Inspired by the answers to What can I add to an oxygen/nitrogen atmosphere to make it unpalatable or poisonous to humans, yet stable and breathable to local creatures?, I am building a planet that currently...

  • Has an iron core, for the magnetic field to help retain the atmosphere
  • Has a surface gravitational acceleration of about 12.2 m/s2, some 25% greater than that of Earth (by virtue of being slightly more massive than Earth as well as somewhat smaller)
  • Is covered by 73.9% land and 26.1% oceans (basically the opposite of Earth)
  • Has an atmosphere consisting of 67.2% N2, 27.4% O2, 4.8% CO2, 0.4% Ar, and 0.2% miscellaneous (which I haven't decided on a complete breakdown as of yet, but which does include 2.4 ppm As)
  • Has a surface atmospheric pressure of 1930 mbar
  • Is highly geologically active, with lots of active volcanoes both on land and under water, as well as active plate tectonics

The planet will have lifeforms not entirely unlike those found on present-day Earth, but obviously not humans as we know them.

Now for the, IMO very much related, questions:

  • Will this atmosphere be stable? If not, then why not?
    • I don't mind the occasional (or even not so occasional; that's a lot of oxygen) wildfire, but I do mind if half the world goes up in flames the first time there's a meteor strike or volcanic eruption.
  • Is the mixture and pressure reasonable given the planet? If not, then why not?
  • Is there anything about the atmosphere that would pose particular problems to indigneous lifeforms? Anything that you can think of which I should keep in mind while designing lifeforms adapted to this atmosphere?
  • 2
    $\begingroup$ Given that there is a large amount of free oxygen in the atmosphere of that planet I would say that the planet most definitely has some sort of photosythetic life, because otherwise that oxygen would quickly find something to oxydise and be gone in a geological blink of an eye. And I seem to remember that arsenic (which is a solid at reasonable temperatures) needs to the quite hot to sublimate into a gas. $\endgroup$
    – AlexP
    Commented May 4, 2017 at 22:03
  • $\begingroup$ Your atmospheric arsenic will oxidize and leave the atmosphere. It will all wind up in the water / soil. Otherwise this seems very similar to Earth. $\endgroup$
    – Willk
    Commented May 4, 2017 at 23:12
  • $\begingroup$ @Will Are you saying that worldbuilding.stackexchange.com/a/78791/29 is wrong? If so, then could you please elaborate there on your reasoning? $\endgroup$
    – user
    Commented May 5, 2017 at 7:24
  • $\begingroup$ Arsenic is solid and will end landing on the floor or waters, but it can take some time. There's public health reports about wind transport of arsenic emissions (mainly from China) and it's worldwide distribution. As long as the arsenic is continuously provided by a certain source (volcanoes seems a likely choice) you can sustain a relatively high proportion of arsenic in the atmosphere. It doesn't really matter if it's only lasting for a few million years if we are exploring that world right now. $\endgroup$
    – Rekesoft
    Commented May 5, 2017 at 8:25
  • 1
    $\begingroup$ @MichaelKjörling: The linked answer talks about salts of arsenic acid H3AsO4 (arsenites and arsenates) not elemental arsenic. $\endgroup$
    – AlexP
    Commented May 5, 2017 at 9:27

3 Answers 3


Started as comment and became an answer.

First bit is the volcanic activity may impact your atmosphere. Depending planetary composition, there's a good chance these volcanic events will add a sulfur component to your atmosphere. Minor change at most. Though do remember if your planet lacks plate tectonics and retain this high volcanic activity trait, these volcanoes will start to grow to the size proportions of Olympic Mons on Mars, potentially longer as it grows for billions of years.

Mixture seems reasonable and the components you have chosen can be sequestered away by the planet or released back into the atmosphere. Fluctuations in make-up can be explained by these processes...I'd say you are good.

Last note as a comment to your indigenous creatures. Remember that respiration depends on oxygen partial pressures, not just the ratio of it in the air. At this pressure and concentration, I believe this atmosphere would cause oxygen toxicity issues in humans and most other creatures found on earth. Using past examples, it would appear Gigantism is the natural path to overcome oxygen toxicity. In short, your creatures are going to be over-sized and have a development cycle that includes massive growth early on in it's development to overcome oxygen toxicity issues.

