The air we need to breathe - Anybody a human physiology specialist?

Kenshin · 2004-02-11 20:16:05

I've been thinking recently about a colony on Mars and oxygen.

Obviously, since the atmosphere is mostly CO2, there isn't a huge problem getting the O2 out. But our atmosphere here on Earth isn't just O2, there's quite a bit of N2 as well.

Of course pure O2 is just fine--we use it in medicine and such--but there is one big problem that we've learned the hard way with Apollo 1: one spark and you're up in flames.

Thus, it doesn't seem to make sense to have long-term residence in a O2-rich environment.

So, on to my question:
Would it be livable long term (and not particularly hard on the human body) to just use (roughly) a 50/50 O2/CO2 mixture on a Mars colony? If not, what are our other options? Bringing large amounts of N2 and 'diluting' the air like that (that we're used to here on Earth)?

RobertDyck · 2004-02-12 01:01:26

CO2 is toxic. On Earth we have 0.03% to 0.04% CO2, but short term exposure to anything below 2% is not harmful. All this is at 1 atmosphere pressure. Concentrations above 3.3% cause increasingly severe symptoms. Several deaths have been attributed to exposure above 20%. You can get more details [http://www.ccohs.ca/oshanswers/chemical … th_cd.html]here.

To breathe without high altitude training, humans require 3.0 psi partial pressure of oxygen. That means you can reduce the other gasses, but the total amount of oxygen must remain the same. High exertion exercise in progressively reduced oxygen for progressively increased time will stimulate the human body to grow more red blood cells. That will enable you to breathe in a lower oxygen environment, but you must maintain it to maintain the concentration of blood cells; and the limit is about 50% of the normal oxygen content. At extremely low pressure, the effect of high-altitude training is less pronounced. You can breathe 3.0 psi pure oxygen without any physical training or 2.5 psi pure oxygen with the training, or even 2.0 psi for 30 minutes if you're in excellent physical condition and prepared with high-altitude training.

There are is another problem with extreme low pressure: the lungs dry out. So 3.0 psi pure oxygen is fine for a spacesuit, for the duration of a space walk, but you don't want to use that for your habitat. Apollo and Skylab used 5.0 psi, with 60% oxygen and 40% nitrogen. That provides you with 3.0 psi partial pressure oxygen and 2.0 psi partial pressure nitrogen. You can replace the nitrogen with any inert gas, such as helium, neon, or argon. Krypton and xenon are also inert, but they're heavy; you don't want to use something heavy on a spacecraft when something light will do. Other gasses are safe to breathe, such as methane, but will burn when mixed with oxygen.

clark · 2004-02-12 07:25:34

I wonder what living, or growing up in such low pressure with these exotic gases would do to children, or people.

What's the highest, unpressuirzed altitude conditions that humans can live in (like if you were living on a plane)?

How difficult would it be to create similar conditions in space, the moon, or mars?

Kenshin · 2004-02-12 10:10:46

Ok, so according to the sources I've seen, Mars has 95% carbon dioxide, 3% nitrogen, and 2% Argon (all approx)

How difficult would it be then to filter the nitrogen and argon into our theoretical colony, if we're going for a 50/50 or 60/40 oxygen/other environment (at the appropriate PSI). Would it be easier to do higher oxygen content, like 70/30, or would that start bringing flamibility levels too high?

(I really need to get more up to speed on some of this chemistry)

RobertDyck · 2004-02-12 12:06:08

Yup, we could make air from Mars atmosphere. To be more precise, Mars has 95.32% CO2, 2.7% N2 (nitrogen), and 1.6% Ar (argon), 0.13% O2 (oxygen), 0.07% CO (carbon monoxide), and 0.03% H2O (water). Atmospheric pressure on Earth at sea level is 14.6925 psi on average (barometric pressure varies with weather). Pure oxygen at high pressure is dangerously flammable, but if the pressure is the same as the partial pressure Earth it isn't much dangerous. An inert gas would act as a buffer, so it would be a bit safer as well as avoiding drying of the lungs. One simple technique to make air is to cool it to -78.5?C (just a little cooler than Mars at night) to freeze CO2 into dry ice snow. If you remove 95% of the CO2, the remaining gasses would be:
2.80% CO2
56% N2
33.2% Ar
2.7% O2
1.45% CO
0.62% H2O
At this level carbon monoxide would be toxic, but a catalytic converter similar to the one on a car will convert CO and O2 into CO2 (one O2 molecule for every two CO molecules). Just add oxygen to that and you have a nice air mix.

