Some background: I'm developing a world that could, theoretically, be inhabitable by humans (with a high enough recruitment success to at least keep a stable population), randomly determining environmental parameters based on the range our physiologies can tolerate. This world has 50 atm of pressure at the inhabited area I'm designing, and a mass of just $0.11M_{\oplus}$ (so a surface gravity of 0.48g). It's too small for plate tectonics, but has enough residual heat to create a magnetic dynamo.
Based on what I've been able to find online, in order for the atmosphere to be breathable, I need something like:
- 90% He
- 4.6% N2
- 3.8% H2
- 0.8% O2
- 0.6% Ne
- 0.18% Ar
- 0.01% CO
- 0.01% CO2
- +0% to 0.2% H2O (based on vapor pressure at habitable temperatures, which are probably on the warmer side since helium conducts heat so quickly)
The reasoning behind such a high concentration of helium is in line with heliox mixtures used in saturation diving, where the inert gas helium is used to "dilute" oxygen to a breathable partial pressure. It's the partial pressure of oxygen (here 50 atm x 8% = 0.4 atm, or almost twice the partial pressure in Earth's atmosphere), not the volume or molar percentage, that determines hypoxia or hyperoxia.
Hydrogen might be a major constituent of the atmosphere if the planet was originally a mini-Neptune that lost most of its hydrogen and a lot of its helium. Oxygen shouldn't be much higher than the noted percentage due to flammability risks and oxidative stresses on life. The big concern with nitrogen, neon and argon is nitrogen narcosis (and similar nasty effects); nitrogen is pretty much maxed out here. CO and CO2 should be more common in a helium-dominant atmosphere, but are directly toxic to life, so we probably can't raise them any more.
(As a minor sidebar, some envelope calculations based on equation 1 in this article give me an atmosphere loss rate (instantaneous $\frac{dm}{dt}$) of $2.61\times10^5$ kg/s, which if projected out geometrically would give about 16 million Earth years until the atmosphere is gone. So uh, definitely not a stable situation in geologic time.)
One of the explanations I've considered for having a thick helium atmosphere despite the current low mass is that the planet was originally a gas giant (mini Neptune-type) that got knocked into a lower orbit by some sort of prehistoric collision/slingshot effect, where it began bleeding off its gas envelope like a giant comet. I'm not sure if the planet could have formed, though - would it make sense for it to have such a small rocky core?
How else could a breathable high-pressure atmosphere form?