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.

 

[snorkack]

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?

No, it is not an issue of censoring or shutting down discussing. Rather, it is a question of exploring the walls of the box. If people do not understand the walls, it is better to talk with them about the walls, rather than shut them down and not explain the walls!
The walls are there. You just need to understand just where they are, what do they consist of, are they hard or soft walls, what are the corners...
You might say "a body with weaker gravity necessarily has a thinner atmosphere".
Not true. It is the case for Moon and Mars - but Venus and Titan have weaker gravity than Earth and denser atmosphere.
Carbon dioxide would pool in places with little wind, and that would include the whole forest floor? This actually happens on Earth... at places with very strong carbon dioxide outgassing from ground. It is not a widespread issue - mostly everywhere else, even gentle winds and diffusion can dilute carbon dioxide to safe levels.
Carbon dioxide would make the atmosphere hot by greenhouse effect? Again no. The OP did not specify the insolation. You could have a combination of dimmer insolation and higher carbon dioxide concentration leading to either moderate temperatures, or cooler temperatures. Mars has more carbon dioxide than Earth (7 mbar, against the 0,4 mbar of Earth) but is cooler.
Next wall or corner you want to talk about?

 

[Baluncore]

Next wall or corner you want to talk about?

So you can kick it down, simply to demonstrate your destructive ability.
I pity the unfortunate OP.

 

[snorkack]

Titan is an example of a moon that has weak gravity and dense atmosphere.
Titan is too cold for plants, or people, and lacks breathable oxygen.
However, a big moon more massive and warmer than Titan might have gravity which is still weaker than Earth, vegetation and breathable air.

 

[Baluncore]

https://en.wikipedia.org/wiki/Lake_Nyos_disaster
https://en.wikipedia.org/wiki/Bogle–Chandler_case

 

[stefan r]

Water worlds can establish oxygen atmospheres without plant life. Hydrogen slowly leaves the atmosphere when water molecules are broken by UV or other stronger radiation.

On a tidally locked “eyeball” planet the atmosphere can collapse. Mars demonstrates atmospheric collapse well with a large portion of the atmosphere snowing out at the poles in each winter. The antipodal point on a planet is far colder than a pole. The solar apex point is also much warmer than a point on an equivalent planet and atmospheric pressure.

The solar apex is likely to be the deepest level of crustal surface. The highest density component of a sphere will sink to the center. Thus it is also an ocean bottom while the antipode is more likely to have the highest mountain ranges. Mountains can collect snow. A deep water ocean can fairly quickly become a vast salt flat while the antipodal side becomes an extensive ice sheet. Salt flats have much lower albedo than ocean water. Water in atmosphere is also a strong greenhouse gas. The blizzards would rapidly make deep snow piles in the terrain on the dark side of the terminator line. Once the vapor component of the atmosphere gets low enough the antipode gets even colder. The cold temperature gases and refrigerants can rain out. This leaves only the oxygen and nitrogen gas.

A very deep ice sheet or glacier is too thick for heat to escape easily. Geothermal processes can liquify the ice from below. This can slowly move material from highland snow caps into valleys. The highlands can also sublime and get carried by wind currents. The under-ice water table (or other fluids) will gradually rise in the deep valleys. If they rise enough they may discover a flow route that allows the ice dam to burst. A new ocean forms and returns the moist state of the atmosphere. That, in turn, mixes warm snow into the carbon dioxide ocean causing it to also burst ice dams and flow under the water and into the valleys. The carbon dioxide or carbonated water can drive a “lake Nycos” or a colossal diet soda event. Fizz launching glacier boulders through the canyon and blowing out onto the ocean plain.

 

[snorkack]

The solar apex is likely to be the deepest level of crustal surface. The highest density component of a sphere will sink to the center. Thus it is also an ocean bottom while the antipode is more likely to have the highest mountain ranges.

I am not sure it follows. How? The dense materials (iron) sink to the centre from all parts of the surface, no matter whether pole or equator.

Mountains can collect snow. A deep water ocean can fairly quickly become a vast salt flat while the antipodal side becomes an extensive ice sheet.

Depends on how deep.

Salt flats have much lower albedo than ocean water. Water in atmosphere is also a strong greenhouse gas. The blizzards would rapidly make deep snow piles in the terrain on the dark side of the terminator line. Once the vapor component of the atmosphere gets low enough the antipode gets even colder. The cold temperature gases and refrigerants can rain out. This leaves only the oxygen and nitrogen gas.

A very deep ice sheet or glacier is too thick for heat to escape easily. Geothermal processes can liquify the ice from below. This can slowly move material from highland snow caps into valleys.

