• 35% O2 (oxygen)
  • 61% Ar (argon)
  • 1.07% CO2 (carbon dioxide)
  • 0.93% As arsenic
  • 2% Other trace elements

On an exoplanet with a mass of 0.284 Earths and a surface gravity of .66 gs (6.44 m/s^2), would this atmosphere be breathable to humans? If so, would there be any long term effects compared to our atmosphere here on Earth? I've heard argon is narcotic, but I'm not sure at what concentration it becomes a noticeable hindrance to human activity.

As an additional question, what color would the sky be on this planet assuming it orbits a star like ours? :)

  • 3
    $\begingroup$ What is the atmospheric temperature and pressure? Arsenic will not be a gas under conditions where humans can live (let alone breath). $\endgroup$ Commented Jun 20, 2016 at 7:02
  • $\begingroup$ Ummm... Earth-ish? xD If you couldn't tell I don't know much about chemistry or atmospheric science. What other inert gas could I safely replace nitrogen with instead of argon? $\endgroup$ Commented Jun 20, 2016 at 8:24
  • 6
    $\begingroup$ Arsenic is quite toxic! en.wikipedia.org/wiki/Arsenic_toxicity - also, a 35% oxygen atmosphere is quite the fire hazard: "many 'non-flammable' textiles will burn fiercely in air containing as little as 30% oxygen" boconline.co.uk/en/sheq/gas-safety/gas-risks/oxygen-gas-risks/… $\endgroup$
    – pjc50
    Commented Jun 20, 2016 at 11:27
  • 1
    $\begingroup$ @Z.Schroeder -- What do you mean by "arsenic"? How did you come up with this mixture? Do you literally mean arsenic, as all these answers assume, or are you going for something different, maybe a compound that contains arsenic that is gaseous in hospitable conditions? $\endgroup$ Commented Jun 20, 2016 at 21:59
  • 1
    $\begingroup$ Please remove that additional question. You could ask a follow-up question at a later stage, but with it, it just makes it too broad. What research have you done towards each of the elements? Wikipedia's page for each would be a start. And why do you want those concentration/elements? $\endgroup$ Commented Jun 21, 2016 at 8:36

6 Answers 6


Well, let's go one by one:

  • Oxygen. 35% is very high. Not something that would kill you outright, but enough to cause a lot of "spontaneous" fires and significant oxidative damage over time (i.e. decreased lifespan, increased incidence of cancer etc.). Oxygen is bad for your health.
  • Argon. Not toxic, but much denser than "our" air, which might make for deadly pockets if the air doesn't mix well (this does happen on Earth, where argon introduced from e.g. oil lines can escape and displace breathable air). Interestingly, it is very similar to oxygen - it will react in similar ways when diffusing, for example, so it might very well be safe in the end. It's hard to say how it would work in practice without knowing where the argon originates from, though.
  • Carbon dioxide. 1% is already quite a bit. In comparison, on Earth, the mean concentration is around 0.04%. It's still not enough to produce much effect on breathing unless you're doing heavy work or you have respiratory problems. However, again, it's important where the carbon dioxide comes from. How much does the concentration change from day to night, or during the year?
  • Arsenic. As a gas. No. Just plain no. Under "standard pressure", you'd need at least 600 °C to have gaseous arsenic. It doesn't matter whether it's breathable or not, your humans are going to die anyway.
  • Trace elements. Depends on what they are, really. 2% is still quite enough for deadly substances, if you know what you're doing. However, since you already have free oxygen in the atmosphere, we can rule out a lot of the worst stuff which wouldn't survive long in such an atmosphere.
  • 1
    $\begingroup$ Note that with 1/5 earths gravity, the air pressure would be smaller making oxygen less dangerous. Pure oxygen is breathable at .2 atmospheric pressure. $\endgroup$ Commented Jun 20, 2016 at 14:30
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    $\begingroup$ @TomášZato -- The planet in question has 2/3 Earth's gravity. Also air pressure can be great even under low gravity, depending on the mass of the atmosphere. $\endgroup$ Commented Jun 20, 2016 at 22:28
  • 2
    $\begingroup$ Density based separation will not be an issue. Carbon dioxide has a molecular weight of about 44, and it does not pool at all due to natural density separation. Oxygen starvation due to argon, etc. is a result of a large localized inert gas production, not settling. $\endgroup$ Commented Jun 21, 2016 at 5:49
  • $\begingroup$ @GaryWalker Yeah, that's why I noted it depends on where the argon comes from. For example, it might be a problem the same way carbon dioxide can on Earth if it is accumulated over time under a lake, and then released all at once. I'm not sure how you could ever get 61% argon atmosphere, so this is an important question. $\endgroup$
    – Luaan
    Commented Jun 21, 2016 at 7:38
  • $\begingroup$ I disagree about the health risk of oxygen. The body doesn't give a hoot what the percentage of oxygen in the atmosphere is. What the body cares about is the partial pressure of the oxygen in the atmosphere. Too low, you die. Too high, you die. $\endgroup$ Commented Jun 27, 2018 at 23:38

