|MadSci Network: Astronomy|
Thanks for the question. It is a difficult one to answer, since the things that affect the color of the sky are complicated. Fortunately, there have been a few answers around this subject already on the MadSci site which I will reference.
There are planets in our solar system and planets outside of it, too (extrasolar). The planets in the solar system all have the same primary light source (the Sun!), while extrasolar planets have their own stars/suns with their own distinctive colors. I will concentrate on the planets in the solar system.
Mercury has a negligible atmosphere, so the sky will look black there. Light needs to be scattered before the sky can have a color. If there is no gas or dust in the sky, the sky cannot have color.
The sky of Venus is problematic, since it is permanently cloudy there. The entire planet is continually covered by its clouds of sulfuric acid. The sun might appear as a "yellow-orange smear".
Earth we know about. Its sky is blue. This is because of something called Rayleigh scattering, where light rays scatter off of the molecules of gas in the atmosphere. [The strength of Rayleigh scattering depends upon the fourth power of the frequency of the light, so blue light with its higher frequency or shorter wavelength is more heavily scattered than red light.]
Mars has a red looking sky. This is due to the reddish iron oxide dust in the atmosphere.
The gas giant planets are Jupiter, Saturn, Uranus,
and Neptune. The
sky gets hazy for these planets since there is probably no
layer that can be called
ground, so it is hard to say where the sky
ends. There are certainly some interesting chemicals
floating in their atmospheres. Complex hydrocarbon molecules might give
light a pinkish or reddish cast as it travels through those atmospheres.
Pluto has a fairly negligible atmosphere so it would have a black sky like Mercury.
Here are some links to other articles in the MadSci archives:
Try the links in the MadSci Library for more information on Astronomy.