Why Is Mars Red? The Rusty Secret of the Red Planet

Glowing orange-red in the night sky, Mars has caught people’s attention for thousands of years. The Romans named it after their god of war, and today we know it as the “Red Planet”. But where does the colour come from? The answer is something you have seen on an old knife or a bicycle chain: rust.

A planet-wide layer of rust

The surface of Mars is covered by an extremely fine layer of dust, usually just millimetres to a few centimetres thick. The dust is as fine as flour and rich in iron oxides.

Iron oxides are compounds of iron and oxygen. On Earth, the most familiar example is the reddish-brown rust that forms on iron left in the rain. Iron oxides reflect red and orange light while absorbing much of the blue and green, so when we look at Mars we see a reddish colour.

Where did the iron come from — and how did it rust?

Like Earth, Mars is a rocky planet, and much of its crust is volcanic basalt, which is rich in iron. When the planet was young and active, giant volcanoes brought this rock to the surface. Olympus Mons, the largest volcano in the Solar System, stands about 22 kilometres tall — more than two and a half times the height of Everest.

For iron in rock to turn into rust, it needs oxygen and usually water. Scientists discuss several possible routes:

• Rusting with water: early in its history Mars had rivers, lakes and perhaps an ocean on its surface. Water slowly oxidised the iron in the rocks.
• Reactions with the atmosphere: ultraviolet light from the Sun splits water vapour and carbon dioxide in the air, producing oxidising chemicals that may have slowly rusted iron over billions of years.
• Crumbling and spreading: meteorite impacts, temperature swings and wind ground the rocks into dust, and winds spread that dust across the planet.

The surprise of 2025: water hidden in the rust

For many years the main mineral thought to give Martian dust its colour was hematite, a dry iron oxide that does not need much water to form. That supported the idea that Mars rusted in a largely dry environment.

In 2025, researchers from Brown University and the University of Bern compared data from orbiting spacecraft and surface rovers with laboratory experiments. The result was striking: the mineral that best explained the colour of Martian dust was ferrihydrite — an iron oxide that contains water in its structure.

Ferrihydrite usually forms quickly where cold water is present. The finding suggests that:

• Mars may have rusted while water still existed on its surface;
• that period may have had a cold but wet climate;
• once formed, the ferrihydrite has survived for billions of years in today’s dry conditions.

The conclusion is not final and needs testing with new measurements. Dust and rock samples planned to be brought back from Mars could answer the question directly.

What colour is the Martian sky?

The dust colours not just the ground but the air as well. The Martian atmosphere is very thin, but fine dust particles are always suspended in it, and they scatter light differently from the gas molecules in Earth’s air:

• By day, the sky looks reddish-brown or “butterscotch”, because the dust scatters red light in all directions while absorbing some of the blue.
• At sunset, a bluish glow surrounds the Sun. The dust scatters blue light most strongly in the forward direction — close around the Sun — and the effect becomes obvious when the Sun is low.

That is the exact reverse of Earth, where the sky is blue and sunsets red. For the details, see our article Why is the sky blue?.

Dust storms that wrap the planet

Mars has no rain and no plants, so dust cannot be held down and is easily lifted by the wind. Local dust storms are frequent, and every few Martian years a global dust storm shrouds the whole planet for weeks.

The global storm of 2018 darkened the sky so much that the solar-powered Opportunity rover ran out of power. After almost 15 years of work, it never sent another signal. During storms, dust is redistributed in layers all over the planet — which is why Mars looks a similar shade of red everywhere.

Mars versus Earth

Mars resembles Earth in many ways, but it is a much smaller and colder world:

Although a Martian day is very close to ours, the thin atmosphere cannot hold heat. On a summer afternoon at the equator it can reach 20 °C, then drop below −70 °C the same night.

What lies beneath the red?

The redness of Mars is really a surface “paint”. When NASA’s rovers drill into rocks or dig into the soil with their wheels, they expose dark grey and blackish rock beneath. In some places, pale clay and salt minerals formed by water have also been found under the dust.

In other words, the colour we see from space is not the colour of the whole planet, but of a thin coat of rust covering it.

In short

Mars is red because of the fine iron-oxide dust coating its surface. Billions of years ago, iron in volcanic rock reacted with water and the atmosphere, rusted, crumbled and was spread around the planet by the wind. The 2025 findings suggest this rust formed while the planet was still cold and wet. The Red Planet’s colour may be a memento of its watery past.