO2 above very small amounts is poisonous to us because our breathing system doesn't detect lack of oxygen, it detects excess CO2. My quick search says that 40000ppm (0.04%) is immediately dangerous.

Of course, if your animals use some other metabolic path, or have evolved to directly detect oxygen deficits this things shouldn't be a problem.

  • $\begingroup$ This is very nice to know, but it doesn't fully answer my question. :) $\endgroup$ Nov 1 at 23:11
  • $\begingroup$ @Silvermidnight you don't appear to have understood. Vertebrates use production of CO2 to regulate respiration, which works because ambient levels are very low. It's the quantity of CO2 which is the issue for humans, not the quantity of O2...which is just what you asked about. $\endgroup$ Nov 2 at 0:09
  • $\begingroup$ @ChristopherJamesHuff I apologize for perhaps not being clear about what I meant. This planet is NOT home to humans, but they HAVE visited, and have suits to counter the effects of CO2. I mention this in the "other info" in my question that life forms have evolved to meet the demands of this planet. $\endgroup$ Nov 2 at 0:20
  • $\begingroup$ @ChristopherJamesHuff I did ask for the quantity of CO2 that is the problem for living forms. However due to only being able to ask one question, I changed this to percent composition. $\endgroup$ Nov 2 at 0:31

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