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I've been trying to create a Earth-like planet but I'm not sure if my planet's atmosphere is breathable. (It's mostly the amount of helium I'm worried about).

I've tried to make it Earth-like while making it alien with respect to Earth's atmosphere. Here are the atmosphere's numbers: atmospheric pressure: 1.22 atmospheres; atmosphere composition: 66% nitrogen, 19.42% oxygen, 14.21% helium, 1.25% xenon, and 0.12% other gases.

Q: Would this type of atmosphere be suitable for human life?

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  • $\begingroup$ The 14% helium won't hang around for long, unless there's a large and continuous planetary source. Helium is so light it quickly drifts to the upper atmosphere and is lost to space. Quoting the Wikipedia article "Atmospheric escape": hydrogen and helium [have] largely escaped from Earth's atmosphere. $\endgroup$ – Bohemian Jun 21 '18 at 16:01
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Given that you've increased the atmospheric pressure, the slight drop in O2 won't be a problem for breathing but your problem in this mix isn't the helium; it's the xenon.

Helium gets a bad rap because some people do stupid things with it; for fun. Yes, breathing helium in large quantities makes your vocal chords vibrate faster because it's less dense than normal atmosphere, so you sound like a chipmunk. The problem is that you need O2 in the air to survive. Breathing in massive amounts of pure helium suffocates you because you don't get any O2 by way of breathing it in. That said, it's not unsafe to breathe as part of the atmosphere and deep sea divers actually do exactly that; Heliox is a gas mixture of both O2 and He2 (instead of O2 and N2) because at pressure, the amount of nitrogen you breathe can make it toxic. Diving to even a relatively shallow depth of 30m using normal air brings on something called Nitrogen Narcosis and it affects your judgement et al like alcohol does.

Xenon, on the other hand, is also safe to breathe in its pure form but it has a singular disadvantage; it can be used to form many different compounds with O2 and many of those compounds are toxic to breathe and very exothermic, read as explosive.

In response to comments, it should be noted that these compounds are VERY hard to form and involve a significant amount of fluorine, more than would be allowed within trace gases. The paragraphs that refer to these reactions have been deleted, however the remainder of the answer is still valid.

The Xenon does however have a narcotic effect in its pure form, just like Nitrogen does in larger quantities than normal atmospheric pressure afford. Xenon is about 25x times more narcotic than nitrogen, so in the quantities listed above your colonists may not be able to function very well, if at all. It would be like living in a mist of anesthetic, and in point of fact xenon isn't used for anesthetic purposes in medicine now only because of its high cost.

Add to that you're dealing with gasses with very different densities. Xenon is heavier than normal air, helium is lighter. If the gasses start separating as a result (very likely) you could actually have a suffocating layer at different altitudes, and mine shafts in particular could be very dangerous as over time they'll fill with much more xenon than normal gasses.

Also, to have that much helium in the atmosphere in a stable form, you've probably got a LOT of exposed decaying uranium. So, while you have helium in abundance, you also have radiation.

All things considered, I'm not convinced this atmospheric model is stable, or safe. The helium isn't the real problem, but the way it's created might be. The xenon has the opposite problem insofar as it's not dangerous in a pure form, but what it becomes might be.

My recommendation? Have your colonists wear rad-safe spacesuits until they've done a more complete study of the atmosphere. Better safe than sorry.

