Lab-made ‘martian dust’

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. This photo shows a mixture of ferrihydrite and basalt made in the lab as part of the research; the mixture was found to best match spacecraft observations of real martian dust.

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.

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

[Image description: A disc of orange-terracotta coloured dust fills the frame. The dust looks quite soft and fine, more like flour than sand.]