# How could the sky be orange in a breathable atmosphere?

I've discovered some ways of coloring the atmosphere through personal research:

• The atmosphere could be colored by particles or colored gases (as on Mars, unless I'm wrong)

• The atmosphere could be more dense to refract more colors.

• The Sun itself could in fact beam a less "colorful" light.

But I might have misunderstood something. So I ask the question here: On a planet other than Earth, how could the sky be orange but the air still be breathable? The more precise the answer the better.

• Bienvenue à Worldbuilding; soyez confiant! "... est-ce que je ne me trompe pas" et "j'ai probablement mal compris" ne sont ne sont pas forcément vrais. (Je juste parle un peu de français, je suis désolé est-ce que c'est pas correct) // Welcome & be confident - don't assume you misunderstood just yet. – Zxyrra Dec 22 '16 at 2:55
• Welcome to Worldbuilding, and thank you for bringing your question here. Your English is fine. Do you mean specifically breathable by humans? – Monica Cellio Dec 22 '16 at 4:19
• At dawn/evening shorter wavelength of visible light gets scattered more easily allowing the sky to appears orange(red). At noon when the star(sun) is directly overhead, brewing sandstorm could do the trick or at least maintain PSI over 400 manually. – user6760 Dec 22 '16 at 4:20
• I wonder if you could have some kind of body near earth that would red shift all light from the sun just enough so that it all was orange? I don't know if that's possible, but maybe someone with more knowledge of physics could think of something. Some kind of a gravity well tidally locked with the earth? Or some spacetime fold that shifts or filters the frequencies of light passing through/across it? – Thom Blair III Dec 22 '16 at 6:14
• @ThomBlairIII The amount of red shift needed either by "some kind of body" or "Some kind of a gravity well" or "some spacetime fold that shifts or filters the frequencies of light" requires a very large black hole. Any planet, ind its star, is likely to fall in quite quickly. Sorry, but no. – a4android Dec 22 '16 at 7:23

# Rule out some things

If the atmosphere is more dense, then it will refract more. However, light refraction is highly wavelength dependent, so if it refracts more light, the sky will be more blue. The equation can be seen here. Different compounds have different scattering cross-sections, so they will scatter different amounts, but all of them will have the same wavelength dependent curve.

Every yellow star will have a blue sky if there are no colored compounds in the air. If the star was redder, then the sky would be shifted to red. If the red, yellow, and blue components to the sky's scattering were equal (a possible result of a red star), then the sky would appear white. If the star were bluer, then the sky would be even bluer or violet.

So basically, we are limited to a tinted sky to get the orange color you want.

# Tinted sky

If your sky was tinted with something orange, then it would be orange. However, there would still be a blue-ish tint to the atmosphere due to reflection from the much more common nitrogen and oxygen. Your best bet is to have planetary cloud cover that is tinted orange; then there is always an orange backdrop at a couple of km altitude.

The list of things that could be in the clouds and are orange is not long. For gasses there are basically none, but Nitrogen Dioxide and Bromine might be close. Nitrogen Dioxide clouds are plausible in some sort of biologically dominated system. Bromine sounds pretty dangerous. While it is very reactive and won't normally be found as a bromine gas, sunlight converts organo-bromide compounds to free bromine at the top of the atmosphere, so that gives us a plausible reason for bromine haze. Unfortunately, free bromine destroys the ozone layer, so that is a bummer.

As far as aqueous compounds suspended in water droplets as a method for coloring clouds, we have Dichromate (Cr$$_2$$O$$_7^{2-}$$) and Cobalt ammine {Co(NH$$_3$$)$$_6^{3+}$$}. I don't know much about either, but neither one sounds very healthy. Depending on your goals for this planet, you could either have the geological and life cycles on this planet be adapted to having high levels of chromium or cobalt around, or just make your human explorers wear masks.

