Could a planet with this atmospheric composition have a fire:

Atmospheric composition

42% nitrogen

53.8457% oxygen

2.5% carbon dioxide

1.5% argon

0.099% carbon monoxide

and 0.0553% chlorine

atmospheric pressure = 22 Atm

Surface temperature = 33 C or 306.15 Kelvins

is it possible?

PS: I'm new to this site (sorry if the post is formatted terribly)

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    $\begingroup$ Just to note, you might want to check if carbon monoxide can exist in that concentration of oxygen at that pressure - my suspicion is that it would instantaneously oxidise. There may be oxides of nitrogen around too. Ask a physical chemist (or another question here). $\endgroup$ Commented Dec 30, 2023 at 7:15
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    $\begingroup$ @Escapeddentalpatient. Doing a bit of math gives an equilibrium vapor pressure for NO of a fraction of a part per million. That said, with this concentration of oxygen, it would almost immediately be converted into NO2. THAT said, at this pressure, NO2 would be a liquid. There would still be a non-zero amount of NO2 vapor pressure at all times, though it's a ferocious oxidizer as well (along with the huge amount of oxygen and free chlorine). For reference, 20ppm of NO2 is fatal to humans and Shanghai (worst NO2 pollution on Earth) averages 0.04ppm. Skies are orange with smog, likely. $\endgroup$ Commented Dec 30, 2023 at 14:53
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    $\begingroup$ I'd say the only way to have fire in that atmosphere is by bringing some combustibles with your spaceship; anything combustible on the planet will have incinerated long ago with that amount of oxygen. $\endgroup$ Commented Dec 30, 2023 at 17:56
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    $\begingroup$ @marcelm even the nitrogen would be a problem for Earth mammals at these pressures, and this is way past the point where the oxygen becomes toxic, not to mention the CO2. For a human, it'd just be a question of which part of the atmosphere would kill you first. It could support something that evolved in that atmosphere, but avoiding spontaneous combustion would be a major evolutionary pressure. $\endgroup$ Commented Dec 30, 2023 at 19:34
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    $\begingroup$ How are you defining "fire"? Specifically oxidation, or rapid exothermic reaction generally? $\endgroup$
    – keshlam
    Commented Dec 31, 2023 at 16:12

4 Answers 4


At 12 bar oxygen partial pressure and 300K temp, most of our everyday combustible substances (wood, fabric, plastic, flour, etc...) will self-ignite if left stacked or piled in a matter of hours.

Self-ignition is pretty much a known problem in our, relatively oxygen-poor atmosphere, but such a feat takes way longer. Objects that are known to self-ignite:

  • piles of coal, especially if somewhat moist. A 100-ton coal pile left alone for a week or two is a recipe for a problem.
  • domestic waste. A truckload of fresh waste is pretty much expected to self-ignite in 3-4 days, if untreated.
  • Clothes, blankets, and other fabrics stacked in a near-pure oxygen atmosphere. Pretty much an ordinary issue to consider in hospitals where patients are treated with oxygen and oxygen-enriched air builds up in some rooms.

On the other hand, 60 times more oxygen (even if we consider the spontaneous oxidation only to be ~60x faster, which probably is a conservative estimate) will outcompete the natural cooling of piled or stacked objects much quicker.

  • $\begingroup$ Wet hay will also self ignite, which is probably why grasslands are so prone to fire, now I think of it $\endgroup$
    – No Name
    Commented Jan 2 at 1:30
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    $\begingroup$ @NoName grassland fires usually start by different mechanism. It is the sunlight focused by a dew droplet. A moist haystack self-heats by fermentation until the spontaneous oxidation takes over. $\endgroup$
    – fraxinus
    Commented Jan 2 at 7:04
  • $\begingroup$ Fascinating! Is that the mechanism for trash fires too? Fermentation --> combustion? $\endgroup$
    – No Name
    Commented Jan 2 at 7:16
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    $\begingroup$ @NoName depends on the scale. An ordinary dumpster is too small to retain enough heat in order to self-ignite by fermentation. When it does ignite it is either because of someone intentionally setting it on fire or because of something specific (e.g. batteries, chemicals). Landfill sites usually self-ignite by fermentation. $\endgroup$
    – fraxinus
    Commented Jan 2 at 8:31

You can certainly have fire in that atmosphere.

