[thedoc (OP)]

"Morgan, Derek, Vodacom South Africa"  asked the Naked Scientists:
   Hi Chris

In your latest podcast on Mars, mention is made of producing oxygen to breathe.

The three methods were splitting carbon dioxide, splitting water or extracting oxygen from the plentiful oxides lying around on the surface (rusty rocks!).  Using energy from the sun.

Good and well, but we can't sustainably breathe pure oxygen. On Mars, what other gases would be available to mix it with to make a sustainably breathable air equivalent? Given the extremely thin and toxic atmosphere, I am guessing it won't be from there.

This is assuming we can breathe other mixtures of gases that contain the correct levels of oxygen but where the components are not the same as our air on earth. I think an alternative is probably possible but not sure about that. So there's a bonus question for you!

Thanks to all of you for an entertaining and informative podcast!

Kind regards
Derek
What do you think?

[alancalverd]

Good and well, but we can't sustainably breathe pure oxygen.

Not a biological problem but a safety engineering concern.

Atmospheric nitrogen acts only as an inert diluent, reducing the partial pressure of oxygen to about 200 hPa. You can breathe pure oxygen at 200 hPa indefinitely, if you add a whiff of carbon dioxide to stimulate the breathing response and a fair bit of water vapor to replace exhaled moisture. We use such "pure" oxygen for high altitude flight and early spacecraft, but it is a fire risk, so for longterm habitation on Mars I would look for a source of nitrogen or even argon to raise the pressure a bit. 

[evan_au]

Adding enough nitrogen to bring a Mars habitat up to Earth-normal would increase stresses on the habitat, and potentially contribute to a "blowout", as shown in "The Martian" movie. Finding a suitable source of nitrogen could be a challenge on Mars - I think the main purpose of nitrogen would be to get plants to turn nitrogen gas into nitrogen compounds which can enrich the soil and allow us to get the protein we need.

Nitrogen has a problem that it is too soluble in the bloodstream. If there is a sudden reduction in pressure, Nitrogen can come out of solution, and cause a painful and damaging condition called "the bends". Mostly it affects divers coming up from deep dives, but it is also a problem on the space station, so astronauts have to flush the nitrogen out of their system before going on spacewalks, by breathing pure oxygen for a few hours. Hardly a productive use of their time!

Deep-sea Divers sometimes use Helium in place of nitrogen, but this has the side-effect that the resonant frequency of the vocal tract is higher, leading to "Donald Duck" speech, requiring an electronic circuit to bring it back into the normal range of speech.

Neon is often used as a filler gas - it would make speech slightly higher than normal, but not as extreme as Helium. Argon (Atomic mass 39) is fairly close to Oxygen (Molecular mass 32) and Nitrogen (28); it would make your voice slightly deeper. There wouldn't be a ready supply of these inert gases on Mars; on Earth they are a byproduct of mass production processes, something that won't appear on Mars for some time!

Divers need a filler gas to compensate for the increased external pressure underwater. Mars explorers (Arienauts?) would not - in fact, minimizing extra gases would reduce the pressure differential between the inside & outside of a habitat, making it lighter (always a big consideration for space travel). Mars explorers can work on mostly low-pressure oxygen.

If you used a reduced air pressure without nitrogen, sound attenuation is higher, so the astronauts would need to talk more loudly to be heard at long distances. But long distances indoors are not likely to be a problem on Mars any time soon.

For typical gas mixtures, see: http://en.wikipedia.org/wiki/Breathing_gas

[Jon Overton]

Hi, it just happens I've spent the last 24hrs intensively reading and researching this very question, so I'm happy to be able to answer in for you!

(Edited to add: I had no idea that I couldn't post links on here, shocking for a science website! so I had to delete all the references! if you'd like the fully referenced version msg me )

The main constituents of Mars very thin air are:
CO2 at 96%, Nitrogen at 2.7% and Argon at 1.6% plus some toxic trace gases such as CO, NO and O3

Pressurise this up to 1 atmosphere and then remove all but the Ar and N2 from Martian air (and ignoring the trace of O2) would result in a mixture of 62.8%  N2 and 37.2% Ar.
This can be used as 79% of the breathing mix, for: 21% O2, 29.4% Ar and 49.6% N2. (Earth air is 21% O2, 78% N2 and 1% Ar, so this provides the same partial pressure of 02 as on Earth.)

How to do this?

First, get rid of the toxic trace gases:
Pass over a catalytic converter Pt or Pt-Pd O(may or may not need heating to 250'C) once to get rid of trace NO and O3 and a second time with added O2 which will oxidise the CO to CO2
Or
Use Hopcalite Catalysts as in scuba and firefighter breathing systems.

[N.B.
Toxic CO is actually useful as a rocket fuel, and can be prepped as by product of producing O2 via e.g. zirconia (solid state) electrolysis ]

Secondly, use cryogenesis to freeze out the bulk CO2 as dry ice, leaving you with a roughly 60/40 N2/Ar gas mix that you can mix with your O2 (derived e.g. from electrolysis of water mined from Martian ice)

This is not just theory - practical work has been done with crickets, mice and humans:

Crickets and later mice breathing a Nitrogen 40% / Argon 40% / Oxygen 20% mix at about 0.85 atmospheric pressure (ambient for the elevation of the experiment) for 16days showed no ill effects, see (work by Boston et al for NASA in early 2000s)

The (Russian) Mars500 isolation experiment actually had a crew of humans successfully breathing a Nitrogen / Argon / Oxygen mix for a number of days if not all 520 days of the mission (it's unclear from what I've been able to find exactly how long and exactly what NitroArgOx mix they used, but it is clear that) - the crew breathed this mix at normal atmos pressure.
There is footage of earlier crews apparently breathing mixes containing high % Argon and reduced O2.


So NitroArgOx looks ok at normal atmos pressure, which is what the ISS is kept at and what Mars habitats will likely be kept at too. So that's great.
However, it's been shown to be likely unsuitable for both a) low pressure EVA work in space (100% oxygen is better) and b) high pressure deep diving underwater on Earth

See:
An argox breathing mix was shown to be unsuitable for deoxygenation prior to low pressure (3.5psi) EVA, owing to increased Decompression Sickness versus both 100% oxygen and  in human volunteers

And:

Mice showed significant narcotic impairment when breathing high pressure ArgOx mix:


Cheers
Dr Jon Overton
Creator of Ultra Frontier Explorer YouTube series on AlwaysAround dot Net