Why is Mars red? Scientists may have been wrong all along

For centuries, Mars has captivated scientists and the public alike with its striking red hue. Early astronomers speculated that the planet’s color came from iron-rich minerals, much like rust on Earth. Today, thanks to a fleet of orbiters and rovers, researchers are closer than ever to understanding what gives Mars its iconic appearance—and what that might reveal about its past.

Iron oxides play a crucial role in the reddish dust covering the Martian surface. On Earth, iron minerals form under different environmental conditions.

Ferrihydrite, an iron oxide mineral, develops in water-rich settings, while hematite, a more crystalline iron oxide, forms in drier, warmer conditions. Understanding which mineral dominates Martian dust can provide insights into the planet’s past climate, water availability, and habitability.

For decades, scientists believed that hematite was responsible for Mars’ red color. Early spacecraft observations suggested the presence of nanophase hematite dispersed throughout the regolith.

Spectral analysis from the European Space Agency’s (ESA) OMEGA spectrometer and NASA’s Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) indicated the presence of ferric oxides, possibly hematite or maghemite, in the planet’s dust. However, recent research challenges this assumption, pointing instead to ferrihydrite as the primary component.

A recent study published in Nature Communications suggests that Mars' reddish dust is dominated by ferrihydrite, a mineral that forms in the presence of cool water.

Led by researchers from Brown University and the University of Bern, the study combines data from Martian orbiters, rovers, and laboratory simulations to build a more accurate model of the planet’s dust composition.

“The fundamental question of why Mars is red has been thought of for hundreds, if not thousands, of years,” said Adomas Valantinas, a postdoctoral researcher at Brown University. “From our analysis, we believe ferrihydrite is everywhere in the dust and also probably in the rock formations.”

Unlike hematite, which forms in arid conditions, ferrihydrite requires water. This suggests that Mars may have experienced a long period of surface water activity before transitioning to its current dry, cold state.

If ferrihydrite formed in ancient Mars’ dust, it implies that the planet rusted much earlier than previously believed—likely when liquid water was still abundant on its surface.

To reach this conclusion, researchers analyzed data from multiple Mars missions. NASA’s Mars Reconnaissance Orbiter, ESA’s Mars Express, and the Trace Gas Orbiter provided detailed spectral data, while rovers such as Curiosity, Pathfinder, and Opportunity delivered ground-based measurements. The study also included laboratory experiments simulating Martian conditions.

One critical finding came from Mars Science Laboratory’s ChemCam instrument, which detected hydrogen in the dust, suggesting water is chemically bound within the particles.

The Alpha Particle X-ray Spectrometer (APXS) revealed that Martian dust shares a composition similar to the planet’s basaltic crust but is enriched with sulfur, chlorine, and iron, hinting at past chemical alterations involving water.

The research team recreated Martian dust in the lab by grinding ferrihydrite and basalt into particles just 1/100th the width of a human hair. When they analyzed the light reflected by these samples, they found a spectral match to what spacecraft observed on Mars.

“We used an advanced grinder machine to simulate the fine-grain size of Martian dust,” Valantinas explained. “The reflected light spectra of our mixtures provide a good match to the observations from orbit and the red surface on Mars.”

If ferrihydrite dominates Martian dust, it suggests that Mars had sustained water activity before its current arid phase. The presence of ferrihydrite indicates conditions where oxygen and water interacted with iron, an environment potentially suitable for microbial life.

“What we want to understand is the ancient Martian climate, the chemical processes on Mars—not only ancient but also present,” Valantinas said. “Then there’s the habitability question: Was there ever life? To answer that, we need to understand the conditions present during mineral formation.”

While these findings offer strong evidence for a wetter past, researchers emphasize that confirmation will come when Martian samples are analyzed directly on Earth. NASA’s Perseverance rover is currently collecting samples for future return missions.

“The study is a door-opening opportunity,” said planetary scientist Jack Mustard, a co-author of the research. “Once we get those samples back, we can test our theories directly.”

For now, the mystery of Mars’ red dust continues to intrigue. While spacecraft observations and lab experiments suggest that ferrihydrite holds the key to the planet’s color, only future sample return missions will provide definitive proof.

Until then, Mars will remain a tantalizing puzzle, its red hue a visible reminder of a planet that may have once been much more Earth-like than it appears today.

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