# What Color Can the Sky Be?

A good friend of mine, who's a doctor, is very well traveled, and he told me he's seen skies of different worlds take all the colors of the rainbow (and the grayscale to boot). I suspect that he might be pulling my leg, since he does tend to exaggerate on occasion.

The welcome green glare of the homeworld's sky

So, is my friend lying or can the sky really be of any color (besides octarine)?

If the whole spectrum is too hard, I'm specifically (and in this order) interested in:

• Green
• Gray
• Purple
• Orange
• Vermillion (I don't actually know what color that is, I just like the name)

To clarify: I don't mean briefly for a minute, I want it that color a significant portion of the day. And no, it need not be Earth, and the system planet and star can vary to be whatever is best for maximizing your color-purposes. And no, you don't need to address all the colors, (I mostly care about green) but extra brownie points if you do.

• Important clarification, coloured, when which species is looking? For any home species the sky would have a neutral colour (but more blue from Rayleigh Scattering, and more red at sunset due to Mie Scattering). – Aron Mar 29 '15 at 21:17
• I'd have to assume human-like retinas and neural color processing. – Serban Tanasa Mar 29 '15 at 21:17
• Oh...thats simple then...it can be any colour you want, just add the right sky colouring. I hear grey is very popular in China right now. – Aron Mar 29 '15 at 21:18
• BTW. Green, could be Fluorine gas, as the sky colouring. Its a cousin to another sky colouring called Chorine gas, very popular during the 20s in Europe. – Aron Mar 29 '15 at 21:27
• @Aron true, but it's unlikely to be a viable homeworld to someone possessing "human retinas" if the atmosphere is comprised of chlorine gas... – Isaac Kotlicky Mar 30 '15 at 0:38

Most colors would be possible. The two items that determine what color the sky is is the color of the sun and what is in the atmosphere. Having a red star will be the easiest in making the the sky shades of red, since that is the primary wavelengths coming from the star.

The rest, it depends on what is in the air to both absorb certain light frequencies and scatter others. Water vapor scatters most frequencies (on a day with even thin clouds, the light is still painful) and when thicker, absorbs most frequencies. I'm not up enough on my chemistry to point out which will do what.

# Most skies will be blue-ish

First off, most gasses are colorless. Taking our solar system as an example, the possible gasses are CO$_2$, N$_2$, SO$_2$, Ar, CO, O$_2$, H$_2$, He, and CH$_4$. That covers Venus, Earth, Mars, Titan and all four gas giants. All those gasses are colorless. Water vapor (trace on Earth and the gas giants) and Neon (trace on Venus and Earth) are also colorless.

Second off, Raleigh Scattering, the blue-shifting of reflected light, happens with all gasses. If one of the dominant atmospheric gas is carbon dioxide or methane (as in Venus and Titan), then the scattering effect will be more prominent than on Earth, since those gasses scatter blue light more efficiently than oxygen or nitrogen. If hydrogen is a dominant gas, as in the gas giants, then the effect will be less.

So basically, all reasonable atmospheric gasses are colorless in and of themselves, and all of them will scatter blue light causing a greater or lesser blue tint in the sky.

Finally, it is relevant the wavelengths of light that stars up out.

Due to various absorption effects, only the brightest and dimmest stars won't have a spectral maximum in the blue-purple range. O-type stars, extremely hot and short lived, will produce a very purple light; M-type red dwarfs will produce a reddish light. All other stars will peak in the 400-450 nm range, just as our sun does, which is sort of blue-purple.

Overall, most skies will be blue-ish. I suspect that given that the peaks of the red dwarf's spectra shown above are on the very edge of the human visible range, even a red dwarf's skies might be more green or yellow than red.

Magenta might be trickier than pretty much anyting else, because there is no magenta wavelength, it only exist as a combination of red and blue (which don't overlap in the light spectrum).

Figure you could fix that by having a binary star system of primarily red and blue light respectively.

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