There are some great explanations on StackExchange of why we see a blue sky on Earth, eg here.
Namely, that it's a combination of :
- (a). the mix of colours from the Sun (eg, more green than indigo), plus
- (b). Rayleigh scattering from gas molecules (preferentially scattering shorter wavelengths, ie indigo/blue) plus
- (c). the sensitivity of our eyes (we see green best, and blue better than indigo).
And, that when the Sun is low and thus light going through ~2000km of atmosphere rather than ~10km (do the trig), Rayleigh scattering means there's very little blue light left, so the end effect is a reddish sky. (A little counterintuitive, but this is a good explanation of how that works.)
My question is why is sky red on Mars.
I've seen two explanations, both dependent on the fact that the much thinner Martian atmosphere means the effect of Rayleigh scattering of light by gas is dominated by the effect of airborne dust.
Explanation one: the airborne dust scatters out all the blue light, so that the sky we'd see is reddish for same reason our sunrise/sunset sky is reddish. (Because there's so much dust?)
Or, simply that because the dust is red (because the surface of Mars is red - ie, lots of iron oxide), we would see a reddish sky.
Can anyone help with this?