## You are correct, the sky can be orange using any of those methods.

• Changing the star: "Sky color of an alien world" describes why the sky on a red-dwarf's planet will appear mostly white due to light scattering. However, subtle changes in the composition of some layers of your atmosphere, or an even redder star, may be able to get around that problem.
• Changing the atmosphere: Using Mars as a model, iron oxide (rust) dust may be able to change the color of the atmosphere, among other substances. Other problems may result from this (the dust or substances may settle on the surface) but they should not severely inhibit life's ability to function. Other, non-DNA or non water-based organisms may also be able to withstand phosphorus or sulfur, which some suggest is responsible for the red spot in Jupiter's atmosphere.
• Coud you explain me how methane would give a reddish color to the atmosphere? I may look dumb but, isn't methane a colorless gas? – Caïn Dec 22 '16 at 5:57
• @Caïn That is correct, but I've read that it may be able to behave differently under certain conditions. Still, that could be false, in which case you could use iron oxide (rust) like Mars has, or sulphur and phosphorus like Jupiter's spot is thought to contain. – Zxyrra Dec 22 '16 at 6:02
• And the air would still be breathable if the atmosphere was saturated with iron oxide? I mean, Mars isn't really a place where I would go for a trip. And Jupiter neither. – Caïn Dec 22 '16 at 6:04
• @Caïn Mars has more problems with radiation and the cold; dust may not be comfortable but life can adapt. Phosphorus and sulfur are some of the building blocks of life, along with carbon, hydrogen, nitrogen, and oxygen - it seems like they'd be harmful but in fact they are not. – Zxyrra Dec 22 '16 at 6:06
• I think you answered my question. I'll wait a bit and mark your answer as accepted. Thanks a lot. – Caïn Dec 22 '16 at 6:13

Airborne 'xeno-algae' that use a close analog of carotene absorb the visible light other than orange.

Here's a way to make the sky look orange, as seen from the surface:
There are several phyla of xeno-algae that use a close chemical analog to https://en.wikipedia.org/wiki/Carotene as their primary photosynthetic absorber.

These xeno-algae are so successful and well adapted that only the light they cannot absorb (orange) reaches the surface. A side effect is that the stars aren't visible -- even the sun is diffused into a mere 'bright patch' in the sky. Surface-based plants use something like chlorophyll, but the surface energy economy is sparse, compared to earth. Fallen xeno-algae* is a significant base nutrient in the ecology.

## Cloud of Gas or Particles Around the Sun

I am not well versed in physics or optics, but I think it might be possible for the Earth's atmosphere to remain the same in terms of elements and still be orange if a cloud of gas or particles were to come between the sun and Earth, thereby coloring the sun's light a different color so that when it enters Earth's armosphere, the primar color seen is orange.

Perhaps if the cloud of elements only had an emission spectrum of orange, i.e. around 600 nanometers (nm), then there would be no other colors coming into earth's atmosphere.

So, if the cloud blocks all the other wavelengths/colors of the visible spectrum of sunlight, then orange would be (I believe) the only color visible.

This scenario would allow you to have an orange atmosphere on modern day earth without changing Earth's atmosphere or the sun, but it would certainly wreak havoc on life on Earth by eleminating all the rest of the wavelengths of visible light.

• sunlight (as we know it) is white with almost all the spectrum and has an emission spectrum that only lacks the hydrogen line, most noticeable as the single, slim "notch" in the yellow graph on en.wikipedia.org/wiki/File:Solar_spectrum_en.svg The lacking parts on red are all atmospheric absorption – Trish Dec 22 '16 at 6:23

Replace the nitrogen in the atmosphere with neon.

Neon is a noble gas, very similar to nitrogen, which is non-reactive in most situations. It also glows red when ionised.

This means that your planet can have a breathable atmosphere, since neon can be inhaled safely, and a mixture of yellow sunlight with a gently glowing red atmosphere should give you the orange sky you're after.

• But plants need gaseous nitrogen which bacteria can fix for them. – RonJohn May 14 '17 at 20:32
• @RonJohn Plants on earth do. Plants which evolved in a nitrogen atmosphere. – Werrf May 14 '17 at 21:31

You can increase the thickness of the atmosphere, so that only the red part of the Sun spectrum is transmitted (like it happens at sunrise/sunset).

• Wouldn't that require higher gravity? – RonJohn May 14 '17 at 20:33