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I am creating an atmosphere for a planet in a science fiction book I am writing and want to make sure that this atmosphere is not too outlandish in regards to sustaining life. I know the oxygen partial pressure is very low in this atmosphere, so normal humans will require a gas mask when visiting this planet, but could human-like aliens that are intelligent species arise on this planet? The atmosphere is as follows:

Atmospheric Pressure at sea level: 0.81 atm Atmospheric composition: 54.34% nitrogen - 17.22% oxygen - 13.91% argon - 7.47% helium - 6.18% xenon - 0.88% other gases (carbon dioxide, water vapor, etc.)

Extra info: Size is similar to earth although has slightly less land mass, one moon orbits planet as well

Goal: I want to make it so that normal humans could survive without a gas mask for a decent amount of time but have a high risk of hypoxia

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    $\begingroup$ Please ask the radiation query separately! You get one question per question in any SE forum! $\endgroup$
    – elemtilas
    Commented Sep 26, 2022 at 1:20
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    $\begingroup$ The noble gas percentages look much less realistic to me than the breathability of the mix. Argon percentage looks way too high and helium and xenon just plain impossible, for helium due to inability of a smaller than earth planet to retain helium for a long time, for xenon - just it being way too rare in the universe $\endgroup$
    – vvotan
    Commented Sep 26, 2022 at 21:14
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    $\begingroup$ Although the amount of oxygen in your atmosphere may not be a problem for humans, the xenon might be. Xenon is sometimes used as an anesthetic. I don't know exactly what effect your 6.18% would have on a human- it's not nearly enough to knock a human out outright; but it might still impair cognition or reflexes to some degree (so humans on your planet would still want to wear gas masks if they're going outside for any reasonable length of time). The native aliens, of course, evolved with that amount of xenon around and would not be impaired by it. $\endgroup$
    – Someone Else 37
    Commented Sep 27, 2022 at 4:03
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    $\begingroup$ @Fogelicious don't forget in the end that Hemocyanin is far more efficient than Hemoglobin. To the point of not even needing lungs or even erythroid cells. $\endgroup$
    – user2284570
    Commented Sep 27, 2022 at 13:43

5 Answers 5

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Yes. Even humans could live there with some acclimation. It might produce some pretty horrible "filtering" of people who can't acclimate.

People have lived for two years at a pressure of 475 millibars of pressure. This is less than half of normal sea level pressure which is just over 1000 millibars. Normal atmosphere is 20.946% Oxygen. So at this altitude the relative percentage is 9.95%.

Your relative percentage is 0.81 * 17.22% = 13.95%..

The partial pressure of oxygen you are suggesting is considerably above what people have lived at for two years. So, yes, humanoids could live there.

As I said, it might produce some horrible filtering. Not everybody can acclimate. Altitude sickness can be quite a serious problem, particularly among those who are genetically pre-disposed not to be able to acclimate.

The Argon, Helium, and Xenon would be interesting. I can't figure out just off-hand what it would do to people's voice. And also, the Helium might tend to stratify in the upper atmosphere, but I'm not sure about that.

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    $\begingroup$ Helium will keep escaping unless either the gravity is rather strong, the temperature is rather low or the planet has some serious internal source of helium. Neither of these things is good for humans. $\endgroup$
    – fraxinus
    Commented Sep 26, 2022 at 11:23
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    $\begingroup$ Aside from genetic predisposition, some types of injuries can cause difficulty acclimating as well. Pretty much anything that produces scar tissue in the lungs reduces pulmonary efficiency, and that in turn makes acclimation more difficult (among other side effects of reduced pulmonary efficiency). $\endgroup$
    – Austin Hemmelgarn
    Commented Sep 26, 2022 at 12:41
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    $\begingroup$ "And also, the Helium might tend to stratify in the upper atmosphere" – My understanding is that once gases have mixed, they don't stratify. At equilibrium, the distribution of each gas is the same as what it would be if that gas were the only gas present. So if you look at the density (not the proportion) of helium as it varies by altitude, you'll find the highest density of helium at the bottom, decreasing approximately exponentially as you go up. $\endgroup$
    – Tanner Swett
    Commented Sep 26, 2022 at 17:31
  • $\begingroup$ @TannerSwett Now that you mention it, I think you are correct. Any stratification would be very tiny at most, and only due to the lower mass atoms having higher speeds at the same temperature. $\endgroup$
    – BillOnne
    Commented Sep 26, 2022 at 20:09
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    $\begingroup$ @TannerSwett unfortunately, that answer is a bit misleading - given enough density difference and no circulation, there should be stratification. Water mixes like gases (the same thermodynamics is a work), yet it stratifies in the oceans, so the bigger differences do indeed allow that effect - so I'm wondering at what point density differences will outweigh diffusion. (Obviously, turbulence will immediately mix anything that flows, so I'm restricting myself to diffusion as mixing process.) $\endgroup$
    – toolforger
    Commented Sep 28, 2022 at 15:24
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Welcome to La Paz

La Paz¹ is the capital city² of Bolivia. It has some 800,000 inhabitants, who are regular normal humans. The city is situated at an elevation of 3,640 meters (11,942 American feet) above sea level, which means that the partial pressure of oxygen is about 0.13 atmospheres.

In the conditions described in the question, the partial pressure of oxygen would be 0.81 × 0.1722 = 0.14 atm, which is higher than in La Paz. The inhabitants of the capital city of Bolivia would feel right at home.

