On Mars, sunsets are blue

The photos from NASA's rovers show it plainly: on Mars, the daytime sky is a pinkish beige and, right around the setting sun, a cold blue halo appears, like a lamp at the far end of a dusty room.

The explanation lies in each planet's air. On Earth, the molecules of the atmosphere scatter mostly blue light: that's why the sky is blue at noon, and why, when the sun is low and its light crosses far more atmosphere, the blue gets lost along the way and the reds and oranges reach us.

Mars has hardly any atmosphere left, but a great deal of fine dust floats in it, just the right size to do the opposite: it deflects red light in every direction — hence that milky-tea sky — and lets the blue travel almost straight towards whoever is looking at the sun. The result: dawn and dusk there are blue.

If someone ever builds a house on Mars, its golden hour will be cold, serene and blue. A reminder that even sunsets are a local custom.