::Edit:: The idea of extracting CO2 from Mars atmosphere by freezing it was Robert Zubrin's.

Bill White · 2004-02-12 12:38:09

Yup, we could make air from Mars atmosphere. To be more precise, Mars has 95.32% CO2, 2.7% N2 (nitrogen), and 1.6% Ar (argon), 0.13% O2 (oxygen), 0.07% CO (carbon monoxide), and 0.03% H2O (water). Atmospheric pressure on Earth at sea level is 14.6925 psi on average (barometric pressure varies with weather). Pure oxygen at high pressure is dangerously flammable, but if the pressure is the same as the partial pressure Earth it isn't much dangerous. An inert gas would act as a buffer, so it would be a bit safer as well as avoiding drying of the lungs. One simple technique to make air is to cool it to -78.5?C (just a little cooler than Mars at night) to freeze CO2 into dry ice snow. If you remove 95% of the CO2, the remaining gasses would be:
2.80% CO2
56% N2
33.2% Ar
2.7% O2
1.45% CO
0.62% H2O
At this level carbon monoxide would be toxic, but a catalytic converter similar to the one on a car will convert CO and O2 into CO2 (one O2 molecule for every two CO molecules). Just add oxygen to that and you have a nice air mix.

Wow! This is far easier than I had thought.

Take in Mars external atmosphere between 1 and 3 am local time and cool slightly. Vent the gas into another chamber leaving dry ice behind.

As you say, almost breathable air. Run this same process a few more times and you also have fairly pure CO2 useful for great many purposes, including my favorite, supercritical CO2 to be used for mining the regolith for water and magnesium.

Waaaay cool!

And they say living on the Moon will be easier. :;):

= = =

CO2 + 2 H20 can become CH4 + 2 O2 right?

Find water ice and there is all the oxygen you need to mix with the above atmosphere.

RobertDyck · 2004-02-12 18:27:54

CO2 + 2 H20 can become CH4 + 2 O2 right?
Find water ice and there is all the oxygen you need to mix with the above atmosphere.

There are a few steps involved, but yes. This is the basis of Robert Zubrin's In-Situ Propellant Production. The details are:
Reverse Water Gas Shift: H2 + CO2 -> CO + H2O
Sabatier Reactor: 3 H2 + CO -> CH4 + H2O
combination of RWGS and Sabatier: 4 H2 + CO2 -> CH4 + 2 H2O
Electrolysis: 2 H2O -> 2 H2 + O2

Kenshin · 2004-02-13 13:27:15

Out of curiosity, is there any other way to get O2 from CO2 (without the use of the H2O)?

Obviously you'd need a colony in an area with water, but still, it seems it might be useful to be able to pull the O2 out of CO2 without using any water.

RobertDyck · 2004-02-13 13:52:40

is there any other way to get O2 from CO2 (without the use of the H2O)?

You can heat CO2 and use a zirconia membrane to convert it to O2 and CO. To quote from a 1992 paper written by Robert Zubrin:

This reaction, which can also be used to produce CO fuel for CO/O2 engine use, can be accomplished by heating CO2 to about 1100 C, which will cause the gas to partially dissociate, after which the free oxygen so produced can be electrochemically pumped across a zirconia ceramic membrane by applying a voltage. The use of this reaction to produce oxygen on Mars was first proposed by Dr. Robert Ash at JPL in the 1970s, and since then has been the subject of ongoing research by both Ash (now at Old Dominion University), Kumar Ramohalli and K. R. Sridhar (at the Univ. of Arizona), and Jerry Suitor (at JPL)16. The advantage of this process is that it is completely decoupled from any other chemical process, and an infinite amount of oxygen can be so produced without any additional feedstock. The disadvantages are that the zirconia tubes are brittle, and have small rates of output so that very large numbers would be required for manned flight vehicle application. (The numbers would not be excessive for rover application only.) Improved yields have recently been reported at the Univ. of Arizona, so the process may be regarded as promising, but still experimental.

