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Apologies for the extremely weird title. Admittedly, it describes my question well.

Now, I'm making a universe where humans have discovered the big secret to space travel. It's actually pretty simple; all you have to do is ████████. Who'd have thought?

Now, the problem with easy space travel in a species with low lifespan is that terraforming takes too long. Humans don't wanna wait for stuff, so instead of letting our machines make the planet perfect, we have a tendency to settle when it's barely livable. That means that not all of the chemicals and stuff are out of the air, but there's nothing so toxic that it'd kill people very fast.

With that in mind, what colors can the sky be if the atmosphere is breathable?

For the sake of the question, please also assume that the gravity, temperature, etc. of the planet is also livable, with modern-day tech. High-altitude stuff is fine -- as long as people can live on the surface, I don't care. Space ships are airtight anyway.

Bonus points if you make it snow- or cold-related.

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closed as too broad by DonyorM, bilbo_pingouin, Separatrix, J_F_B_M, overactor Feb 15 '16 at 11:24

Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

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    $\begingroup$ I don't understand what you're asking. The sky is blue because Rayleigh scattering affects blue light more than red. If you have very thin atmosphere, the color will tend towards black, and if you have very thick atmosphere, the color will tend towards white. A thin atmosphere where the light travels a long way (such as during sunrise/sunset) causes the scattered blue light to scatter all the way to the ground/space before it gets to us, making it red. Any other colors are from chemicals in the atmosphere, which can probably be any color. $\endgroup$ – MichaelS Feb 15 '16 at 4:41
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    $\begingroup$ @MichaelS "Any other colors are from chemicals in the atmosphere, which can probably be any color." What I'm asking is what chemicals can I put in the air to turn the sky different colors without killing people. I'd imagine that, while you could make an atmosphere any color by putting the right stuff in, not all of it is breathable. If that made sense, feel free to edit the question to clarify; I just realized how tired I am. $\endgroup$ – Nic Hartley Feb 15 '16 at 4:42
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    $\begingroup$ But it's basically asking for a negative answer. "Prove that no safely-breathable chemicals in the galaxy can produce a yellow atmosphere", etc. But a further complication is that high-altitude chemicals could cause the weird colors without affecting the humans on the ground, depending on the atmosphere of this random planet. So it's not even about safe chemicals, just chemicals that might possibly exist safely. $\endgroup$ – MichaelS Feb 15 '16 at 4:45
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    $\begingroup$ I feel like this is a little too broad. There are thousands (if not millions) of possible chemicals that could be in the atmosphere, and their effect on light may not be known to us. This would produce list-based answers or incomplete answers as best. If you want a particular atmospheric color for a planet, appropriate amounts of handwavium always make it the right color. $\endgroup$ – DonyorM Feb 15 '16 at 5:40
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    $\begingroup$ I've given an answer to your question illustrating the wide array of colors already known to be possible. My concern with the question isn't proving a specific color is possible. My concern is that basically any color is possible, so the only sensible answers to the question besides "whatever color you want" involve very careful analysis of every possible known chemical that could naturally exist in an oxygen-rich environment in every known atmospheric type to demonstrate that certain colors aren't possible. $\endgroup$ – MichaelS Feb 15 '16 at 6:14
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As I mentioned in a comment that may or may not get deleted, I think the question itself is very broad. There's just no way to reasonably prove that some particular color isn't possible. That said, we can show that most colors are possible, as long as you don't need it to be some really specific shade.

TL;DR

Red from dust, yellow from dust, green from storms, and blue from Rayleigh scattering are all perfectly normal here on Earth under conditions that could conceivably be permanent on another planet. Purple is probably possible with a slightly hotter sun and some atmospheric dust that absorbs yellow and green light.

Light Source

The reason our sky is blue is because of Rayleigh scattering, which preferentially scatters shorter-wavelength (higher frequency, higher energy, blue-er) light. That will be true of any atmosphere, regardless of composition, but only for the colors that actually survive the trip.

The first part of having colors survive the trip, is they need to exist at the beginning. A star's temperature determines what colors it outputs. See black body radiation topics for more information. A "cold" star (around 3000 K or 5000 °F) will be red. Meaning your sky (and everything else you look at) will be red. As the star gets hotter, the color will progress through orange and yellow, then start becoming whiter and whiter, but the sky will always tend towards the shortest wavelengths visible, so it could look red, yellow, green, or blue. If you make the sun hotter than ours, you could conceivably get a purplish sky, although oxygen tends to absorb light in that part of the spectrum (and we can't see a huge range of purple colors -- most purple colors are the absence of green).

Note that for most of these sky colors, the planet won't be incredibly nice visually, because without artificial light sources, you'll get monochrome or duochrome everything. So if you want normal-ish world lighting with a different-colored sky, you have to rely on chemicals in the atmosphere.

Chemical Filters

There are two methods of changing the atmospheric color through chemicals. One is by changing the gases in the atmosphere itself (fluorine turns the atmosphere yellow, while chlorine turns it green). The other is by constantly kicking up a bunch of dust of a specific color (red dust will absorb the blue and green light, so only red light is able to refract down to observers on the ground).

Both of the gases listed seem to be toxic in the concentrations needed to change the sky color, so it might be safer using dust particles. Mars is a reddish-orange color because of the oxidized iron in its dust. So adding extra oxygen won't change the color much. Sand on Earth is generally a desaturated yellow color (also known as shades of brown), so anything in that range is clearly possible. Green skies can occur on Earth near thunderstorms (it seems particularly prevalent if there's hail but a quick Google search says we're not really sure what causes it), so a planet with a lot of upper-atmosphere storminess could give green (of note, this is thought to relate to ice in the atmosphere, which could be related to your very cold planet).

Again, the further you are from an Earth-like blue sky, the more of an effect you'll have on anything at the ground level. So bright, pure yellow skies will mean red and green stuff shows up decently, but blue objects will look black. But a faded yellow (which means there are other colors besides yellow, but yellow is the brightest) will be perceived as a yellow sky while still making ground objects relatively normal. Note that even our sky isn't really blue -- it's a faded (whitish) cyan/blue color. But you can still get a reasonably pure sky color without sacrificing ground visuals too much.

Of note, the denser the atmosphere, the more Rayleigh scattering has an effect (and the less relative effect the dust particles have). So for humans to live, we need something more dense than Mars, which means the atmosphere can't be as red, but it can still be fairly red to orange, especially with a mildly colder star. Purple skies can exist with any compound that mostly absorbs green, but I'm not sure what that would be. All the purple compounds I looked up react with oxygen to turn some other color.

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  • $\begingroup$ So, because I ended up not having the chance to edit my question and I've since forgotten what I originally wanted to ask, I'm accepting this answer, because it's detailed and answers what I asked. $\endgroup$ – Nic Hartley Apr 17 '16 at 15:48

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