Why Mars Is Red

Mars's crust is mostly basalt, a hardened volcanic rock rich in iron. The same basalt lies in Iceland and Hawaii, and it rusts too. The difference is that on Earth, rust is quickly hidden: water washes it away, vegetation covers it, and plate movement grinds it up and pulls it down. On Mars, there is nothing to clear it away, so the oxidized layer has remained on top.

The oxygen needed for this is not the kind you breathe: there is almost none in the Martian atmosphere. But water acts as an oxidizer - ultraviolet light splits its molecules, and the released oxygen binds to iron. Scientists still debate exactly how this happened: in the classic version, oxidation was very slow and dry, stretching over billions of years. But a 2025 study showed that the spectra of Martian dust are best explained by ferrihydrite - a mineral that forms in cold water. That would mean the main rusting happened early, while water was still present. The color does not change, only the date does.

Then the wind takes over. Rusty particles have been ground down to just a few micrometers - this is powder, not sand. Dust storms lift it up and spread it across the whole planet, sometimes covering the surface completely for weeks. That is why Mars is uniformly reddish: it does not have reddish regions, it has a reddish coating over everything.

Color is about what a surface does not reflect. Iron oxides eagerly absorb short wavelengths, the blue and green parts of light, while barely affecting long ones. From about 600 nanometers, reflectance rises sharply - what reaches your eye is the remaining warm light.

Still, "red" is a strong word. In person, Mars is more reddish ocher, closer to the color of rusty sheet metal; in bright images, processing often further boosts the shade. The same dust colors the sky: during the day on Mars, it is yellowish-brown, while around the setting sun it is bluish - exactly the opposite of ours.