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Earth's sky appears blue, due to the composition of the atmosphere and refraction. Mars' sky is reddish, due to the high iron-oxide particle content of the air.

If a planet had enough vegetation and humidity to keep the air relatively dust-free, and if the air composition was similar to that of Earth, would that planet necessarily have a blue sky, like ours? Under what conditions could such a planet have a different color sky (yellow, for example) and still support terrestrial-type life?

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  • $\begingroup$ Under those conditions the only thing that might make a difference is the sun, if the star is red then maybe the color of the sky would be different. $\endgroup$ – Santiago Nov 10 '16 at 17:07
  • $\begingroup$ Before the Earth had oxygen it was orange. $\endgroup$ – Keith McClary Nov 17 '16 at 3:28
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Not necessarily blue, although pure nitrogen, ozone, methane and hydrogen- cyanide all appear as various shades of blue (assuming the incoming starlight is white). Nitrogen dioxide would be a deep brownish red. Some organics and chlorine dioxide are yellow, whilst chlorine is yellowish green. There are many other coloured gasses, but they are not likely to form planetary atmospheres. However, the question implies vegetation, and hence Earth-like photosynthesis, in which case it also implies an Earth-like atmosphere, in which case a normally pale-blue sky is inevitable.

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