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    $\begingroup$ Xenon is a noble gas, I think to make it react you need some weird condition (weird = not something you achieve in plain atmosphere), or not? $\endgroup$ – L.Dutch Jun 18 '18 at 5:21
  • $\begingroup$ No, you're right. It's a noble gas, but that 'weird condition' can often be something as simple as the application of heat. I'm not sure what goes on with xenon to make it compound with oxygen, but I know it does happen and the result can be both toxic and explosive. Leave it with me @L.Dutch and I'll check it out and update my answer. $\endgroup$ – Tim B II Jun 18 '18 at 5:30
  • $\begingroup$ A quick check on Wikipedia informed me that Xe oxide forms starting from XeF6... but also that Xe is used as narcotic... $\endgroup$ – L.Dutch Jun 18 '18 at 6:05
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    $\begingroup$ Xenon is very non-reactive, it's orders of magnitude less than Nitrogen. Its products, on the other hand, are very reactive, and as a consequence can not survive for long in the atmosphere. $\endgroup$ – Alexander Jun 18 '18 at 6:08
  • $\begingroup$ @Alexander so does that mean that even with fluorine in the atmosphere (and sunlight), the reaction to create xenon - fluorine compounds won't happen at all or just that it won't happen in amounts that will deplete the xenon gas? My understanding was that it would react with very little energy supplied with fluorine and the reason why it doesn't do that so much on Earth is because there's not a lot of fluorine for it to react with. Is this wrong? $\endgroup$ – Tim B II Jun 18 '18 at 6:14
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That seems perfectly breathable to my knowledge: helium, xenon, and other noble gasses don't tend to interact with anything (hence "noble"), and so the only problems they cause have to do with them displacing other things. For instance, a person can breathe 100% helium without it causing any health problems—the issue is the 0% oxygen.

However, your planet will need stronger gravity than Earth in order to hold helium in the atmosphere. Earth's gravity isn't strong enough to keep hydrogen and helium around, so they'll dissipate off into space over time. (We get helium nowadays from nuclear fission happening deep underground, and hydrogen from breaking apart water, rather than extracting them from the atmosphere.)

You'll also want CO₂ for the plants to use, but that would fall under "other" (and you don't need very much). And you probably want some water vapor too, but that doesn't directly impact its breathability.

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  • $\begingroup$ We also need $CO_2$ for breathing (if the inhabitants are human). It is counterintuvitive, but just a concentration is required for proper diffusion AFAIK. $\endgroup$ – Kavi Vaidya Jun 18 '18 at 6:12
  • $\begingroup$ The human breathing reflex isn't triggered by a lack of oxygen, but by a buildup of carbon dioxide. If there isn't any in the air, humans forget to breathe. (This is why nitrogen and helium "preservation" atmospheres are often spiked with a bit of carbon dioxide: so anyone breathing them feels like they're suffocating and leaves, rather than just dropping dead from lack of oxygen.) $\endgroup$ – Mark Jun 21 '18 at 1:10
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TL; DR -> No

This atmosphere is lethal to humans.

It has too little oxygen partial pressure 11 kPa vs 21 kPa (Normal) vs 16 kPa (Minimal safe) vs 13.3 kPa (hypoxia).
That is because you have too much helium partial pressure (66 kPa). And that is because helium has too little molecular weight (4) compared to oxygen (32) which increase a lot the helium value.

You can solve this by: increasing oxygen percentage (a lot), reducing helium percentage (also a lot), increasing atmosphere pressure (a lot again) or changing helium to a heavier sustance (great!).

May I suggest replacing Helium wirh Argon?. That would raise oxygen from 11 kPa to 22 kPa making it literally perfect. The best part is that 13 kPa of argon won't produce negative effects and the 86 kPa of nitrogen will be close to nitrogen narcosis but won't produce it!.

Finally, Xenon isn't lethal in that amount, but remember that it's heavy. Without wind it can accidentally settle on a plain or valley and asphyxiate everyone.

Long version

First I will calculate your atmosphere partial pressures:

You said $1.22\text{ atm}$ but I don't like atmospheres measure, so I'll convert them to kilopascals: $$1.22\text{ atm} \times 101.325 \text{ kPa} = 122.3965 \text{ kPa}$$

Also, since I don't know what is that $0.12\%$ "other elements" I'll assume it's more nitrogen.

$$\begin{array}{|cr|r|cc|c|} \hline \text{Chemical} & \text{Percent} & \text{gr/mol} & \text{Mols} & \text{Fractal Mol} & \text{Partial Pressure (kPa)} \\ \hline \text{N}_2 & 66.12\% & 28.0134 & 2.3602 & 0.3616 & 44.2617\\ \text{O}_2 & 19.42\% & 31.9988 & 0.6068 & 0.0929 & 11.3809 \\ \text{He} & 14.24\% & 4.0026 & 3.5501 & 0.5439 & 66.5753 \\ \text{Xe} & 1.25\% & 131.293 & 0.0095 & 0.0014 & 0.1785 \\ \hline \text{Total} & 100\% & - & 6.5269 & 1 & 122.3965\text{ kPa} \\ \hline \end{array}$$