Well, since arsenic is a gas above 887 K​ (615 °C, 1137 °F) which is hotter than a pizza oven, I would have to say no, humans could not breathe that.

If arsenic is present as some kind of lingering particle or a chemical that contains arsenic, my immediate thought is smog. As an analog to phosphorus it might be used on that planet in ways we see phosphates here. But I don’t know what kind of compounds might exist or how they get suspended in the air. Assuming it’s a particulate and not a gas, simple filters can separate it out.

  • 1
    $\begingroup$ Arsine would be the most likely gaseous compound containing arsenic, and it would be a kind of opposite of smog. Smog is oxidized nitrogen while arsine is reduced arsenic. $\endgroup$ Commented Jun 20, 2016 at 22:01
  • $\begingroup$ There's a thermodynamic meaning of fluids being in solution with each other and that has corresponding phase boundaries. There are indeed different ways the particles can be engagued intimately with the gas, beyond the fact that mechanical filters are able to separate them. $\endgroup$
    – JDługosz
    Commented Jun 21, 2016 at 10:17
  • $\begingroup$ Particulates, yes. 0.93% of the atmosphere as floating particulates? Good grief NO $\endgroup$
    – user79911
    Commented Nov 5, 2020 at 9:32
  • $\begingroup$ P.S. if that arsenic is in the air as particulates. EPA states 300 microgram per m3 as hazardous. Your air is 12 000 microgram per m3. This is ignoring that Arsenic is a lethal poison, of course. Just as non-poisonous "smoke" it will kill you in minutes. If you breathe oxygen, and your cellular metabolism runs on ATP, you cannot tolerate Arsenic. $\endgroup$
    – user79911
    Commented Nov 5, 2020 at 9:43

Well, your main question was answered, here is my stab at the last question.

First, a remark. This graph shows the relative escape velocity needed for a gas to leave a planet:

enter image description here

Attribution: By Cmglee - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=42449252

So for your suggested planet, I think that 35% oxygen combined with 61% argon (which is much heavier than oxygen as already stated) might not be feasible, due to oxygen escaping the atmosphere. If the arsenic actually gaseous, it would definitely be too hot for oxygen to remain in the atmosphere.

so, the color. Both nitrogen, oxygen and argon are colorless. So there would not be that much of a difference in color purely from the atmospheric contents. However, since the volume of the planet is only roughly 1/4th, the radius also decreases dramatically, which means that there's less diffusion of sunlight from the atmosphere, since that's smaller as well. It's likely the sky would be a much deeper blue due to this, since the shorter wavelengths are less diffused.

Time for maths! I'm going to dump most units since these formulas are known to work.

The formula for gravity is F = G(mass1*mass2)/D squared. We don't know D (the distance), and we'll assume that mass2 (our weight) is 60 kg. mass1 is equal to .234 Earth masses, which is 1.39749246e24. G is 6.67408e-11. Assuming the g-force is also 1/3 of Earth at 640, we get a radius of roughly 3000 km according to Wolfram Alpha. This planet would be roughly 1/8th the size of Earth, but the mass is only 1/4th, so the density would be double that of Earth.

We can also see that this planet cannot possibly have an atmospheric pressure similar to that of Earth. Pressure is equal to mass per surface area. The mass is a relation between density and volume. The problem is that this planet does not have enough gravity to have an atmosphere that's higher than that of Earth, and the density required for a similar atmosphere would require a much lower temperature than is livable. So this is literally an impossible planet.

