[MarianaM (OP)]

Robert is wondering...

There is a lot of discussion about the need to find water sources for extraterrestrial colonies both for use as water and to make oxygen for respiration. The air we breathe on Earth is only about 20% oxygen, though. The vast majority is nitrogen.

Are readily available sources of nitrogen easy to find on other planets, is it expected that another gas would be substituted, does the lower gravity on smaller planets allow us to safely breathe higher concentrations of oxygen, or is there some other explanation for how we can create a breathable environment off planet Earth?

What do you think?

[Hayseed]

Being that the lungs are built for certain gases, I say stick to those gases and pressures.

Off earth habitation is still a dream.  We have to have gravity.  So redesign of craft is necessary.  This is easily done.....and say ~ 25% of earth's gravity might be all that's needed for health.  We don't know......and for some reason, an adjustable gravity structure has not been installed.

We also need radiation protection.  Which is still lacking and a big problem.

I'm sure that improvements and work on radiation shielding is being done.  Have not heard one word on gravity.

We should have a gravity section on the space station now, at least for sleeping.

I would not allow transport to Mars without a gravity craft.

[evan_au]

As humans, what we need is a certain pressure of oxygen, to keep our blood oxygenated.
- And a low level of CO₂ to allow us to exhale this waste product, and keep our blood pH in balance
- Other gases like nitrogen are just a "filler".
- Nitrogen is a bit of a problem: if pressure suddenly reduces, nitrogen "fizzes" in our blood, creating bubbles in your joints and blood vessels, causing severe pain and nerve/brain damage.
- You don't want to have 100% oxygen at sea-level atmospheric pressure, as this can cause vision problems (hyperventilation), and causes normally non-flammable items to sustain a flame. NASA discovered this as a result of the Apollo 1 fire.
- Some space vehicles have not used a "filler": they just use pure oxygen at 20%-27% of sea-level pressure. This has the advantage that your spacecraft does not need to be quite as strong (it is holding in less pressure). It also lightens the vehicle, as you don't need to carry pressurized tanks of nitrogen to replenish the air.
- Spacesuits tend to balloon into a sphere. If you are doing something decidedly non-balloon-like (like operating a tool), your fingers have to work against the pressure in the suit. Just having oxygen in the suit means your fingers have to fight less pressure, and so you can function for longer in a spacesuit. The ISS spacesuits use this technique.
- The ISS uses a 21% oxygen/79% nitrogen mixture. So, before going for a spacewalk, astronauts spend several hours breathing pure oxygen to get the nitrogen out of their system. Cutting down the time to do an emergency spacewalk would be an advantage!

So, oxygen will be needed, but a filler gas is somewhat optional (and nitrogen is somewhat problematic).

See: https://en.wikipedia.org/wiki/Space_suit#Operating_pressure

[rcbates]

Thanks, evan_au. Is there another gas, maybe a noble or other inert gas, other than nitrogen, that would work as a filler gas?

[Kryptid]

Thanks, evan_au. Is there another gas, maybe a noble or other inert gas, other than nitrogen, that would work as a filler gas?

Argon, neon and helium should work okay if the atmosphere is at normal pressure. Krypton and xenon can cause narcosis or anesthesia, so I would avoid using them. Getting them in large enough quantities to fill the atmosphere of a planet could be challenging. Helium could be acquired from the gas giants, I suppose, but it is very light and as such easily escapes from the atmospheres of Earth-sized planets.

[Bored chemist]

Helium and, to a lesser extent neon would make your voice funny.
Much less of a problem with argon.
I wonder if anyone has experimentally verified this with neon. It would be a rather expensive experiment.

Assuming you took the gas with you from Earth, nitrogen would be much cheaper and a bit lighter than argon.

[alancalverd]

Oxygen at 0.2 atmospheres pressure will support life indefinitely and works OK in small aeroplanes but it is a fire hazard, which is why larger structures like the ISS use normal air. You can recycle the nitrogen if there's no obvious alternative source.

[evan_au]

Helium and, to a lesser extent neon would make your voice funny.

Deep-sea divers have used Helium as a filler gas.
- The higher speed of sound in Helium produces higher resonant frequencies in the mouth, producing high-pitched speech.
- A bit of digital signal processing changes these higher frequencies generated by the mouth into a more normal range of frequencies to which our ears are attuned.

Oxygen at 0.2 atmospheres ... is a fire hazard

Can you explain the rationale here?

I thought that the flammability of a gas was due to its partial pressure?
- So oxygen at 0.2 atm (on its own) was just as flammable as oxygen at 0.2atm mixed with nitrogen at 0.8atm?
- Or is it a thermal inertia thing? With 80% Nitrogen, the flame has to heat up 5x as much air to ignition temperature?

[Bored chemist]

Or is it a thermal inertia thing? With 80% Nitrogen, the flame has to heat up 5x as much air to ignition temperature?

Yes,
and also a rate of diffusion thing- the nitrogen gets in the way.

[Kryptid]

Speaking of low-pressure, pure oxygen atmospheres, is it known if such atmospheres have a negative impact on human beings over a very long period of time? Like years? What about other organisms? Plants at least would have an issue with nitrogen fixation if some terraformed planet didn't have any nitrogen in the atmosphere. If we're talking about a sealed colony, then I suppose that could be fixed manually with artificial fertilizer.

[alancalverd]

We know that high pressure nitrogen is deleterious, but it would be ethically difficult to conduct research into longterm pure oxygen exposure  because of the known fire risk. AFAIK NASA has abandoned 0.2 bar O2 for space flight and accepted the weight penalty associated with "8000 ft normal air", which is well tolerated by fit humans. Low pressure pure oxygen is still used for small aircraft but if people have to move around the cabin, pressurisation is safer and more convenient.

[Bored chemist]

it would be ethically difficult to conduct research into longterm pure oxygen exposure

Difficult, but not impossible
It's been done (because there are patients receiving long term high oxygen concentrations.).
Oxygen  at high concentrations is toxic.
https://en.wikipedia.org/wiki/Oxygen_toxicity
So when Michael Jackson announced that he planned to live in an oxygen tent, the grown ups were able to explain that it would be a bad idea.
https://www.telegraph.co.uk/culture/music/michael-jackson/6008206/Michael-Jacksons-oxygen-chamber-found.html

[alancalverd]

The theoretical interest is in longterm exposure to 0.2 bar oxygen with no nitrogen. There would be some weight saving in transit and fewer problems of maintaining pressurisation and preventing leakage on a planet with a lower standard atmospheric pressure, but the fire risk remains high in a large enclosure with multiple activities - you can get away with it in a glider or even a fighter cockpit  but NASA ditched it after Apollo 1.

Longterm medical oxygen concentrators are prescribed for people whose respiratory uptake is poor: the blood is not normally driven above 95% saturation. You wouldn't select a respiratory-compromised person as an astronaut. Hyperbaric treatment attempts to oxygenate hypoxic tissue (particularly, solid tumors) by brute force and generally only lasts an hour or so.