system
Given the following post, respond as if you're commenting on Reddit:
human
Why is Mars sky opposite of Earth (red during day, blue at sunset/sunrise) ?
gpt
TL;DR:  Earth: Rayleigh scattering by gas molecules in thick atmosphere = Blue daytime sky, red(dish) sunset.  Mars: Mie scattering by fine dust suspended in thin atmosphere = Red(dish) daytime sky, blue sunset.

^(^.)

The Earth has a thick atmosphere composed of nearly 80% N₂ and 20% O₂.   As light passes through the Earth's atmosphere a portion of it is scattered in random directions by encounters with its gas molecules and atoms.  

This is known as **Rayleigh scattering**, which approximates how light is scattered by particles which are small relative to its wavelength.   The wavelength of visible light ranges from about 390-700 nm.  The molecular sizes of diatomic Nitrogen or Oxygen is about 150 pm,  3 orders of magnitude smaller.

There is strong wavelength dependence of Rayleigh scattering (~λ^−4) meaning that shorter (blue) wavelengths are scattered more strongly than longer (red) wavelengths.  This results in the indirect blue light coming from all regions of the sky.

At sunset, sunlight must travel a much longer path through the atmosphere to reach an observer.  Each meter of the atmosphere that the light passes through preferentially scatters more and more  shorter wavelength light, gradually reddening the light that ultimately reaches the sunset observer.

Because Mars has a much thinner atmosphere, the amount of Rayleigh scattering is much less.  But Mars also has a dry, dusty surface, and weaker surface gravity, so its atmosphere is often filled with fine dust particles.  Were it not for this dust,  the Martian  sky would be nearly black.  However, these dust grains are comparable in size to the wavelengths of visible light, so most of the light is scattered by **Mie scattering**. 

One of the main differences between Rayleigh and Mie scattering is that Rayleigh scattering tends to occur in all directions, while Mie scattering varies with scattering angle: longer wavelengths (reds) tend to scatter more uniformly, while shorter wavelengths (blues) tend to scatter at slight angles.   

Put simply, with Mie scattering, blue light tends to be deflected **less than** red light – the opposite of Rayleigh scattering on Earth. This means Mars can have a dusty reddish daytime sky, and a blue sunset.