Now, lets start to analyze!:

  • Good nitrogen value! Nitrogen can produce nitrogen narcosis but only at high values. You'll be perfectly fine.
  • Warning! Lethal (low) oxygen value: Your oxygen partial pressure is $11.38 \text{ kPa}$ while our current partial pressure is $21 \text{ kPa}$.
    Humans can survive values below to normal but they won't be happy.
    However, the minimal safe partial pressure of O2 is $16 \text{ kPa}$, and values bellow to $13.3\text{ kPa}$ are lethal.
    So your people will die from hypoxia.
    In this answer I explain about hypoxia sympthoms, bassically they are: fatigue, numbness / tingling of extremities, nausea, anoxia, ataxia, confusion / disorientation / hallucinations / behavioral change, severe headaches / reduced level of consciousness, papilloedema, breathlessness, pallor, tachycardia, and pulmonary hypertension eventually leading to the late signs cyanosis, slow heart rate / cor pulmonale, and low blood pressure followed by death.
    In other words, a painful and slowly death.
  • A bit risky and funny helium:
    Your helium value aren't too high so I don't think it'll be a problem, but just in case here I wrote a bit of information:
    • Helium and voice: The higher resonant frequencies caused by the gas (see the link to know how) cause a change in timbre, resulting in a reedy, duck-like vocal quality.
    • Helium asphyxiant gas: Inhaling helium can be dangerous if done to excess, since helium is a simple asphyxiant and so displaces oxygen needed for normal respiration. Note that at a only $14.24\%\text{ He}$ I'm not sure if it may be lethal by itself (combined by your low oxygen value it will make your death just a bit quickler because it will reduce the oxygen of your lungs).
    • Helium narcopsia: I don't know much about it but I'm quite sure you won't suffer helium narcopsia because you have an important amount of helium, but no so high (helium is one of the noble gases with less "narcopsia power").
  • Interesting effects of Xenon:
    You don't have much Xenon so this effects are almost not noticeable. However remember that Xenon is the heaviest sustance of your atmosphere by a lot. If there isn't wind (e.g: maybe between mountains or in depressions of terrain) Xe may decant to the lower part of the atmosphere increasing dangeously it's percentage while the oxygen may rise to the upper part. That can be lethal (but really rare). Here I wrote which problem may raise in that situations.
    • Xenon and voice: Xenon vibrates more slowly in the vocal cords when exhaled and produces lowered voice tones, an effect opposite to the high-toned voice produced in helium. You don't have much xenon so it won't be noticeable, but if you raise it, the mixture of He and Xe effect would be quite interesting.
    • Xenon isn't toxic by itself: Xenon is non-toxic, although it does dissolve in blood and belongs to a select group of substances that penetrate the blood–brain barrier, causing mild to full surgical anesthesia when inhaled in high concentrations with oxygen.
    • Xenon anesthesia: Xenon has been used as a general anesthetic, I think (quote needed) that a 30% Xe is used like anesthetic.
      Dense gases such as xenon can be breathed safely when mixed with at least 20% oxygen, sadly you have much less oxygen (remember your partial pressure). Xenon at 80% concentration along with 20% oxygen rapidly produces the unconsciousness of general anesthesia.
    • Xenon asphyxiant gas: Like argon and other inert gasses it can be a gas asphyxiant because it has more weight than O2 and can fill the surface of a planet with 100% inert gas (O2 will fly upper).

Now, that you have read all my long explanation please read my TL;DR part, there is explaining of what is going on.

