Why is Mars red?

A rusty world

Look up on a clear night and you might spot Mars. It looks like a bright orange-red star. People have noticed its colour for thousands of years. The ancient Romans thought it looked like fire and blood, so they named it after Mars, their god of war. Today we often just call it the Red Planet.

So what makes it red? The answer is rust. Mars is covered in rocks and dust that contain lots of iron. When iron meets oxygen, it slowly turns into a reddish-brown material called iron oxide. That is exactly what happens to an old bike or a nail left out in the rain here on Earth.

Over billions of years, wind has ground this rusty rock into fine dust and blown it all over the planet. The dust even hangs in the air. From the ground, the daytime sky on Mars looks hazy yellow-orange instead of blue.

A clue hidden in the dust

For a long time, scientists thought the red dust was a dry kind of rust called hematite. But in 2025, a team led by researchers at Brown University in the United States and the University of Bern in Switzerland took a closer look. They mixed up copies of Mars dust in a lab and compared them with what spacecraft see on Mars.

The best match was a different rusty mineral called ferrihydrite. Here is the exciting part: ferrihydrite forms quickly in cool water. That means Mars probably rusted long ago, when it still had rivers and lakes on its surface.

Other clues agree. Rovers have found dried-up riverbeds and minerals that only form in water. The Perseverance rover is exploring Jezero Crater, where a river once spilled over the crater wall and filled it with a lake, more than 3.5 billion years ago. Today, most of Mars's water is frozen as ice under the ground near its poles, with a little in thin clouds.

Smaller, colder and farther away

Mars is the fourth planet from the Sun, one place further out than Earth. It is about 228 million kilometres from the Sun, around one and a half times as far as we are. That makes it a cold place. The average temperature is about minus 60 degrees Celsius, colder than the inside of your freezer.

Mars is about half as wide as Earth. A day on Mars, called a sol, is 24.6 hours long, only a bit longer than ours. But a year on Mars takes 687 Earth days, almost two of our years, because Mars has a much longer journey around the Sun.

The air, or atmosphere, is very thin and is mostly carbon dioxide, so people could not breathe it. Sometimes the wind whips up dust storms so big that they cover the whole planet. Mars also has two small, lumpy moons, Phobos and Deimos. Phobos is only about 27 kilometres long.

Giant mountains and canyons

Mars has the tallest volcano we know of anywhere in the solar system. Olympus Mons rises more than 20 kilometres above the plains around it, well over twice the height of Mount Everest. Its base is nearly 600 kilometres wide, but its slopes are so gentle that you could walk up them without noticing much of a climb.

Not far away is a canyon so long it would stretch most of the way across Australia. Valles Marineris is more than 3,000 kilometres long and up to 8 kilometres deep. Olympus Mons and its giant neighbours are the round bumps on the left of the photo above.

Robots on Mars, and a big dish near Canberra

Mars is the only planet we have landed robot rovers on. Two are working there right now. Curiosity has been exploring since 2012. Perseverance landed on 18 February 2021 and is collecting rock samples and looking for signs that tiny living things once lived on Mars. It even brought a little helicopter, Ingenuity, which made 72 flights, the first aircraft ever to fly on another planet.

When a rover takes a photo, the picture has to travel across space as a radio signal. NASA catches these signals with three giant dish stations spaced around the world: in California, in Spain, and at Tidbinbilla, just outside Canberra. The Canberra station is run by CSIRO for NASA, and its biggest dish is 70 metres across. As the Earth turns, each station takes its turn, so Canberra's dishes spend part of every day listening to Mars.

📚 Words to know

Iron oxide

The scientific name for rust. It forms when iron joins with oxygen, and it is what makes Mars red.

Atmosphere

The layer of gases around a planet. Mars's atmosphere is thin and mostly carbon dioxide.

Sol

One day on Mars, 24.6 hours long.

Rover

A robot vehicle with wheels that explores the surface of another world and sends pictures home.

Crater

A bowl-shaped hollow made when a space rock crashes into a planet or moon.

🧪 Try this: grow your own Mars rust

1. Ask an adult to help. You will need two small jars, a piece of plain steel wool (the kind without soap), some water and a pinch of salt.
2. Tear the steel wool in half. Put one half in a dry jar and leave the lid off. Put the other half in the second jar, sprinkle in the salt and add just enough water to make it damp, not soaking.
3. Leave both jars somewhere safe for two or three days and look at them each morning. The damp steel wool turns orange-brown. It is rusting: the iron in the steel is joining with oxygen from the air, with help from the water.
4. The dry wool stays shiny for much longer. That is why matching Mars dust to ferrihydrite matters: this kind of rust needs water, so Mars was probably wet when it formed. Wash your hands afterwards, and do not pick up the rusty wool with bare fingers, because it can be sharp.