Science Puzzle
Why Is Mars Red?
Mars has been called the Red Planet for thousands of years, because it shines with an orange-red glow in the night sky. Close-up photographs from orbiters and rovers show a reddish-brown landscape of dusty plains and rocks. Yet Mars is cold, with an average temperature of about -60 °C.
What gives Mars its red colour?
The Answer
Rust. The surface of Mars is coated in fine dust rich in iron oxides: the same sort of reddish-brown compounds that form on an iron nail left out in the rain.
Mars's rocks contain plenty of iron. Over billions of years that iron has reacted with oxygen, much of it during a period when the planet was wetter than it is today, producing iron oxides. Wind and rock grinding against rock broke the rusty material into dust finer than flour, and huge dust storms, some big enough to cover the whole planet, spread it everywhere. Underneath the dust, many Martian rocks are actually dark grey or black.
For a long time scientists thought the red came mostly from haematite, a dry form of iron oxide. A study published in 2025 suggested that much of it is ferrihydrite instead, a rust that forms quickly in cool water. If that's right, Mars rusted largely while it still had liquid water on its surface, which matters for the question of whether it could once have supported life.
The dust even colours the Martian sky. Rover cameras show a butterscotch daytime sky and, oddly, bluish sunsets, the opposite of Earth's.
The principle: Oxidation. Iron combines with oxygen to form reddish-brown iron oxides, and a planet covered in rusted, iron-rich dust looks red.