dickbill · 2004-02-13 16:42:14

from Robert:

2.80% CO2
56% N2
33.2% Ar
2.7% O2
1.45% CO
0.62% H2O

The CO2 is still too high at 2.8%. On earth at 1tm, we have 0.035%, at lower pressure we can probably tolerate higher, though I don't know exactly how much. CO is definitively a poison which has to be removed.
I checked in PUBMED, apparently breathing O2/Ar argon mixture, called ARGOX is not toxic, at least in the short term. That might be an experience for the future, I mean the present sorry : putting rats or other animals in the low pressure atmospheres, prefrably one that would match a compressed martian air-based atmosphere. The goal of such experience is to see what would happen in the long term. Often, long duration experiments bring surprise.
The Mars Society sponsors the Mars Gravity Experiment, I guess they could sponsor such series of small experiments, if they don't already think about it.

RobertDyck · 2004-02-14 09:03:36

The idea is to freeze Mars atmosphere to remove 95% of the CO2, then take the remainder and add some oxygen and heat it and pass it over a catalyst to convert CO and O2 into CO2. Add more oxygen and the result will be:
4.09% CO2
45.07% N2
26.7% Ar
21.1% O2
0.50% H2O
Pressurize this to 1 atmosphere and you have air perfect for a greenhouse. The plants or air recycling system can convert CO2 into O2. If you want to use this for a habitat, the oxygen is a little too high and you will definately have to convert CO2 into O2; or you can increase oxygen to 60% and reduce pressure to 5 psi. In a greenhouse the nitrogen can feed bacteria to create nitrates/nitrites/amonia also known as fixed nitrogen. Do you want me to detail the concentration of trace gasses: neon, krypton, xenon, and ozone? We have those same trace gasses on Earth. By the way, Earth's atmosphere is 0.934% argon.

Rxke · 2004-02-14 11:04:54

I thought i read about a breakthrough in the zirconium membrane technology recently, (not on the 'net)
Basically they found a way to produce the membranes in a more efficient way (dunno if it was in Arizona, though, but it does ring a bell, so could very well be the case)

Ian Flint · 2004-02-17 11:22:37

I like the idea of freezing the CO2, but a little red flag just went up.

At -78.5 degrees C I know for sure that water is frozen. So the water would be frozen out along with the CO2. It's pretty insignificant, I know.

Are there any other gasses that would freeze at -78.5 degrees?

I suppose that we could "freeze out" each gas in sequence. Or, we could liquify them. Liquids are easier to handle than solids.

How much pressure does it take to liquify CO2.

Ian · 2004-02-17 12:49:39

Why can't you just seperate the oxygen atoms from the hydrogen atoms in a H2O molecule? I think that this can be done using electricity. It's probably how submarines get their oxygen for extended periods of time underwater. What do you think of this idea?

Bill White · 2004-02-17 14:03:07

One membrane [http://science.nasa.gov/headlines/y2003 … list993593]link. This process may allow theseparation of CO2 from Mars air through an easier and lower energy process compared with freezing out the CO2.

Bill White · 2004-02-17 14:12:22

CO2 + 2 H20 can become CH4 + 2 O2 right?
Find water ice and there is all the oxygen you need to mix with the above atmosphere.
There are a few steps involved, but yes. This is the basis of Robert Zubrin's In-Situ Propellant Production. The details are:
Reverse Water Gas Shift: H2 + CO2 -> CO + H2O
Sabatier Reactor: 3 H2 + CO -> CH4 + H2O
combination of RWGS and Sabatier: 4 H2 + CO2 -> CH4 + 2 H2O
Electrolysis: 2 H2O -> 2 H2 + O2

If we start with CO2 and H2O collected on Mars and skip the step where H2 is brought from Earth (original Mars Direct) then the cracking of H2O into 2 H2 and O2 appears to be the most energy intensive step of the process.