With a partial pressure of oxygen of over ten atmospheres, the problem is more likely to be avoiding everything that can burn being on fire.

  • $\begingroup$ Including some of the air! $\endgroup$
    – No Name
    Commented Jan 1 at 14:58

Yes, fire is highly probable on this planet given its atmospheric composition. The oxygen level at 53.8457% is more than twice higher than that of Earth, significantly increasing the likelihood of combustion, given some source of energy (sparks, lightning, etc).

The high atmospheric pressure of 22 Atm could further intensify and accelerate a fire. The 2.5% carbon dioxide and traces of carbon monoxide and chlorine are unlikely to prevent combustion. The 33 C surface temperature is also favorable for fire initiation.

I wonder if there is an actual danger of having spontaneous ignition of fires and/or chain reactions involving oxygen on this planet. If there are perhaps volcanoes say.

Also, please reconsider the CO concentration. On Earth, it is a product of incomplete combustion and has a lifetime of about a month before turning into $CO_2$. On Earth, CO is replenished by volcanoes and, of course, fires. So, to have such a concentration on your planet, there needs to be a reason for its existence.

Edit: Considering the comment below (thank you), another way to look into the oxygen situation is through partial pressures (Dalton's law). On Earth, oxygen's partial pressure is about 0.21 Atm, since it's 21% of air. But on the suggested planet, with about 53.9% oxygen in a much thicker atmosphere (22 Atm), oxygen's partial pressure is around 11.86 Atm which is indeed more than 55 times higher than Earth! This means fires could start more easily and burn more intensely. It's not just about having more oxygen; it's about how much contribution it has in processes such as combustion.

  • 1
    $\begingroup$ Re, "oxygen level...more than twice higher...high atmospheric pressure..." Can't you just roll that up by saying that the partial pressure of oxygen in the proposed atmosphere is more than 55 times the partial pressure of oxygen at the surface of the Earth? Does the ratio of oxygen to other gasses really matter? $\endgroup$ Commented Jan 1 at 1:44
  • $\begingroup$ @SolomonSlow thanks for the comment. I added a small paragraph at the end to include the partial pressures discussion which is indeed crucial as you pointed out!! $\endgroup$
    – cconsta1
    Commented Jan 1 at 8:55
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    $\begingroup$ thanks guys for making me fix the atmosphere $\endgroup$
    – user107608
    Commented Jan 2 at 10:03
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    $\begingroup$ While a number of things are determined mainly by the pO2, the ratio of oxygen to other gases does matter for things such as flame temperature. If 1/n molecules in the air are oxygen, then for a fire to use 1 oxygen molecule, it will also (roughly) interact with (n-1)/n non-oxygen molecules. This cools the fire, lowering the temperature. Eliminating this "nitrogen cooling" is why plenty of torches have their own oxygen supply. $\endgroup$
    – AI0867
    Commented Jan 3 at 14:53

As others have said, there most certainly could be a fire, and it would be hard to avoid one if combustible materials were around. And this actually makes it hard for such an atmosphere to even exist. Oxygen is almost always found bound to other elements in nature. The only reason the earth has lots of free oxygen is because of photosynthesis. It would be an amazing kind of photosynthesis to generate that much oxygen while the oxygen itself is fighting back by recombining with carbon and hydrogen atoms in whatever molecules the photosynthesis was generating.

This is not to say it's bad to put such a planet in a story, but maybe you should have hyper-efficient photosynthesizers with quartz shells or something, or if explorers find it have them be really surprised. The constant danger of fire would make it an interesting background for a story of human presence for sure.

  • $\begingroup$ thanks for the feedback $\endgroup$
    – user107608
    Commented Jan 2 at 10:04

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