Since here on Earth we have a decently large city featuring a partial pressure of oxygen lower than that required by the question, one can immediately conclude that (most) ordinary humans would be able to adapt quite quickly.

¹) Nuestra Señora de La Paz, Our Lady of Peace.
²) La Paz is the executive and legislative capital of Bolivia. The judicial and constitutional capital is Sucre.

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  • $\begingroup$ According to WeatherMX, the current atmospheric pressure at La Paz is 1037 mb or 1.02 atmospheres. You sure you got that number correct? It's lower than the death zone $\endgroup$
    – JBH
    Commented Sep 26, 2022 at 15:54
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    $\begingroup$ @JBH: The death zone is defined as beginning at 8000 meters ASL, so that air pressure is less than 0.356 atmospheres, corresponding to 0.075 atmospheres partial pressure of oxygen. La Paz has an average air pressure of 0.64 atmospheres. I have no idea what WeatherMX has been smoking, but 1 atmosphere at 12,000 feet above sea level is plain impossible. (It would be the mother of all anticyclones! Maybe they mean the village in Indiana, USA?) (Fun fact: La Paz, Bolivia, has only one fire station, because it is really really hard to start a fire.) $\endgroup$
    – AlexP
    Commented Sep 26, 2022 at 16:45
  • $\begingroup$ 12,000 feet is only 3,657 meters, which is still way below the death zone. According to this air pressure calculator, the air pressure at 12,000 ft is 0.64 atm. Granted, that's quite a bit lower than WeatherMX is reporting. but it's the closest I've found to an official report. But I do apologize for putting a tempest in a tea cup. I misread your first paragraph to read 0.13 atm total pressure at La Paz, not the partial pressure of oxy as you mention (0.64 atm * 21% oxy = 0.13 atm partial pressure). Sorry for the ruckus. $\endgroup$
    – JBH
    Commented Sep 27, 2022 at 4:06
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    $\begingroup$ @JBH Weather reports give the pressure extrapolated to sea level, not the pressure at the actual surface. $\endgroup$
    – John Doty
    Commented Sep 27, 2022 at 13:11
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    $\begingroup$ @JBH and AlexP, I guess WeatherWX is extrapolating the pressure to sea level. That's not an unreasonable thing to do, but it is unreasonable to do it without saying so. Put in Kathmandu or Denver and you get >1atm as well (1027 mbar for Denver which at 1600m altitude should be more like 850mbar). barometricpressure.app/denver allows you to switch between "sea level pressure" and "station pressure", and makes sense, but is in inches of mercury $\endgroup$
    – Chris H
    Commented Sep 27, 2022 at 13:14
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You're way inside humanity's ability to breathe

The "Death Zone" when climbing mountains (about 8km) is 0.35 atm/356 millibars and Earth's oxygen percentage is 21%. Simplifying a bit, that means that at the death zone there's only 1/3 of the oxygen we'd be conditioned to at sea level.

Ugly fractional math aside and simplifying. That's more-or-less equivalent to 7% oxygen at sea level.

You're proposing 0.81 atmospheres and 17% oxygen. Same simplification: 13.78% oxygen vs. 7% at the death zone.

Humans from outside the world would need a bit of acclimation, but not much. At worst it'd be like living your life smoking cigarettes.

In other words, there's really no threat here at all from an oxygen standpoint. You're a long way away from the threat of hypoxia.

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A very simple way to accomplish this: The inhabitants have a larger lung area and thus can get more oxygen from the same biology. We reach our altitude limit at the point where we can't take in enough oxygen, our blood will still transport it.

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    $\begingroup$ I believe that when the partial pressure (pO2) gets too low, the equilibrium in the blood shifts. While the lungs will still happily equalize the pO2 in the air with the dissolved oxygen in the blood, below (very) roughly 8 kPa of pO2, the ability of hemoglobin to then bind that oxygen to make room for more starts to drop off pretty quickly. If you need humans to live at 5 kPa of pO2 or lower, you'll need to modify their hemoglobin so it will do its job at lower partial pressures. $\endgroup$
    – AI0867
    Commented Sep 27, 2022 at 13:25
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Yes, but also Maybe Not.

Yes: 13% oxygen is low for humans, but not crazy. Arbitrary humanoids could do just fine. There is even a good chance that any given regular human wouldn't die.

Unfortunately, hypoxia doesn't work like you want. Any given person is likely to either die quickly or be unhappy-but-fine indefinitely.

Maybe Not: Some of your other stuff is pretty nasty. Argon is already an asphyxiant if it gets the opportunity to settle. Xenon is way, way worse. Stagnant air is going to be a terrifying death zone anywhere on your world, and there's a good likelihood that your atmosphere will stratify into a xenon layer and an everything-else layer.

Aside from that, look hard at your CO2 level. That's going to cause very mild hypocapnia, which is not exactly bad, but will make exercise weird.

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  • $\begingroup$ "Argon is already an asphyxiant if it gets the opportunity to settle. Xenon is way, way worse"; Nitrogen is also an asphyxiant, and people don't generally asphyxiate in stagnant air in our world. BTW: no, Xenon wouldn't stratify; Xenon atoms won't do anything to prevent Oxygen molecules from being there as well. $\endgroup$
    – poncho
    Commented Sep 27, 2022 at 22:12

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