Not iron-oxide but Ferrihydrite: Scientists reveal new reason behind Mars’ iconic red color

Feb 27, 2025

For centuries, Mars has been known as the Red Planet, with its striking hue long believed to be the result of iron oxide—essentially rust—coating its surface. However, a groundbreaking study now suggests that a water-rich mineral called ferrihydrite may be the real reason behind Mars’ red dust, challenging long-held scientific assumptions.

Published in Nature Communications, the study by researchers from Brown University and the University of Bern combines advanced spacecraft data with laboratory simulations to propose a new theory: Mars’ red dust is better explained by ferrihydrite, an iron oxide mineral that forms rapidly in the presence of cool water.

READ: SpaceX to send first high-speed internet satellite into space in 2019 (November 20, 2017)

This discovery carries major implications. Ferrihydrite only forms in wet conditions, meaning that its presence suggests Mars once had significant water, reinforcing theories that the planet may have been habitable in its early history. On Earth, ferrihydrite is commonly found in weathered volcanic rocks and aquatic environments, unlike hematite—the previously suspected cause of Mars’ redness—which forms under drier conditions.

Experiments match data

Planetary scientist Adomas Valantinas and his team analyzed data from multiple Mars missions, including NASA’s Mars Reconnaissance Orbiter, ESA’s Mars Express, and the Trace Gas Orbiter. Ground-level measurements from NASA’s Curiosity, Pathfinder, and Opportunity rovers further confirmed the presence of ferrihydrite in Martian dust. By comparing spacecraft observations with laboratory simulations, the researchers identified ferrihydrite as the best spectral match for Mars’ red dust.

To validate their findings, the team replicated Martian dust in the lab using an advanced grinder to reduce ferrihydrite and basalt—a volcanic rock abundant on Mars—to submicron sizes, mimicking the planet’s fine dust. They then analyzed how light interacted with these particles, revealing that ferrihydrite, not hematite, best matches the spectral data collected by Mars orbiters and rovers.

“We were trying to create a replica Martian dust in the laboratory using different types of iron oxide,” Valantinas explained. “We found that ferrihydrite mixed with basalt best fits the minerals seen by spacecraft at Mars.”

The discovery of ferrihydrite on Mars suggests that Mars once had cool, wet environments where liquid water was stable. It also strengthens the case that Mars could have been a habitable world in its distant past. “Mars is still the Red Planet,” said Valantinas. “It’s just that our understanding of why Mars is red has been transformed. Because ferrihydrite could only have formed when water was still present, it means Mars rusted much earlier than we previously thought.”

While the new finding suggests that Mars transitioned from a wet world to a dry one billions of years ago, the pertinent question remains — how long did water persist on the surface and whether it had supported microbial life in the past.

While the study provides strong evidence for ferrihydrite’s role in Mars’ coloration, the ultimate confirmation will come when Martian samples are brought back to Earth for direct analysis. As NASA’s Perseverance rover is already collecting rock and dust samples, the upcoming Mars Sample Return mission of NASA and ESA is expected to throw the final verdict on Mars surface.

“Some of the samples already collected by Perseverance include dust,” said Colin Wilson, ESA’s Trace Gas Orbiter and Mars Express project scientist. “Once we get these precious samples into the lab, we’ll be able to measure exactly how much ferrihydrite the dust contains and what this means for our understanding of the history of water—and the possibility of life—on Mars.”

READ: Elon Musk’s SpaceX eyes $350 billion valuation, could surpass ByteDance (December 3, 2024)

Not exciting as moon’s Helium-3

Ferrihydrite, a water-formed iron oxide mineral found in soils, sediments, and iron-rich waters, plays a crucial role in volcanic rock weathering and iron cycling in aquatic systems. While not widely used in industry, it holds promise in emerging fields such as energy storage—particularly in lithium-ion batteries—and contributes to water purification and environmental cleanup efforts.

However, unlike the moon, Mars’ ferrihydrite is unlikely to drive future missions.

The moon holds a far more lucrative resource — Helium-3 (He-3), a rare non-radioactive isotope on Earth that could one day fuel nuclear fusion reactors. With an estimated 100 tons of He-3 potentially capable of powering Earth for a year, even a single ton — when fused with 0.67 tons of deuterium — could generate 1,000 MW of energy, enough to sustain global power needs annually.