THE SKY ON MARS

Since the first NASA landers arrived on Mars, it’s been known that the sky on that planet has a predominantly red tint. This is also due to atmospheric scattering, but not of the Rayleigh kind. Rayleigh scattering is produced by air molecules, which are smaller than the wavelength of light, and these are less important in the thin atmosphere of Mars than wind-blown dust. The latter is made up of much larger particles, and for these a different scattering mechanism comes into play, resulting in the distinctive reddish-orange colour of the Martian sky.

This photograph from Opportunity shows the characteristic orange-red colour of the Martian sky