If you follow my suggestion, this will be your new atmosphere:

$$\begin{array}{|cr|r|cc|c|} \hline \text{Chemical} & \text{Percent} & \text{gr/mol} & \text{Mols} & \text{Fractal Mol} & \text{Partial Pressure (kPa)} \\ \hline \text{N}_2 & 66.12\% & 28.0134 & 2.3602 & 0.7082 & 86.6911\\ \text{O}_2 & 19.42\% & 31.9988 & 0.6068 & 0.1821 & 22.2906\\ \text{Ar} & 14.24\% & 39.948 & 0.3557 & 0.1067 & 13.064 \\ \text{Xe} & 1.25\% & 131.293 & 0.0095 & 0.0028 & 0.3496 \\ \hline \text{Total} & 100\% & - & 3.3324 & 1 & 122.3965\text{ kPa} \\ \hline \end{array}$$

  • Nitrogen: High but not so high! You won't suffer nitrogen narcosis.
  • Oxygen: One word: Perfect.
  • Argon: Is heavier than oxygen, so there is always the risk of asphyxia on places without air circulation, but that is very difficult to find on a planet.
  • Xenon: The same as before, still isn't a real danger.
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  • $\begingroup$ I think there's something wrong with your math. Our atmosphere is 20.95% oxygen and (obviously) 1 atm pressure, so .2095 atm partial pressure. The proposed atmosphere is 19.42% oxygen and 1.22 atm, so 19.42% * 1.22 = .2369 atm partial pressure. There's slightly more oxygen, not less. $\endgroup$ – Bohemian Jun 21 '18 at 15:55
  • $\begingroup$ @Bohemian. I'm sorry but I think you are actually wrong. Partial pressure isn't calculated by multiplying O2 per atm. To calculate it you have to determine the fractal mol of each gas and multiply that by the total pressure. Fractal mol is calculated by divide the gas percent with molecular weight of the gas, sum up all the mols, and divide each gas's mol with the total amount. If you do that, our PP O2 will be 19 kPa - 21 kPa (depends on presition). That's why O2 is lower in his atmosphere, beacuse He has too little molecular weight and that increase the fractal mol of He, decreasing O2 mol. $\endgroup$ – Ender Look Jun 21 '18 at 16:10
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Let's take a step-by-step approach. Thankfully for you, there is a lot of information on the various effects of different gas combinations on the human body, most of it somehow relating back to SCUBA diving.

1.22 atmospheres

This is around 17 PSI.

66% nitrogen

This doesn't pose a significant problem. Earth's atmosphere is already ~70% nitrogen, so this isn't too far from the norm. There does exist a condition called "nitrogen narcosis" (see xenon), but at below 2 atmospheres, there are hardly any noticeable effects.

19.42% oxygen

Long exposure to oxygen at increased partial pressure results in a lot of bad things happening to your body.

Normally, more extreme effects are experienced in very high oxygen concentrations and pressure several times normal. However, your colonists will always be exposed to more oxygen than they need. The effects of breathing such a mixture for your entire life is hard to research, but some of the effects of oxygen toxicity (retinal damage, hormonal abnormalities, etc.) can be anticipated.

14.21% helium

Helium is very light. Unless your planet has much, much higher gravity, all your helium will drift off into space. Even if the helium stuck around, it would be most concentrated near the top of the atmosphere, and doesn't have many interesting effects on humans anyhow.

However, the fact that your atmosphere is not evenly mixed would mean that traveling to high altitudes would be dangerous, because after a certain point the air would not be able to sustain human life.

1.25% xenon

This part causes trouble. Xenon is significantly heavier than Earth air, and will sink to the bottom of the atmosphere. Xenon is a noble gas and is not reactive, but will act like a narcotic at higher concentration (and especially at the elevated pressure). To quote Wikipedia:

The noble gases argon, krypton, and xenon are more narcotic than nitrogen at a given pressure, and xenon has so much anesthetic activity that it is a usable anesthetic at 80% concentration and normal atmospheric pressure.

Although 1% xenon may seem reasonable, remember that your atmosphere is noticeably higher pressure than Earth, and that your colonists will always be breathing this air. Without some other source of air, it would be pretty hard for your colonists to function well.

Overall, xenon has plenty of interesting effects on the human body. There is plenty more information on its dedicated page on Wikipedia.

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