A sunset on Mars is blue

The Martian daytime sky is butterscotch. Around the setting sun it turns blue. Fine dust scatters the red light out of the beam and lets the blue continue forward.

On Earth the molecules of the air scatter blue light in every direction, so the sky is blue, and the setting sun, seen through a long path of that air, is what remains: red. Mars has a thin carbon-dioxide atmosphere that would do little of this, and a load of fine dust that does something else. Particles about the size of the wavelength of light scatter red strongly off to the side and scatter blue preferentially forward, into the line of sight. Look near the sun and the forward-scattered blue is what you see. Look elsewhere in the day and the sky is the red that was thrown aside, a salmon or butterscotch dome.

Landers have photographed it since Viking, and again from Pathfinder and Curiosity: a pale blue glow hugging the sun as it sits on the rim of a crater, and a sky the colour of rust above. Dust storms thicken the effect or hide it. The blue is a band around the sun, not a blue vault, and it is not a sign of oceans. It is the spectrum of the dirt, sorted by angle.