A rusty planet
Why Is Mars Red?
Mars is red for the same reason an old nail is: iron plus oxygen equals rust.
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Step 1 of 5
Mars is covered in fine dust.
The surface is not solid red rock. It is coated in a layer of powder as fine as talcum, blown everywhere by planet-wide dust storms.
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Step 2 of 5
That dust is full of iron.
Mars's rock formed rich in iron — the same metal in a bicycle frame or a nail. On Mars, it is spread through the surface everywhere.
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Step 3 of 5
Iron plus oxygen makes rust.
The same chemistry as a rusty nail on Earth. When iron meets oxygen, the two lock together into a new substance called iron oxide. Iron oxide is orange-red. That is what rust is.
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Step 4 of 5
The whole surface did this, long ago.
Mars once had more water and more oxygen. Over billions of years the iron in its dust rusted, and the planet turned the colour of an old nail.
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Step 5 of 5
You can see the colour from your garden.
Mars is bright enough to spot without a telescope, and the orange tint is visible to the naked eye. You are looking at rust from 200 million kilometres away.
Rovers on Mars come home covered in the stuff — their solar panels slowly go orange.
The short version
Mars is red because its iron-rich dust rusted, exactly like a nail left out in the rain.
Try it yourself
Leave a steel nail in a saucer of water for a week. The orange powder that appears is the same substance as the Martian surface.
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