Mars is famous for its red hue, which comes from rusted iron minerals in its dust. These minerals are formed when iron reacts with water or oxygen, just like rust on Earth.
Over billions of years, this iron oxide has been broken down into dust and spread across the planet by winds. Different types of iron oxides exist, and scientists have debated the exact chemistry of Martian rust because it provides clues about Mars’s past environmental conditions and potential habitability.
Based on spacecraft observations, previous studies didn’t find water in Martian dust, leading researchers to believe the iron oxide was hematite, formed under dry conditions after Mars’s early wet period.
However, new research combining data from ESA and NASA spacecraft with lab experiments on replica Mars dust has revealed that Mars’s red color is better matched by ferrihydrite, an iron oxide containing water. This suggests Mars rusted early in its history when liquid water was more abundant. Ferrihydrite has retained its watery signature despite being spread around the planet over time.
Lead author Adomas Valantinas and his team confirmed ferrihydrite as the best match through lab experiments and space mission data. They created a replica of Martian dust using an advanced grinder to match the tiny dust grain size (1/100th of a human hair) and analyzed it using the same techniques as spacecraft.
Colin Wilson from ESA explains that Mars Express’s analysis found water-rich minerals in dusty regions. Thanks to TGO’s unique orbit, the team could accurately analyze dust particle size and composition.
Data from NASA’s Mars Reconnaissance Orbiter and ground-based measurements from Curiosity, Pathfinder, and Opportunity rovers supported the presence of ferrihydrite.
Upcoming missions like ESA’s Rosalind Franklin rover and NASA-ESA Mars Sample Return will help further understand why Mars is red. Samples collected by NASA’s Perseverance rover include dust that will help measure ferrihydrite content and learn more about Mars’s water history and potential for life.
Mars’ red hue will continue to intrigue scientists, offering new insights into the planet’s past and its potential for habitability.
Journal Reference:
- Valantinas, A., Mustard, J.F., Chevrier, V. et al. Detection of ferrihydrite in Martian red dust records ancient cold and wet conditions on Mars. Nat Commun 16, 1712 (2025). DOI: 10.1038/s41467-025-56970-z
