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This planet's atmosphere has oxygen at a partial pressure that is breathable to humans, and the rest of the atmosphere is, at least, not made up primarily of Nitrogen (that is, it's not just Earth's atmosphere with marginally different partial pressures). The atmosphere must be breathable for at least long enough to mount a rescue mission (say a few days without any ill effects, and a few months before death), although I'm also happy for it to be breathable indefinitely.

The atmosphere must be made of gases that are reasonable to exist in the quantities required. For example, it's unlikely to be made of Xenon gas, since that's fairly rare (as far as I know), even though Xenon is probably breathable.

I am aware that, at high enough pressures and low enough partial pressures of oxygen, some really exotic gases are breathable. I'm good with that, although one final stipulation is that the atmosphere cannot be so dense that gas narcosis sets in (I just really don't want to deal with that in my story is all).

So, what's my atmosphere made of?

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Argon might be a good replacement for Nitrogen.

It's the 3rd most common element in Earth's atmosphere, although only at a 1 % ratio. Argon is also heavy enough to sidestep the issue of losing most of it to space like some of the other noble gasses. It is however a little heavier than oxygen, which could pose the danger of it settling in lowlands. Argon is breathable and as far as I can tell shouldn't have any significant adverse effects at the quantities described, so long as the planet/area is windy enough to prevent the argon from accumulating too much.

Argon-40 seems to make up the vast majority of the element on Earth, produced primarily due to potassium-40 decay at a roughly 11 % ratio (the rest turns into calcium-40). Even though potassium itself is a fairly common element (it's the 8th most common element on Earth at 0.2 %), potassium-40 only makes up 0.012 % of the element's isotopes, so far too uncommon (at least on Earth) to produce the amounts of argon you would need.

For the planet you describe, a significant amount of potassium-40 in its crust could explain a high enough percentage of argon in the atmosphere. Since it's produced by decay, some math would be needed to calculate the amount of potassium-40 needed, taking into account the age of the planet itself and the potassium-40 half life (1.251×109 years). There's also an interesting question on Astronomy StackExchanged which has some further details on these calculations.

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    $\begingroup$ This answer is worthy of a bounty. $\endgroup$
    – The Square-Cube Law
    Commented Sep 11, 2019 at 15:16
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    $\begingroup$ Atomic Argon is only slightly heavier than mollecular Oxygen (40g/mol vs 32g/mol) similar to how much heavier Oxygen is to molecular Nitrogen (28g/mol) - it won't stratify. $\endgroup$
    – Dale M
    Commented Sep 13, 2019 at 1:13
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Helium.

It is so safe to us, it is used in an array of different kinds of surgery to inflate your abdomen so it is easier for doctors to get to hard-to-reach organs. This article is from 1995 - a lot has changed since then, helium is much more commonly used today because of its non-toxicity.

Helium may be rare on Earth, but scientists think it may make up for up to 23% of the mass of the known universe. Most of the helium here is a product of radioactive decay, but is lost to space through various processes. I think it wouldn't be a stretch for a planet with a gravity higher than Earth's to have much more helium in its atmosphere, though.

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    $\begingroup$ As far as I can work out, you should be able to get away with approx 1.6 * Earth gravity. +1@MichaelStachowsky $\endgroup$
    – Escaped dental patient.
    Commented Sep 11, 2019 at 13:24
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    $\begingroup$ Or perhaps a younger planet wouldn't have lost it all yet? $\endgroup$
    – Shane
    Commented Sep 11, 2019 at 22:12
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    $\begingroup$ You could have a similar gravity to Earth if the planet is bigger; this raises the escape velocity without raising the surface gravity. Saturn, for example, has a surface gravity of only about 1.1G, but is large enough to retain both helium and hydrogen in its atmosphere. $\endgroup$
    – Skyler
    Commented Sep 12, 2019 at 14:27
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    $\begingroup$ Exactly. And the results of such an atmosphere are hilarious $\endgroup$
    – Machavity
    Commented Sep 12, 2019 at 14:46
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    $\begingroup$ @Skyler To have a larger planet with 1G surface gravity, you have to reduce its density quite drastically as the volume grows faster (O(n^3)) than the surface gravity drops (O(n^-2)). The problem is, there are limits to the density of rocky planets. Of course, replacing the iron core with some silicon based rock is thinkable, but I doubt that it will give you the amount of density reduction you require. And if you make the planet so poor in iron, it will be a planet without blacksmiths (in case that matters...). $\endgroup$
    – cmaster - reinstate monica
    Commented Sep 12, 2019 at 21:19
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what's my atmosphere made of?

Nothing else in significant amounts.

As you state, oxygen has the same partial pressure of Earth. Thus around 0.2 bar.

Let it be that, plus traces of other oxides, like water and carbon dioxide.

In this way you don't have to worry about adverse effects, and you just need to take care of higher pressures because of oxygen poisoning.

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – L.Dutch ♦
    Commented Sep 12, 2019 at 14:17
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Methane.

http://aetinc.biz/newsletters/2010-insights/october-2010

Methane gas is relatively non-toxic; it does not have an OSHA PEL Standard. Its health affects are associated with being a simple asphyxiant displacing oxygen in the lungs. Miners previously placed canaries in deep mines to check methane gas levels. Reportedly, canaries keeled over at about 16% oxygen indicating it was time to leave.