Solar or nuclear powered electrolysis is the brute force way to create H2 and O2 however photo-catalysis is another way. Google reveals significant research into both bio-photo-catalysis (bacteria that crack water into H2 and O2 in the presence of sunlight) and inorganic photocatalysis, chemicals that crack water into H2 and O2 in the presence of sunlight.

Collect water from a Mars well. Fill a glass jar with water and add one or other form of photo-catalysis. Set in the sunlight, perhaps with mylar concentrators to focus the meager Marsian insolation. Siphon off the resulting O2 and H2.

Easy as pie.

Ian · 2004-02-17 14:42:18

Yes. But you have to make sure that the H2 doesn't explode when an electric charge is applied to it.

RobertDyck · 2004-02-17 15:05:15

Are there any other gasses that would freeze at -78.5 degrees?
I suppose that we could "freeze out" each gas in sequence. Or, we could liquify them. Liquids are easier to handle than solids.
How much pressure does it take to liquify CO2.

According to [http://www.airliquide.ca]Air Liquide
CO2 - freeze -78.5?C, tripple point (can be liquid) -56.6?C @ 517.3kPa (5173 mbar, or 75.1 psi)
CO - liquid -191.5?C @ 1 atmosphere, tripple point -205.1?C @ 15.2kPa (152 mbar, or 2.2 psi)
N2 - liquid -195.8? @ 1 atm, tripple point -210?C @ 12.5kPa (125 mbar, or 1.81 psi)
Ar - liquid -185.9?C @ 1 atm, tripple point -199.3?C @ 68.9kPa (689 mbar, or 9.99 psi)
O2 - liquid -182.96?C @ 1 atm, tripple point -218.8?C @ 0.148 kPa (1.48 mbar, or 0.02147 psi)

Ian · 2004-02-18 12:45:16

"Collect water from a Mars well. Fill a glass jar with water and add one or other form of photo-catalysis. Set in the sunlight, perhaps with mylar concentrators to focus the meager Marsian insolation. Siphon off the resulting O2 and H2."

What will happen to the Hydrogen that will come out of it? It will be left over from the seperation of Hydrogen and Oxygen. I think that on the way to Mars, it could be used as rocket fuel. People would have to mine less on the moon or mars when all you have to do is seperate the hydrogen from the oxygen.

RobertDyck · 2004-02-18 13:18:44

Yes. But you have to make sure that the H2 doesn't explode when an electric charge is applied to it

With electrolysis, you run electricity through water so there is no spark. Furthermore, hydrogen is formed on one electrode and oxygen on the other, so you only need a cone over each electrode to collect the gasses separately. Hydrogen will not burn or explode if there is no oxygen mixed with it. With photo-catalysis I don't know if oxygen is kept separate from hydrogen.

markapp · 2005-04-15 12:15:48

CO2 is toxic. On Earth we have 0.03% to 0.04% CO2, but short term exposure to anything below 2% is not harmful. All this is at 1 atmosphere pressure. Concentrations above 3.3% cause increasingly severe symptoms

Recheck those numbers you are off by x100 earths atmosphere is .03 to .04 or 3-4 percent co2
CO2 may be toxic in high enough concentrations but i seem to remember it is also essential to human life and is used througout the body as well as produced in surplus and excreted by breathing. it is essential to plant life as well.

The CO2 is still too high at 2.8%.

an atmosphere of 2.8 percent should be perfectly breathable but may me just a little on the weak side for plants since they prefer as low as 3% or as high as around 8%. One of the mysteries of science is why do plants like up to 3x current co2 levels? One theory is at one time earth may have had as much as 3x current levels. some cities get as much as 4-5% so i doubt people will suffer much from a little higher than normal levals but i am sure research does exist.
I believe argon is used by deep sea divers to compensate for high pressures as a signifigant portion of the mix. Researching deep sea scuba equipment schemes should shed additional light on exactly what atmospheres and co2 levels are usable to humans although it may not give info on corosiveness or explosiveness of said atmospheres. Also any longer term colonization efforts will likely require some sybiotic life forms such as plants to recyle air as well as provide sustenance Actually suficient plant life could concievably reform the co2 to oxygen provided controlled amounts were applied.