On Earth, the East Antarctic ice sheet is about -60 degrees near summit surface (unfortunately the Chinese do not winter at Kunlun station) but at melting at bottom (lake Vostok). Antarctic ice sheet obviously flows.
In Quaternary Ice Age, the ice sheets of North America and Europe took up around 40 million cubic km of water, which managed to lower the level of ocean by just 120 m - only 3% of seawater. Flow of the glaciers prevented runaway freezing of Earth.
In contrast, although the polar caps of Mars are almost as big as Greenland ice sheet, they obviously do not flow. There are detailed investigations of the layering of ice at the steep slopes of trenches cut in the caps, and they show absence of flows.
When can ice sheets be frozen (i. e. stop solid flow like the Martian caps)? When can ice sheets dry up ocean without flowing back into it?

A new ocean forms and returns the moist state of the atmosphere. That, in turn, mixes warm snow into the carbon dioxide ocean causing it to also burst ice dams and flow under the water and into the valleys.

Carbon dioxide ocean takes over 5 bars of carbon dioxide. Which will be strongly warming.

 

[stefan r]

I am not sure it follows. How? The dense materials (iron) sink to the centre from all parts of the surface, no matter whether pole or equator.

That happened on terrestrial planets in our solar system too. There still heavy and light sides. The Procellarum faces Earth. On Earth the Pacific is denser crust. Continents are less dense. The southern half of Mars is much higher altitude than the northern half.

Depends on how deep.

On Earth, the East Antarctic ice sheet is about -60 degrees near summit surface (unfortunately the Chinese do not winter at Kunlun station) but at melting at bottom (lake Vostok). Antarctic ice sheet obviously flows.
In Quaternary Ice Age, the ice sheets of North America and Europe took up around 40 million cubic km of water, which managed to lower the level of ocean by just 120 m - only 3% of seawater. Flow of the glaciers prevented runaway freezing of Earth.
In contrast, although the polar caps of Mars are almost as big as Greenland ice sheet, they obviously do not flow. There are detailed investigations of the layering of ice at the steep slopes of trenches cut in the caps, and they show absence of flows.
When can ice sheets be frozen (i. e. stop solid flow like the Martian caps)? When can ice sheets dry up ocean without flowing back into it?

A planet’s total water inventory strongly effects if/when it can dry out. The internal heat from other planets’ causing tides, radioactive decay etc will effect the warming from below. Mars’ crust has cooled very deep but there are still detections of possible brine pools.

Both Earth and Mars rotate. We get Summers where there is 24 hours of daylight at the poles. Near the terminator line equator the planet might get brief low angle light. Slightly further the shadow is permanent darkness except deep space and the glow of other planets. The crater poles of Mercury have temperatures not quite cold enough to freeze methane, oxygen or nitrogen.

The water mass on Earth is almost 300 times the atmospheric mass. The solubility of air in water is too low for that to dissolve away. Snowflakes can bring considerably more gas. Under enough pressure they become bubbles and then clathrates.

Tidal locked planets still have Coriolis effect winds. Wind blown dunes might recycle some of the water during the cold dry lock periods.

Carbon dioxide ocean takes over 5 bars of carbon dioxide. Which will be strongly warming.

If Earth like gravity 5 bar is only 55 meters of water ice. I am picturing more like 550 or. 5,500. Also likely to be carbonated water. Alcohols or ammonia could be present in levels below the flammability.

 

[snorkack]

The water mass on Earth is almost 300 times the atmospheric mass. The solubility of air in water is too low for that to dissolve away. Snowflakes can bring considerably more gas. Under enough pressure they become bubbles and then clathrates.

More but by my estimate not very much.
For fresh water at 0 degrees, I see the solubility of nitrogen quoted as 0,03 volumes and oxygen as 0,07 columes. I interpret these values as applicable to 1 bar partial pressure. Cold water in air would then dissolve roughly 30x0,8=24 l nitrogen per ton, and 70x0,2=14 l oxygen. Total 38 l air components per ton.
Snow initially encloses a lot of air, but the pores are connected and the air stays at ambient pressure. I read estimates that the pores close around density 820...840 g/l. Since the density of ice is 917 g/l at freezing and shrinks to about 934 g/l at low temperatures, assuming about 920 g/l would mean that 1 t ice (volume of ice under 1100 l, the enclosed volume around 1200 l when pores close) would trap 80...100 l air per cubic m, that is 95...120 l per ton of water. At most 3 times the amount it dissolved as liquid.
The volume of atmosphere is equivalent to 8000 m air at sea level pressure. The volume of sea is equivalent to about 2750 m water spread over whole Earth. Since converting water into ice traps up to 6...8 % its bolume of air into bubbles, freezing the whole sea into glaciers made from dry snow would remove maybe 165...220 m equivalent volume of air. That is just 2...3% of all atmosphere.
Can anyone refine my estimates?