Methane is extremely flammable and can explode at concentrations between 5% (lower explosive limit) and 15% (upper explosive limit). These concentrations are much lower than the concentrations at which asphyxiant risk is significant.

Your world has an unstable atmosphere of 15% oxygen, 84% methane, and 1% (and rising) CO2. Oxygen is 10% so at the very low end of what humans need. Unstable because the methane does consume the oxygen and vice versa. The world is in flux - a methane world which terraformers seeded with photosynthetic algae in the recent past. Oxygen is being rapidly produced by the greening oceans of this world. Explosions happen but are rare - there is too much methane to easily explode.

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    $\begingroup$ Out of curiosity, why wouldn't it explode above 15%? $\endgroup$
    – Michael Stachowsky
    Commented Sep 11, 2019 at 19:07
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    $\begingroup$ @Michael Stachowsky: I did not know. But I found it on the chemistry stack exchange! chemistry.stackexchange.com/questions/49261/… What this means is that the methane atmosphere world is a lot safer than I thought it would be. There is too much methane to explode! $\endgroup$
    – Willk
    Commented Sep 11, 2019 at 19:21
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Sulfur hexafluoride

This is a real oddball and probably a sign that the planet is a terraformed low gravity planet, because this is when sulfur hexaflourite is actually useful. No natural planet would have this in its atmosphere. However looking at its description from Wikipedia:

Sulfur hexafluoride (SF6) is an inorganic, colorless, odorless, non-flammable, non-toxic extremely potent greenhouse gas, and an excellent electrical insulator. SF6 has an octahedral geometry, consisting of six fluorine atoms attached to a central sulfur atom. It is a hypervalent molecule. Typical for a nonpolar gas, it is poorly soluble in water but quite soluble in nonpolar organic solvents. It is generally transported as a liquefied compressed gas. It has a density of 6.12 g/L at sea level conditions, considerably higher than the density of air (1.225 g/L).

It would work quite nicely and gives you an interesting plot hook namely that the world the crew has stranded on has been terraformed.

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  • $\begingroup$ And as a bonus, you get the reverse-helium effect in that it makes everyone's voices super-deep. $\endgroup$
    – Darrel Hoffman
    Commented Sep 12, 2019 at 19:14
  • $\begingroup$ Ah, you beat me to it. In addition to its mostly inert nature, and voice changing characteristics, SF6 can cause irritation of the lungs and throat, as well as pulmanory edemas if you breath too much of it. In an SF6-laden atmosphere, the rescuers would operate under a ticking clock on their time to complete the rescue. $\endgroup$
    – asgallant
    Commented Sep 12, 2019 at 19:20
  • $\begingroup$ @asgallant Out of interest, how much would be too much SF6? What concentrations can breath indefinitely? $\endgroup$
    – TheDyingOfLight
    Commented Sep 12, 2019 at 20:30
  • $\begingroup$ @TheDyingOfLight wish I could say, but my knowledge of SF6 comes from watching Periodic Videos and skimming wikipedia. $\endgroup$
    – asgallant
    Commented Sep 12, 2019 at 20:44
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    $\begingroup$ "probably a sign that the planet is a terraformed low gravity planet" <== is potentially a really interesting bonus feature / side plot / sequel hook! (Who were the Ancient Aliens who made this world...?) $\endgroup$
    – dgould
    Commented Sep 12, 2019 at 23:42
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There are other inert gasses with reasonable cosmic abundance... neon and argon spring to mind. Neither are particularly common on earth (argon makes up 1% of the atmosphere, and neon is much rarer than that) but given that you're handwaving away the nitrogen in the air then handwaving in noble gas replacements isn't entirely farfetched. Both are safely breathable at STP (unlike xenon, which is used as an anaesthetic), and neon is slightly more common than nitrogen in space and indeed the solar system so there's lots of it about. You can also manage at least 4% water vapour (that's what you'll get in a steamy jungle on earth, for example, and I'll bet steam baths get higher than that).

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  • $\begingroup$ Why is it odd to wave away Nitrogen? Does it have some special properties that would make it common? $\endgroup$
    – Michael Stachowsky
    Commented Sep 11, 2019 at 14:54
  • $\begingroup$ @MichaelStachowsky it is one of the most abundant elements in the body of any living being. Life as we know would not evolve in an atmosphere with low levels of nitrogen. $\endgroup$
    – The Square-Cube Law
    Commented Sep 11, 2019 at 15:07
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    $\begingroup$ @StarfishPrime Two bodies with N2 rich atmospheres? Three. Earth (0.78 atm) , Titan (1.5 atm) and Venus (3.3 atm) (approximate values). $\endgroup$
    – TheDyingOfLight
    Commented Sep 12, 2019 at 13:47
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    $\begingroup$ @TheDyingOfLight that's a very good point, thanks. $\endgroup$
    – Starfish Prime
    Commented Sep 12, 2019 at 14:10
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    $\begingroup$ @MichaelStachowsky, unlike oxygen and hydrogen, nitrogen mostly doesn't form many non-gaseous compounds. Earth's nitrogen (and presumably Venus's and Titan's) is mostly present in the atmosphere, not the crust. $\endgroup$
    – Mark
    Commented Sep 12, 2019 at 20:17

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