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It is my understanding that Rayleigh scattering depends on both the length of the particle as well as the wavelength. Due to the similar lengths of molecular nitrogen and oxygen it is blue light that gets scattered down to Earth giving the terrestrial sky its blue color. But what about Titan? The Titanian atmosphere has a vastly different chemical composition than Earth(on Titan it is mostly nitrogen dioxide and methane). So is there evidence that Nitrogen Dioxide and Methane tend to scatter longer wavelengths than molecular nitrogen and oxygen do? Hence why extraterrestrial skies are not always blue like Earths.

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    $\begingroup$ The size of the molecules has nothing to do with it. If I'm allowed to self-publicise, see the explanation on my blog post on rogerjbarlow.com $\endgroup$ – RogerJBarlow Jul 8 '18 at 21:16
  • $\begingroup$ Interesting @RogerJBarlow, but the fact is that the sky on other celestial bodies is NOT blue. They sky on Mars is pale yellow, and on Titan(which has a fairly thick atmosphere) it is deep orange during midday. How do you explain this? $\endgroup$ – SavedbyZer0 Jul 8 '18 at 21:24
  • $\begingroup$ @Roger You are allowed to cite your own work, we simply expect you to be up front about it (as you have been). And you could write an answer with that link in it, though you would be expected to provide a substantial enough taste here on physics to be some kind of answer on it's own (but of course it can also be less then you have already written so that it serves as a teaser). $\endgroup$ – dmckee Jul 9 '18 at 1:32
  • $\begingroup$ @RogerJBarlow I beg your pardon, but Rayleigh scattering occurs when the particles have sizes that are shorter than the wavelength of the incident light. When the particles are much larger, it is called geometric scattering. Which is why the sky turns deep orange-red during sandstorms. $\endgroup$ – Mr X Jul 23 at 5:26
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The blue sky on Earth is because of Rayleigh scattering, and because the particles in the air scatter blue light more then red.

This is the Tyndall effect, he discovered that when light passes through a fluid with particles in suspension, blue light is scattered more then red.

The amount of light scattered is inversely proportional to the fourth power of the wavelength.

This works if the particles are small. Much smaller then the wavelength.

The sky on titan is blue.

Please see here:

https://lightsinthedark.com/2011/12/18/the-colors-of-titans-sky/

You are right, that the size of the particles in the atmosphere does decide the color of the sky, because with larger particles, Rayleigh scattering is not dominant, but Mie scattering is.

On Mars, Rayleigh scattering is usually a very weak effect; the red color of the sky is caused by the presence of iron(III) oxide in the airborne dust particles. These particles are larger in size than gas molecules, so most of the light is scattered by Mie scattering. Dust absorbs blue light and scatters longer wavelengths (red, orange, yellow).

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  • $\begingroup$ Arpad, that photo you linked to is a photo of blue light scattering off of Titan's upper atmosphere & it was taken from space. This is what the sky looks like on the surface of Titan: esa.int/var/esa/storage/images/esa_multimedia/images/2005/01/… $\endgroup$ – Mr X Jul 23 at 4:38
  • $\begingroup$ Might I add: Titan's atmosphere is MUCH thicker than Earth's atmosphere. So the noonday sun on a clear Earth day is 3000x as bright as it is on Titan. This is partly due to the fact that unlike Earth, Titan's atmosphere is dense enough to absorb light whereas the Earth's atmosphere merely scatters light. $\endgroup$ – Mr X Jul 23 at 4:41

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