  • $\begingroup$ Olympic Mons at 1.25 g and an erosive atmosphere? You wish. $\endgroup$
    – user58697
    Commented May 5, 2017 at 6:08
  • $\begingroup$ Part of the reasoning that led to both the high oxygen content as well as the large atmospheric pressure is precisely to at least approach the partial pressure range for oxygen toxicity in humans with a still-reasonable oxygen content in the atmosphere. (I even poked around some NASA diagrams for just that part.) I hadn't considered gigantism but it's actually not a bad thing for some of what I have in mind. Yes, the planet has plate tectonics. $\endgroup$
    – user
    Commented May 5, 2017 at 7:22
  • 1
    $\begingroup$ @user58697 What about the atmosphere would be erosive? Are you referring to the large amount of oxygen? Please do elaborate a bit. $\endgroup$
    – user
    Commented May 5, 2017 at 7:23
  • $\begingroup$ More likely lifeforms will evolve with an increased tolerance for oxygen. So gigantism for that reason won't necessary. An erosive atmosphere that's due to increased weather activity. Possibly any higher sulphur content will be in the form of sulphates and sulpuric acid. Acid rain will contribute to erosion. More acid rain, more erosion. $\endgroup$
    – a4android
    Commented May 5, 2017 at 10:19

Partial Pressure

Note that I don't know the $\text{gr/mol}$ of $\text{Others}$ so I tryied to make an average with your actual gases.

$$ \left| \begin{array}{cc|ccc|c|c} \text{Gas}&\text{%}&\text{gr/mol}&\text{Mols}&\text{Fractal Mol}&\text{Partial Pressure (kPa)}\\ \text{N}_{2}&\text{67.20%}&28.01&18.83&\text{62.88%}&121.37\\ \text{O}_{2}&\text{27.40%}&32.00&8.77&\text{29.29%}&56.53\\ \text{CO}_{2}&\text{4.80%}&44.01&2.11&\text{0.53%}&13.62\\ \text{Ar}&\text{0.40%}&39.95&0.16&\text{0.06%}&1.03\\ \text{As}&\text{0.024%}&74.92&0.02&\text{0.02%}&0.12\\ \text{Others}&\text{0.18%}&29.94&0.05&\text{0.18%}&0.34\\ \text{Total}&\text{100%}&248.83&29.94&\text{100%}&193 \end{array} \right| $$

  • Nitrogen (N2): 121.37 kPa
    • Nitrogen Narcopsia: No, don't worry, you don't have nitrogen narcopsia because it's developed under pressures above 240 kPa and 354 kPa, and you only have 121.37 kPa of N2.
  • Oxygen (O2): 56.53 kPa:
    • Oxygen toxicity: When O2 partial pressure is above 50 kPa oxygen become toxic. Also you would suffer hyperoxia.
      • Disorientation, breathing problems, vision changes such as myopia.
      • Prologed exposures of higher O2 PP or shorter exposure but very higher, can cause oxidative damage to cell membranes, collapse of the alveoli in the lungs, retinal detachment, and seizures.
      • A lot more, click in the link for more info.
      • In this question you can get more information, also you can see symptoms diagram.
  • Carbon dioxide (CO2): 13.62 kPa
    • Hypercapnia (Carbon dioxide poisoning): Severe hypercapnia is cause by an increment of 10 kPa CO2 and you have 13.62 kPa. You will die in hours.
      • If you want to see all the symptoms of high CO2 values or a table about CO2% and lethality you can see the same question of above.
  • Argon (Ar): 1.03 kPa
  • Arsenic (As): 0.12 kPa:
    • In this question I said that it's poison but he has 0.93% of As, I your case As is too low. Arsenic toxicity is above 10 μg/m³ and you don't have that, don't worry.
  • Others: 0.34 kPa
    • It's others, I don't know what it's...

Your animals have to be capable of:

  • Support the respiratory acidosis, they have to be capable of support high pH levels in blood or have an inner system to reduce the pH.
  • Support the oxygent toxicity, they will have some ability to support the cell membrane damage (e.g: lungs and eyes) and the free radical of oxygen: ROS that destroy the DNA or organels of cells.

