Ferrihydrite vs. hematite as a match for martian dust (IMAGE)
Caption
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.
The team of researchers created mixtures of ferrihydrite and hematite in the lab and measured how much they reflected light over a wavelength range of 500–840 nm. These plots show that the reflection of light by the ferrihydrite mixture (left graph) matches spacecraft observations of real martian dust much closer the hematite mixture (right graph) does.
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: Two graphs side by side comparing the lab-made ferrihydrite and hematite dust mixtures with measurements of real martian dust by spacecraft. The left graph compares the ferrihydrite mixture with spacecraft data, showing that this type of dust closely matches. The right graph compares the hematite mixture with spacecraft data, showing that this type of dust does not match so closely.]
Credit
A.Valantinas/ESA
Usage Restrictions
Credit must be given to the creator. Adaptations must be shared under the same terms.
License
CC BY-SA