RobS · 2005-04-16 00:49:02

The Earth's atmosphere is three hundredths of one percent carbon dioxide; in other words, 3 parts in 10,000 (Stephen Dole, Habitable Planets for Man, 2d edition, page 17). People cannot safely breathe 2% CO2 for very long; Dole puts the maximum upper limit at 1%.

-- RobS

RobertDyck · 2005-04-19 19:22:44

Since this thread was recently referenced, I thought I would update it. Here is my idea for a Mars atmosphere machine.

Take Mars air at night and pressurize it to 10 bar. Earth pressure as sea level is 1.01325 bar, and Mars is roughly 7 millibar. Mars Pathfinder recorded between 6.77 and 7.08 millibar. Then freeze it to 170°K (-103.15°C). Use a HEPA filter on the intake to remove dust and fines. Although the catalytic converter of a car will convert CO with O2 into CO2, it requires the heat of car exhaust. NASA created a catalyst that works at room temperature, intended for a breathing mask when evacuating a burning building. Any such catalyst will also decompose ozone (O3) into molecular oxygen (O2). Heat the catalyst at the top of the canister while chilling freezer coils at the bottom. This will decompose O3 and CO as CO2 is frozen to dry ice frost. As pressure drops from freezing CO2, the pump will activate to maintain pressure at 10 bar. Once the pressure stops dropping, close the intake valve and turn off the pump, but keep the freezer and catalyst warmer going. Once a sensor is no longer able to detect CO, turn off the catalyst warmer but keep the freezer going. Chilling will drop pressure further. Once pressure stabilizes, open a separate valve to release remaining gas into a holding tank. Use a scavenger pump to quickly pump out remaining gas. Do it quickly to prevent sublimating dry ice. Once a reasonable vacuum is achieved, close the second valve and turn off the scavenger pump. Them open a third valve to the CO2 holding tank and reverse the freezer to warm the dry ice. As dry ice sublimates it will create pressure. No need for a CO2 pump, just let gas pressure push CO2 into its holding tank.

The result in the first holding tank is something I call "diluent gas". It will consist of: (by volume)
CO2 - 0.74935%
N2 - 61.0%
Ar - 36.1%
O2 - 2.1%
Ne - 0.0056%
Kr - 0.00068%
Xe - 0.00018%

That "diluent gas" can be mixed with oxygen (to dilute oxygen). If you add oxygen to bring the total to 2.7psi partial pressure of oxygen, and total pressure to 8.43333psi the result will be:
CO2 - 0.52%
N2 - 42.4%
Ar - 25.1%
O2 - 32.0%
Ne - 0.0039%
Kr - 0.00047%
Xe - 0.00013%

This is still rather high in CO2, but anything less than 2% will not be noticeable by humans. The CO2 sorbent system of the habitat can reduce CO2 content in this "new" air. A freezer can not practically reduce CO2 below this level so only a sorbent could. Rather than duplicating a system, just let the normal habitat sorbent system do it.

The reason for choosing habitat pressure of 8.43333psi is to maximize pressure while ensuring zero pre-breathe time to go outside in a suit. This assumes the suit is 3.0psi pure oxygen. The ratio of habitat N2 partial pressure to suit total pressure for zero pre-breathe time is 1.2:1 according to NASA. Lower pressure permits greater suit flexibility, and is particularly important for MCP suits. Suit pressure could drop by 10% by a leak and it would still equal O2 partial pressure that astronauts would be used to in the habatit. I kept the ratio of N2:Ar the same as Mars atmosphere because it would take energy to separate them.

SpaceNut · 2020-09-12 09:17:47

Bump another large post on co2....

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