Also, take in mind that you wold make the nightmare of all the people, the most fear of ever... ... ... BIGGER INSECTS!. Do you think that we already have big spiders, like this? Well, I don't want to visit your planet.... In this answer I explain it, insect haven't respiratory system*, they breath throught their skin so if you make them bigger they won't have enought cm2 of surface per gramme of insect, they will suffocate. In your planet there is the double of oxygen in air so insect could be a lot bigger without suffocate.
* Insect have respyratory system but they don't have lungs or gills, they use other things.


Sorry, I don't know much of this but I know that CO2 and O2 aren't stables.
What this means?
This means that you need a constant flow of them.

  • Oxygen (O2): Oxygen has the passive ability of make oxides, this means that if you don't have a source of oxygen, is some millions of years (or less...) all the metals of the surface would be oxides and the oxygen would dissapear of the atmosphere.
    My only idea to get a source of oxygen is the photosyntesis who make oxygen from carbon dioxide.
  • Carbon dioxide (CO2): But now we have another problem.
    • Carbon dioxide has the passive ability of combine slowly with the water to make it more acid, well, this is really slow so you don't have problem, but...
    • Photosystesis uses carbon dioxide so you will lose it very quicly, luckly you can us volcanos to get a constant source of CO2.
  • Arsenic (As): I didn't know about it but from the Kingledion answer I learn it. (You have my upvote).


Mmm, yes, I think it's reasonably, this is almost a personal opinion but I see that possible, obviously it's only possible if you have a photosystesis (plants) and volcanoes from me personal opinion.


The atmosphere should be stable

Carbon Dioxide

This Carbon Dioxide/Oxygen ratio should be stable, so long as that ratio is maintained by the carbon cycle on your planet. Having volcanoes to keep re-adding carbon to the atmosphere is definitely good. You will need photosynthetic life (or some other oxygen producing surrogate) to keep that free oxygen in the air; otherwise it will quickly end up in the rocks and what have you.


Arsenic will not hang around in the air. From Wikipedia,

It oxidises readily in air to form arsenic trioxide and water...

As$_2$O$_3$ is in turn hygroscopic and will end up in solution eventually. You would end up with your oceans being a weakly acidic with ionic arsenites in solution.

Oxygen at high pressure

Regarding the oxygen level, it was probably at least 27% for two periods (of tens of millions of years) in Earth's history, including during much of the Mesozoic.

However, the high pressure does give me some pause. If the pressure is 1.9 times that of Earth, and the oxygen concentration is 27%, then the oxygen partial pressure is $$1.9*\frac{27}{20} = 2.6$$ times that of our Earth, a partial pressure of about 510 mbar. There are claims that air pressure was higher in the Mesozoic, which would render this problem moot by showing that oxygen partial pressure in addition to concentration had been higher in the past. But those claims do not seem legitimate to me, and I'm going to have to find some hard evidence before accepting them.

Equilibrium constants for gaseous reactions depend on the partial pressure of that gas, so it is partial pressure, not percentage composition that determines if things will spontaneously combust, at least at higher pressures. Figure 5 on page 10 of this NASA report shows that flammability decreases for constant oxygen partial pressure as inert gas (N$_2$) pressure increases, but that this effect stops around 800 mbar. Since the difference between our atmosphere and yours is above that limit, we can expect that flammability will increase as a function of oxygen partial pressure.

Getting exact numbers on what will combust in what oxygen pressures is pretty tough. The linked NASA paper shows that for gaseous fuel air mixtures, 21% oxygen (~230 mbar) is sufficient for flammability and that increasing oxygen partial pressure has almost no effect (page 7).

However, a more reasonable approach to the atmosphere would be to consider that as oxygen concentration increases, oxidation reactions with the various materials of the lithosphere will increase. I think these would be more important than spontaneous combustion reactions. Without any other evidence, I propose that it is logical that there is some 'upper limit' for oxygen partial pressure, at which point the oxidation of atmospheric oxygen with surface minerals outstrips the ability of the biosphere to create that oxygen.

For our planet, that upper limit was probably around 30%, or 300 mbar oxygen. Applying the same to your planet at 1900 mbar would give you an oxygen concentration of about 16%. I suggest that 27% is too high an oxygen partial pressure to develop naturally, and that because of chemical weathering of the lithosphere the oxygen concentration on your planet should not go above 20% at the highest.


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