18 "Two worlds, one Sun" - sunset on Earth and on Mars.On Earth, our atmosphere scatters the blue of the Sun's light more than the red. So the sky directly overhead is blue, but when sunlight travels the long path along the horizon, it's mostly the red light that's left. Mars has very little atmosphere, but what it has is dusty. At sunset on Mars, sunlight is traveling through more fine dust particles. These particles scatter the red light more than the blue.
[Credit: NASA / JPL-Caltech / MSSS / Damia Bouic][ Brian Koberlein]