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Why is Mars Red?

Look at Mars in the night sky, and you’ll see a distinctive red glow. Through a telescope, it’s even more clear. And when both rovers and probes send back images of an alien red surface, we know for certain: Mars, for whatever reason, is red. So what’s behind this unique coloration?

1. Mars is covered in iron.
When the planets formed, dust and debris containing a variety of metals was swirling around the early solar system and became the materials that make up the inner, rocky planets. Mars and Earth both got a good amount of iron, but while Earth’s iron sunk down into the core, much of the iron on Mars stayed on the surface. That iron is now a fine, dusty powder.

2. When iron oxidizes, it rusts.
Iron isn’t usually red, though, so what is it that changes the color of the surface materials? The answer is so commonplace, you experience it all the time on Earth: rust. Rust is formed when iron, water vapor, and oxygen combine to form iron oxide (the technical term for rust). Rust is a reddish color, and clearly there’s enough of it on Mars to paint the entire landscape the same shade.

3. But how did it oxidize?
Today, there’s very little oxygen in Mars’ atmosphere, and the only water is at the polar caps. So, what caused the iron to oxidize and turn to rust? We don’t know for sure, but chances are there was a lot more oxygen in Mars’ early history, and that oxygen contributed to all the rusting. There’s a chance that the vast majority of Mars’ oxygen was used to rust the surface, and that’s where most of it went. With more research, we just might know for sure.