Q&A

Is this planet's atmosphere stable and reasonable, and anything to keep in mind for lifeforms living in it?

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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?
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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

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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