Gasses that can be present on a planet habitable by humans

[capslfern]

TL;DR Summary: what kinds of gasses can be in the atmosphere of a planet without killing them or not being naturally possible

I have been creating a fictional star system and ran into an issue, most of the planets atmospheres are not all the interesting, feels wrong to have most be oxygen, nitrogen, and some noble gas, what gasses could be in a planets atmosphere that aren't too rare in nature or will kill humans, or is just oxygen-nitrogen fine. I would rather they at least be somewhat possible to occur naturally on a planet

 

[Baluncore]

Welcome to PF.

Only oxygen is needed by humans.

Any reactive gasses will be removed from the atmosphere by the excess oxygen and starlight. That includes hydrogen, methane, ...

Oxygen concentration will become stable when the planet's surface cannot be oxidised further. Water and most rocks are oxides.

 

[capslfern]

so no reactive gasses, what gasses could exist, and how many would be toxic?

so anything that can be oxidized with oxygen in the atmosphere pretty much has been

 

[Baluncore]

so no reactive gasses, what gasses could exist, and how many would be toxic?

Probably the most toxic would be CO₂, that will dissolve in your blood, making it acidic with carbonic acid, which will kill you.
https://en.wikipedia.org/wiki/Hypercapnia#Tolerance

Vegetation will spontaneously burn in oxygen, which releases mostly CO₂ and water. The CO₂ gradually dissolves in water, (sea water), and slowly precipitates as carbonates such as CaCO₃, limestone.

CO₂ is used by plants, so you will need vegetation on the planet for a long period before you get there, that should remove most of the remaining toxic CO₂ from the atmosphere.

The oxygen concentration of the atmosphere must be less than about 25% or spontaneous fires will break out, all over your body. That will require "inert" filler gasses, such as argon or nitrogen, to stabilise the atmospheric pressure.
https://en.wikipedia.org/wiki/Apollo_1

Without sufficient atmospheric pressure, water will boil from your body at your body temperature and you will dehydrate.
https://en.wikipedia.org/wiki/Armstrong_limit

You will need water dissolved in the air, or breathing through your lungs will dry out your body. The atmosphere must be cooler than your body, or you will suffer heat stroke. The warmer air you breathe out will contain more water than that you breathe in, so you will need to drink water.

 

[snorkack]

Probably the most toxic would be CO₂, that will dissolve in your blood, making it acidic with carbonic acid, which will kill you.
https://en.wikipedia.org/wiki/Hypercapnia#Tolerance

Vegetation will spontaneously burn in oxygen, which releases mostly CO₂ and water. The CO₂ gradually dissolves in water, (sea water), and slowly precipitates as carbonates such as CaCO₃, limestone.

CO₂ is used by plants, so you will need vegetation on the planet for a long period before you get there, that should remove most of the remaining toxic CO₂ from the atmosphere.

That depends on geology - how efficient the biosphere is in removing carbon from circulation.
High carbon dioxide atmosphere with native life adapted to it but poisonous for people is plausible.

The oxygen concentration of the atmosphere must be less than about 25% or spontaneous fires will break out, all over your body.

No, they won´t.
Spontaneous ignition depends on thermal runaways in decomposing organic matter, like oil, hay, manure, cotton. Human body is not spontaneously decomposing by thermal runaway.

That will require "inert" filler gasses, such as argon or nitrogen, to stabilise the atmospheric pressure.
https://en.wikipedia.org/wiki/Apollo_1

Without sufficient atmospheric pressure, water will boil from your body at your body temperature and you will dehydrate.
https://en.wikipedia.org/wiki/Armstrong_limit

I think that there is roughly the minimum amount of exhaled stuff. About 63 mbar water and 50 mbar carbon dioxide. And then the needed oxygen must be over and above these 110 mbar.

You will need water dissolved in the air, or breathing through your lungs will dry out your body. The atmosphere must be cooler than your body, or you will suffer heat stroke. The warmer air you breathe out will contain more water than that you breathe in, so you will need to drink water.

Which is why you don´t need water vapour in the air. Because you will need to drink anyway. Air in airplanes is about 2% relative humidity. A man would do just as fine at exactly 0%, provided he drinks.
Also: nitrogen is poisonous. Argon, krypton and xenon even more so.

 

[Baluncore]

Also: nitrogen is poisonous. Argon, krypton and xenon even more so.

Under what conditions are those gasses poisonous ?

Maybe, rather than telling me I am wrong, you can be constructive, and put some effort into answering the OP's question.

 

[capslfern]

the planet does have quite a bit of plants, with its lower gravity the trees grow quite tall, also its not a planet but a large moon

 

[snorkack]

Under what conditions are those gasses poisonous ?

Maybe, rather than telling me I am wrong, you can be constructive, and put some effort into answering the OP's question.

I thought the answer was already offered. The only options are oxygen, nitrogen and inert gases. Plus carbon dioxide.
Inert gases are liable to cause narcosis. Nitrogen gets people noticeably drunk over 3 bar. Ar, Kr and Xe increasingly more so, at lower pressures.
The exceptions here are He, which appears to not cause narcosis, and Ne, which is needed in high pressures to cause narcosis.

 

[DaveC426913]

...what gasses could be in a planets atmosphere ...

I can think of a few.
Gaseous iron, nickel and lead and most other metals will go hand-in-hand with dead humans - although that's a correlation, not a causation.

 

[capslfern]

Gaseous iron, nickel and lead and most other metals will go hand-in-hand with dead humans - although that's

yea that is a bit warm, I have a planet that might be bad enough to have some of those, at least in the day

 

[Baluncore]

... what gasses could be in a planets atmosphere that aren't too rare in nature or will kill humans, or is just oxygen-nitrogen fine. I would rather they at least be somewhat possible to occur naturally on a planet

If gravity was low, and the trees were tall, then the lighter gas molecules would tend to be lost to space. O2 would be lower than in Earth's atmosphere. As it is relatively heavy, concentrated CO2 would remain pooled near the forest floor, where there is little wind. That is not good news for human habitation, unless it lived in the upper tree canopy, eating fruit and leaf tips, while drinking soda. The atmosphere would be hot due to greenhouse effect, so lighter gas would be lost even faster than the low gravity would suggest.

 

[snorkack]

If gravity was low, and the trees were tall, then the lighter gas molecules would tend to be lost to space. O2 would be lower than in Earth's atmosphere.

Does not follow. Note that N₂ molecules are lighter than O₂.

As it is relatively heavy, concentrated CO2 would remain pooled near the forest floor, where there is little wind.

It does not do so in Earth forests. It is very far from doing so. The gentle winds under forest are much more than capable of mixing the gases. Actually, on annual basis, even diffusion alone would.

That is not good news for human habitation, unless it lived in the upper tree canopy, eating fruit and leaf tips, while drinking soda. The atmosphere would be hot due to greenhouse effect, so lighter gas would be lost even faster than the low gravity would suggest.

Does not follow. The OP did not specify insolation.
Note that other things being equal, low gravity increases greenhouse effect.

 

[Baluncore]

Does not follow.

Rather than mobbing any novel approach, this is a time to think outside the box.
What have you got against the OP, that makes you want to censor or shut down any discussion of the expansion of the possible atmospheres?

Maybe, rather than telling me I am wrong, you can be constructive, and put some effort into answering the OP's question.