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I'm starting a whole new planet for a story and would like it to have an orange sky during the day.

At a basic level (I am not a smart man) what chemical composition would be most conducive to Orange sky colouring? What colour would that appear during sunrise/sunset (Our blue turns orange due to some science magic, what effect would this same process have on an Orange atmosphere?

The air doesn't have to be breathable, I am fine with whatever implications this will have on human characters re: Breathing gear, space suits, etc.

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  • $\begingroup$ This might be a better fit on the Physics SE. That being said, I would like to see answers to this question and, beyond that, a variety of sky colors. If Physics SE doesn't have something like this, maybe we could compile one here... $\endgroup$ – Frostfyre May 22 '15 at 3:04
  • $\begingroup$ For some reason this came to mind. $\endgroup$ – Neil May 22 '15 at 7:42
  • $\begingroup$ Related: Why does the sky change color? on Physics. $\endgroup$ – a CVn May 23 '15 at 23:04
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Mars has a CO2 atmosphere, which is not orange but a reddish-brown similar to its surface. CO2 is fairly nonreactive, so there aren't a lot of questions about "what would X material do". The biggest difference is that things wouldn't burn, and metal wouldn't rust.

But it's not exactly orange, and nonreactivity is BOOO-RING. If you want something that's really orange, I would go for bromine. It exists as a liquid under normal conditions, but easily evaporates like water, so an atmosphere of it is reasonable. It is also very nasty stuff. It is a halogen in the same family as chlorine and fluorine, which means it reacts violently with most metals. It also tends to attack organic materials by replacing the hydrogen. These properties would make for an interesting space suit design challenge. One other thing to consider is that bromine gas is not transparent, so at the bottom of an atmosphere it could get very dark.

As for sunrise and set, they are ruled by Rayleigh Scattering, the same process that makes the sky blue, and that would still be true with either CO2 or bromine. I would expect even deeper reds and oranges in either case, and no blues because blue light would be absorbed. You would need large molecules similar in size to visible light (400-750 nanometers) to get a different type of scattering, while most gas molecules are a few tenths of a nanometer.

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  • $\begingroup$ Bromine sounds like a really cool option. I like the idea of the air itself challenging everything that the characters have to do. $\endgroup$ – Wompguinea May 22 '15 at 3:28
  • $\begingroup$ Always fun when the world itself fights back :) . Another nasty reactive one is nitrogen dioxide, which explodes and creates strong acids in water, but I like bromine better. $\endgroup$ – evankh May 22 '15 at 3:42
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The skies of Earth turn a red colour during sunsets as the sunlight passes through longer light paths in the atmosphere before reaching your eye, which means most of the shorter wavelengths are being absorbed by the air before reaching you. This suggests that rather than a normal planet, you are on a tidally locked planet circling red dwarf star, and are somewhere between the 45 degree latitude mark and the twilight equator (with the Hot Pole being the 0 degree mark and the sun directly overhead).

During the 1960's and early 1970's, sunsets were often "enhanced" by the pollutants that were routinely released in the air, especially oxides of nitrogen and sulphur. If these elements were in the atmosphere due to natural processes or human activity, your permanent "sunset" on the red dwarf world would also be more colourful.

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  • $\begingroup$ I like that! Might be a bit easier to work into a story than air that eats you. Would that also mean that the sky would change colour the further towards the Cold Pole? Sounds cool. $\endgroup$ – Wompguinea May 23 '15 at 5:05
  • $\begingroup$ Since the planet is not rotating, the cold hemisphere will be in darkness. Around the twilight equator you would have a glowing sky, but the cold pole will be always dark. $\endgroup$ – Thucydides May 23 '15 at 11:52
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Nitrogen dioxide. Hella poisonous and brown orangey. And making an atmosphere of it is pretty easy, set off a nova or other powerful gamma ray source near earth or another similarly atmosphered body. The gamma rays will tear apart the o2 and n2 our atmosphere is made of and it will recombine into the more favorable NO2. An atnosohere of this would likely mean your planet was likely earthlike at some point.

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