English: The Red Planet Mars got its iconic colour from a combination of rusting and erosion over its 4.6-billion-year history.
Mars was a once wetter place than the dry, barren world we know today. During its early history, iron in its rocks reacted with oxygen and water to create rust. The rust was washed into the rivers, lakes and seas that used to cover Mars, becoming incorporated into the underlying rocks. Volcanic activity could also have triggered ice-melting events, further contributing to this process.
Once Mars dried out, this rusty rock was broken down into dust over billions of years. Strong winds blew this dust all over the planet’s surface, gradually turning Mars red.
Signatures of the water-rich environment in which the rust formed are still preserved in the dust, as analysed by spacecraft studying Mars today.
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[Image description: Graphic showing how Mars turned from a grey, wet planet into a dusty red planet. From left to right, four steps are illustrated in a single image. First, iron in the planet’s rocks react with oxygen and water to create rust. Then the rust is washed into rivers, lakes and seas, and becomes incorporated in the underlying rocks. A volcano is also shown to represent a heat source that may have melted ice, further washing the rust into pools. Over billions of years the rusty rock is broken down into dust. Finally, winds blow the dust around the planet. A rover is shown on the surface, representing the direct analyses of this rusty dust. An orbiting spacecraft surveys the scene from above.]