Why Mars Is Red: Rust Dust, and a New Twist on Which Rust

Rust on a planetary scale

The short answer is rust. NASA states that iron minerals in the Martian dirt oxidize, or rust, and that this makes the surface look red.^[1] The color does not stay on the ground. Winds lift the fine dust into the atmosphere, and from a distance it makes the planet appear mostly red.^[1] That is why Mars shows up in the night sky as a bright red point of light.^[5]

Iron is no rarity on the planet. NASA says Mars has a dense core, 1,500 to 2,100 kilometers in radius, made of iron, nickel and sulfur, inside a world about half the size of Earth.^[1] The red coloring, however, belongs to the surface materials, which are exposed to a thin atmosphere of carbon dioxide, nitrogen and argon and to temperatures that range from about 20 degrees Celsius down to about minus 153.^[1]

Dust storms spread the effect around. NASA notes that Martian winds are sometimes strong enough to raise storms covering much of the planet, and that it can take months for all of the dust to settle afterward.^[1]

Which rust? A newer answer

For a long time, the favored culprit was hematite, a dry, rust-like mineral that forms under warmer, drier conditions. A 2025 paper in Nature Communications challenged that idea. Its authors reported that poorly crystalline ferrihydrite is the dominant iron oxide phase in Martian dust and matches the planet's ochre color best in spectral data.^[2]

Ferrihydrite is a water-rich iron oxide that forms in cool, wet environments.^[3] According to the paper, the dust grains are smaller than a micrometer and contain roughly 20 to 33 percent ferrihydrite by weight, mixed with basalt and sulfates.^[2]

The team, from Brown University and the University of Bern, combined observations from orbiters (NASA's Mars Reconnaissance Orbiter, ESA's Mars Express and Trace Gas Orbiter), measurements from rovers, and laboratory experiments with simulated dust at submicron sizes.^[3] Lead researcher Adomas Valantinas said that others had considered ferrihydrite before, but that it had "never been proven the way we proved it now."^[3]

What the color says about the past

The mineral matters because of how it forms. The study argues that ferrihydrite records a cold, wet period on early Mars under oxidative conditions, before the planet shifted to the hyper-arid state seen today.^[2] Researchers quoted by SETI also pointed to canyons and clays as evidence that liquid water was once abundant.^[4]

The result is not final. Direct confirmation would require Martian samples brought back to Earth for analysis.^[4] One press account says the answer awaits the return of samples now being collected by NASA's Perseverance rover.^[3] Until then, the safe summary is that rusting iron makes Mars red, while the exact mineral remains under debate.