We are all told at school water vapour and carbon dioxide are the top two greenhouse gases, and that they absorb thermal infra red radiation, trap heat and warm up the Earth. My question is how do they do that? Why can't Oxygen or Nitrogen or any other gas not absorb infra red radiation as well as water vapour or CO2?
Atmospheric Science – How Does Carbon Dioxide or Water Vapour Absorb Thermal Infrared Radiation from the Sun?
atmospheric science
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There are other wavelengths of light than visible and infrared. The venusian atmosphere is transparent to some of them, which also warm the surface of the planet. As the surface is warmed by these wavelengths, the infrared that it emits is trapped by the atmosphere. Over time this causes a runaway greenhouse effect, which is why the surface of Venus is hot enough to melt lead.
Since the atmosphere and surface have different densities, and the density of a substance is what determines which wavelengths of light can interact with it, it's clear that there will always be wavelengths which can interact with the surface but pass right through the atmosphere. If electromagnetic radiation can interact with a substance, it will cause that substance to warm up. When the atmosphere is composed of a substance which is opaque to infrared light (which warm objects emit), a greenhouse effect will result.
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.
Best Answer
The absorption of IR radiation is due to vibrations of molecules. When a vibration causes change in charge distribution (or dipole moment to be more specific) the IR radiation is absorbed.
Generally, hetero-polar molecules, like $\rm H_2O$ and $\rm CO_2$, have permanent dipole moment. The external oscillating electric field in this case perturbs the Hamiltonian and causes IR absorption. Hence, they contribute to "Green House effect" by absorption of heat.
$\rm H_2O$, which is non-linear molecule, has three fundamental modes of vibrations. Symmetrical Stretching, asymmetrical Stretching and scissoring (bending). $\rm CO_2$, which is linear molecule, has four fundamental modes of vibrations. Symmetrical stretching, asymmetrical stretching and two degenerate scissoring modes, in planes perpendicular. The symmetric stretching mode in $\rm CO_2$ does not produce or absorb any IR, as it does not cause change in dipole moment, but other modes do change charge distribution causing absorption of IR.
Well, the homo-polar molecules, like $\rm N_2$ and $\rm O_2$, does not have any permanent dipole moment. The external oscillating electric field does not perturb the Hamiltonian for nuclear motion, and does not absorb IR, though it perturbs Hamiltonian for electronic motion. Hence, do not contribute for "Green House effect".