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The sky on Mars is sort of a butterscotch/rust color. The sky on Earth is blue because gas molecules scatter blue sunlight better than red sunlight, so the blue light fills the sky. On Mars, there is much less of an atmosphere—the surface pressure is less than 1% of Earth’s. So, the sky would be a very dark blue, with stars shining, if the atmosphere were clear. So far, though, every Mars landing has found a dusty atmosphere. The dust may be some sort of a clay-like mineral, but the color is because it contains small amounts of oxidized iron. And oxidized iron is a fancy way of saying rust. We know Mars has global and regional dust storms, but even away from the dust storms there are dust devils that throw dust into the sky. So, away from a dust storm, there is not a lot of dust in the sky, but there seems to be enough to keep the sky a bright rusty color.
An interesting thing about the sky’s appearance is related to the sunset. On Earth, sunsets are red and orange because all of the blue light has been scattered away from the Sun into the sky. On Mars, all colors are scattered away, but a lot of the blue light is absorbed by the dust (making the sky yellowish-red). The blue light that is scattered stays close to the Sun, and the result is a blue sunset in a rusty sky, instead of Earth’s rusty sunset in a blue sky.