Why is Mars Red?

Iron oxide-hydroxide minerals in Martian dust provide important clues into the planet’s past climate and potential for life. Previous research suggested that Mars’ red color comes from anhydrous hematite formed by recent weathering.

However, Adomas Valantinas, University of Bern, Switzerland, and Brown University, Providence, RI, USA, and colleagues have identified ferrihydrite (Fe₅O₈H · nH₂O)—a poorly crystalline, hydrated iron oxide mineral—as the dominant iron-bearing phase in Martian dust.

Their study, which combines visible near-infrared (VNIR) spectral analysis, orbital and in-situ observations, and laboratory studies of submicron ferrihydrite-basalt mixtures, shows that a fine-grained mix of ferrihydrite, basalt, and sulfate best matches Mars’ color. Ferrihydrite remains stable under simulated present-day Martian conditions (UV irradiation, 6 mbar pressure, CO₂ atmosphere) and does not transform into other iron oxide-hydroxide phases. Its persistence suggests it formed during a cold, wet period under oxidative conditions in Mars’ late early history, before the planet transitioned to its current hyper-arid state. This challenges previous models of continuous dry oxidation, indicating that ancient Mars underwent aqueous alteration before becoming a desert.

The upcoming Mars Sample Return (MSR) mission may bring regolith samples to Earth that potentially contain ferrihydrite-bearing materials. Detailed analyses, including Fe, H, and O stable isotope measurements, could provide crucial insights into Mars’ redox conditions, past water activity, and habitability, the researchers suggest.

Detection of ferrihydrite in Martian red dust records ancient cold and wet conditions on Mars,
Adomas Valantinas, John F. Mustard, Vincent Chevrier, Nicolas Mangold, Janice L. Bishop, Antoine Pommerol, Pierre Beck, Olivier Poch, Daniel M. Applin, Edward A. Cloutis, Takahiro Hiroi, Kevin Robertson, Sebastian Pérez-López, Rafael Ottersberg, Geronimo L. Villanueva, Aurélien Stcherbinine, Manish R. Patel. Nicolas Thomas,
Nature Communications 2025.
https://doi.org/10.1038/s41467-025-56970-z