Why is Mars red?

Mars is immediately recognisable in the night sky by its red hue, but where does its iconic colour come from?

Martian dust is mostly rust!

Mars’s famed colour has captivated humankind for centuries, earning its nickname of the ‘Red Planet’. Romans named Mars for their god of war because its colour was reminiscent of blood, while Egyptians called it ‘Her Desher’, meaning ‘the red one’.

Thanks to the fleet of spacecraft that have studied Mars over the last decades we know that the red colour is due to iron minerals in the soil rusting. That is, iron bound up in the chemistry of Mars’s rocks has at some point reacted with water and oxygen in some form, just like everyday rust forms on Earth. Over billions of years this rusty material – iron oxide – has been eroded down into dust and spread all around the planet by winds, a process that continues today.

Exciting new research, published in February 2025, has shown that this rusty dust has a much wetter history than previously thought.

Because of the absence of liquid water on Mars’s surface today, its rusty red minerals were thought to arise from dry iron oxides present in the dust, such as hematite.

However, new analysis of spacecraft observations in combination with novel laboratory techniques suggests that Mars’s red colour is better matched by iron oxides containing water, known as ferrihydrite. Ferrihydrite typically forms quickly in the presence of cool water, and so must have formed early on ancient Mars when the planet was still wet. It has remained stable under present day conditions on Mars.

The stunning image of Mars featured here shows off the Red Planet’s renowned colour from the viewpoint of ESA’s Rosetta mission as it flew past on 24 February 2007, en route to Comet 67P/Churyumov-Gerasimenko. It is a composite image created by combining near-infrared, green and near-ultraviolet colour information obtained by the OSIRIS Narrow Angle Camera. The polar ice cap at the south pole is particularly bright, and wispy clouds are seen most clearly around the planet’s curved horizons.

Read more about how Mars got its iconic colour in Have we been wrong about why Mars is red?

[Image description: The full disc of Mars is seen with the polar ice caps slightly off centre to the top left and bottom right. Clouds wrap around the planet’s curved horizons. Dark surface markings are clearly seen against the characteristic red tones of the dusty martian surface.]