Ferrihydrite in Martian dust may explain red planet’s colour – and hint at water-rich past

Mars’s distinctive red hue has long been attributed to haematite, a rust-like iron mineral, formed under dry atmospheric conditions. New research now points to ferrihydrite, a hydrated iron oxide, that forms in aqueous conditions, challenging conventional beliefs and hinting at a wetter Martian past.¹

Led by Adomas Valantinas, a planetary geologist at Brown University, US, a team of planetary scientists collated data from multiple Mars orbiters, rovers and used laboratory simulations to suggest that ferrihydrite could be widespread in Martian dust. ‘There was this idea that haematite is forming slowly by minimal, gas–solid interactions on the surface over billions of years, but we found ferrihydrite which needs brief interactions and rapid kinetics to form, requiring liquid water,’ explains Valantinas. ‘This told us that the ferrihydrite must have formed early on when there was liquid water. It’s not a modern product, but an ancient one.’