Mars Is Red Because of Rust

For millennia, humans have gazed at the sky and seen a distinct, fiery point of light. The ancient Romans, associating its blood-red color with conflict, named it for their god of war. This iconic hue is the result of a vast, planet-wide chemical reaction. The Martian (Review) surface is rich in iron-bearing minerals, and over billions of years, this iron has oxidized, creating a fine layer of what we commonly call rust.

The process required ingredients that were once more plentiful on ancient Mars: water and a different atmosphere. In the planet's distant past, liquid water flowed across the surface, and a thicker atmosphere held more oxygen. This environment allowed the iron in the volcanic basalt rock to slowly "rust." Scientists theorize that much of this oxygen may have been released as ultraviolet radiation from the sun split water molecules apart, freeing the oxygen to react with the iron-rich dust and rock.

This rusty coating is surprisingly superficial. It's not that the planet is a solid red rock, but rather that it's covered in a layer of incredibly fine, reddish dust, almost like talcum powder. This dust is then whipped up and distributed globally by some of the most powerful dust storms in the solar system. These massive storms can blanket the entire planet for weeks at a time, ensuring that the Red Planet lives up to its name by constantly re-coating its surface in a thin veneer of rust.