  • $\begingroup$ Not sure why you think the oxygen would escape. The escape velocity of the planet in question is 6.86 km/s. Also, under lower gravity, it takes a more massive atmosphere to get the same surface pressure/density. That means a deeper atmosphere and thus more scattering. (And where did you get a volume of 1/4th?) $\endgroup$ Commented Jun 20, 2016 at 22:52
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    $\begingroup$ I don't think Argon is much heavier than oxygen. Molecular mass of O₂ is around 32g/mol whereas the atomic mass of Argon (monoatomic gas) is 40g/mol. $\endgroup$
    Commented Jun 21, 2016 at 8:26
  • $\begingroup$ @MackTuesday I assumed that the planet would have a similar density to Earth. 1/4 of the mass is 1/4 of the volume. He also didn't say the surface pressure would be the same. $\endgroup$
    – Nzall
    Commented Jun 21, 2016 at 8:29
  • $\begingroup$ @Nzall -- Oh OK. It's just that you're talking about the volume of the atmosphere, not the planet. Regarding pressure, I see you answered before he said in the comments that it was "Earth-ish". My bad. $\endgroup$ Commented Jun 21, 2016 at 23:37
  • $\begingroup$ @MackTuesday I edited my answer to clarify and also add extra math that points out contradictions in the question. $\endgroup$
    – Nzall
    Commented Jun 22, 2016 at 14:15

Yes it is breathable , but with 61% of Argon people would have always Demonic-Like voices , this gas is the opposite of Helium.

plus the solid arsenic powder in the air could cause damage over time to human lungs , but it might take as much as 30 years for anything relevant.

  • 4
    $\begingroup$ The atomic weight of argon is 40. Xenon gives a booming voice and it's 131 (and too expensive. Sulpher hexaflouride is 146). Nitrogen, being diatomic, is 28. So argon won't have that much of an effect on your voice. $\endgroup$
    – JDługosz
    Commented Jun 20, 2016 at 13:11

I don't know the atmospheric pressure of your world, so I'll assume that it is like Earth's: 101.325 kPa.

Atmosphere: 101.325 kPa

  • 35% oxygen.
  • 61% argon.
  • 1.07% carbon dioxide.
  • 0.93% arsenic.
  • 2% other trace elements.


Arsenic is:

  • Deadly poison. Here you can read about the symptoms and more information.
  • Impossible for it to be a gas in your current atmosphere (you need to change the pressure and temperature of the atmosphere a lot to 615°C).


  • 2% other trace elements.

To calculate the partial pressure of gases I need know the molecular mass of other trace elements, because I don't know them I will replace them with N2 (a really common gas).

Partial Pressure

$$ \left| \begin{array}{cc|ccc|c|c} \hline \text{Gas}&\text{%}&\text{gr/mol}&\text{Mols}&\text{Fractal Mol}&\text{Partial Pressure (kPa)}&0.66 \text{ g}\\ \hline \text{O}_{2}&\text{35%}&32&1.09&\text{40.08%}&40.61&26.8\\ \text{Ar}&\text{61%}&39.95&1.53&\text{55.96%}&56.7&37.42\\ \text{CO}_{2}&\text{1.07%}&44.01&0.02&\text{0.89%}&0.9&0.6\\ \text{As}&\text{0.93%}&74.92&0.01&\text{0.45%}&0.46&0.3\\ \text{N}_{2}&\text{2%}&28.01&0.07&\text{0.45%}&2.65&1.75\\ \hline \text{Total}&\text{100%}&218.891&2.73&\text{100%}&101.325&66.8745\\ \hline \end{array} \right| $$

  • High oxygen value: Humans need around 21 kPa of oxygen to "work" properly, you have the double, your people would suffer hyperoxia. Also when oxygen is above 50 kPa it becomes toxic, luckily your O2 isn't toxic but would be annoying for your population.
  • Argon Asphyxia: Although argon is non-toxic, it is 38% denser than air and therefore considered a dangerous asphyxiant gas in closed areas. It is difficult to detect because it is colorless, odorless, and tasteless.
  • Argon narcopsia: I don't know much about it but I think it can cause narcopsia like nitrogen (56.17 kPa of argon is very much, maybe it could produce some dizziness). Also, I am not sure but Xenon weakens the blood-barrier brain and this increase the probability of infections in the brain, argon and xenon are inert gases, anaesthesic and narcotic, maybe argon also weakens the barrier.
  • CO2 slightly above the normal: maximum amount of CO2 in air can be 1% without visible problems, at 1.5% you would die in a month, you have 1.07%, maybe it could take years to kill you or your body will adapt to survive.
  • Lethal arsenic: see above.

