Although this is a hypothetical, I think it is nevertheless a legitimate physics problem. I would very much appreciate any answers you can give.

What colour would the sky be, both at daytime and at nighttime, on a small (little over mars-size) moon located 150.3 million km from its yellow dwarf binary star with an unusually high atmospheric temperature and an atmosphere with the following composition: Argon- 80% Oxygen- 12% Nitrogen- 5% Hydrogen- 0.73% Neon- 0.22% Carbon dioxide- 0.05%

There are also thin-spread particles of Zincite floating around in the outer atmosphere due to volcanic eruptions millions of years ago. The sky of this world is high in water-vapour, the average humidity being about 70%. The atmospheric density is 50x that of earth.

  • $\begingroup$ There was a little discussion of this in chat. The calculations for this are horrendous and evil, but they can be done. That suggests that maybe this could be rephrased somewhat and posted to Physics SE, where there are bound to be lots of people who know how to answer accurately. This isn't off-topic, but I worry that you won't have a lot of luck getting a clean answer here. $\endgroup$
    – CAgrippa
    Commented Mar 16, 2016 at 19:45
  • $\begingroup$ Okay, thank you. Could you recommend how I reword it? $\endgroup$
    – IJoinedCozIcan
    Commented Mar 16, 2016 at 20:10
  • $\begingroup$ "Although this is a hypothetical, I think it is nevertheless a legitimate physics problem." {insert question text here} That'd probably do it. $\endgroup$
    – CAgrippa
    Commented Mar 16, 2016 at 20:29
  • $\begingroup$ I think you'll need to specify the atmosphere's density as well as its composition. At one extreme, if the density is 0.00001% of the Earth's atmosphere, the sky will be black. $\endgroup$
    – Mike Scott
    Commented Mar 16, 2016 at 20:45
  • 1
    $\begingroup$ IS there a reason these numbers are specific, also, how far would you be willing to bend these numbers? The 2 main factors of the color of the sky is the color of the Star and how well the atmosphere scatters its light. Really, though, because the gases are probably colorless, the biggest difference is probably the yellow star itself, Rayleigh scattering of yellow light in argon gas is probably the color of your sky, if you can even see it in all that gas. $\endgroup$
    – Ryan
    Commented Mar 16, 2016 at 20:58

1 Answer 1


I don't claim to be a physics or astronomy expert or anything, but here's what I found. Someone asked a question similar to yours, with similar percentages of elements in the atmosphere (80% argon, 20% oxygen). Obviously the answer to the question given will not be perfect, given that the question asked there refers to a planet orbiting a white star, but here it is:

Still blue. Argon is colorless, and a thick enough layer of any colorless gas looks blue because it scatters shorter wavelengths more efficiently. On very rare occasions holes in different layers of Jupiter's clouds happen to line up, and thus provide a glimpse into clean hydrogen below. Such holes look blue (from outside).

Since that answer is for a planet orbiting a white star and you're asking about a yellow star, I would figure that your moon who have a blue-greenish tinted sky. Also, you state that nitrogen is present in the atmosphere, but as nitrogen gas is colorless (at least to humans), it wouldn't affect the sky color. The other gases you mention are in very small quantities, so their impact on the color of the sky would be unnoticeable.


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