For more information about gases in the atmosphere you can check this answer (effect of several gas with an emphasis in O2) and this answer (effect of several gas in extreme doses with an emphasis in CO2 intoxication).

Your world has 0.66 g gravity and you don't indicate the pressure of your atmosphere. The information above assumes that the pressure is equal to Earth's but I don't know if it's same pressure or same amount. If it's the second option then its partial pressures will be found in the last column of the table, in that case you won't have hyperoxia, but argon could still be dangerous.

Sorry, but I don't know about your additional question, (I'll compensate for that with a free check of atmospheric stability!.

Calculating if gases will escape!

1) Calculation of the escape velocity:

In physics, escape velocity is the minimum speed needed for an object to escape from the gravitational influence of a massive body.

  • Escape Velocity = $\text{v}_\text{e} = \sqrt{\frac{2\times\text{G}\times\text{M}}{\text{r}}} = \sqrt{2\times\text{g}\times\text{r}}$
  • Where:
    • G is the gravitational constant: ($\text{G} ≈ 6.67 \times 10^{11} \text{ m}^3 \times \text{kg}^{-1} \times \text{s}^{-2} ≈ 0.0000000000667$)
    • M is the mass of the body to be escaped (planet) in kg.
    • R is the distance from the center of mass of the body to the object in metres.
    • g is the gravity in m/s2.

The problem is we don't know the radius of your planet:

Gravity can be calculated: $${\displaystyle g={\frac {m}{r^{2}}}}$$ Where g is the surface gravity of the planet, in a multiple of the Earth's, m mass, in multiples of the Earth's mass (5.976·10^24 kg) and r its radius, expressed as a multiple of the Earth's (mean) radius (6,371 km).
So, to calculate the radius I can do: $$r = \sqrt{m \times g}$$

On an exoplanet with 0.284 Earth-mass and a surface gravity of 0.66 g (6.44 m/s^2).

$$r = \sqrt{0.284 \times (0.66)} = \sqrt{0.18744} = 0.43$$ $$0.43 \times 6,371 \text{ km} = 2758.28 \text{ km}$$


$$v_e = \sqrt{2gR} = 5,960 \text{ m/s}$$

2) Now if the RMS (Root-mean-square speed) velocity of any gas in your atmosphere is equal or greater than escape velocity of the planet then that gas will escape rapidly and will be absent.

  • $\text{RMS} = \text{v}_{\text{rms}}=\sqrt{\frac{3\times\text{R}\times\text{T}}{\text{M}_{\text{m}}}}$
  • Where:
    • $\text{Vrms}$ is the root mean square of the speed in meters per second.
    • $\text{Mm}$ is the molar mass of the gas in kilograms per mole.
    • $\text{R}$ is the molar gas constant. $\text{R} = 8.3144598(48)\text{ J}\times\text{mol}^{-1}\times\text{K}^{-1}$
    • $\text{T}$ is the temperature in degrees kelvin (K = °C + 273.15). I'll use 25°C (298.15 K), I think that is the "normal" temperature used in gas calculations where it's specified.

$$ \left| \begin{array}{c|c|c} \hline \text{Gas}&\text{kg/mol}&\text{RMS}\\ \hline \text{O2}&0.032&482.08 \text{ m/s}\\ \text{Ar}&0.039&431.45 \text{ m/s}\\ \text{CO2}&0.044&411.07 \text{ m/s}\\ \text{As}&0.074&315.06 \text{ m/s}\\ \text{N2}&0.028&515.27 \text{ m/s}\\ \hline \end{array} \right| $$

Your atmosphere is stable! (or at least for short-term, I don't know how to calculate the Boltzman distribution for geological ages).


It is theoretically breathable because of its percentage of oxygen gas, but it will take years to humans to adapt bodies to those conditions and live like on the earth. Argon and arsenic are the biggest problem and their percentage is really high to be accepted by an human body. In some millennia human people will mutate their organism to survive to the new living conditions.

  • $\begingroup$ Welcome to the site, Alberto. Please note that there are three questions being asked here. If you would please edit your answer to include material concerning the other questions, including any required research, we would appreciate it. We prefer answers be more than one-liners, to ensure sufficient information is provided to fully address the OP's problem(s). $\endgroup$
    – Frostfyre
    Commented Jun 20, 2016 at 12:23

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