A new hypothesis has been proposed to explain why Mars has a reddish hue.

Mars is easily recognizable in the night sky due to its distinctly bright red hue. Thanks to spacecraft that have studied the planet over the past few decades, scientists have discovered that this color is caused by rust—particles of iron oxides found in the dust.

The surface of Mars is rich in iron-containing minerals. At some point in time, iron reacted with liquid water or oxygen and oxidized. As a result, iron oxides were formed, which have a reddish-orange color. Rust forms in a similar manner on Earth.

Over billions of years, they have been ground down and dispersed by the winds across the surface. When dust rises into the atmosphere, the iron oxide particles scatter light, enhancing the planet's reddish glow.

However, there are different types of iron oxides. Scientists are still debating the exact chemical composition of Martian rust. If researchers can determine it, they will be able to understand what conditions existed on Mars in the past and whether the planet could have been suitable for life.

In previous scientific studies, researchers analyzed data on Martian dust obtained from spacecraft and concluded that its main "red" component is the iron mineral hematite (Fe₂O₃). The specialists found no signs of water in it, leading them to believe it formed during a "dry" phase in Mars' history, when the planet had already lost its water. The researchers concluded that hematite formed when iron came into contact with oxygen, suggesting that Mars began to "rust" after the wet period.

An international team of cosmo-geologists led by Adomas Valantinas (Adomas Valantinas) from Brown University (USA) discovered that a completely different iron mineral accounts for Mars' red hue.

Initially, Valantinas and his colleagues conducted an experiment to recreate Martian conditions in the laboratory. They ground volcanic basalt to a size 1/100th the thickness of a human hair (which is the texture of Martian dust) and mixed it with various types of iron oxides.

By comparing the spectral data of the laboratory samples with observations from orbital missions (TGO, Mars Express, Mars Reconnaissance Orbiter) and rovers (Curiosity, Pathfinder, and Opportunity), the scientists found that the spectral "fingerprints" of the dust most closely matched ferrihydrite ((Fe3+)2O3·0.5H2O).

This mineral forms quickly when iron comes into contact with cold water and retains traces of H2O in its structure even after drying. During the formation of the mineral, H2O molecules become "embedded" in its crystalline lattice, becoming part of its chemical structure. Even when external water evaporates, these molecules remain "trapped" inside the mineral—as if in a cage.

Mars remains the Red Planet, but scientists now have a better understanding of why. It was previously thought that its color was associated with hematite, but it turns out that another mineral—ferrihydrite—plays the primary role in this. Since ferrihydrite could only have formed during a wet Martian period, rust on the planet emerged much earlier than researchers had assumed. Moreover, ferrihydrite does not decompose under present-day Martian conditions (cold, dry, no water): it has persisted, and the planet retains its characteristic hue even after billions of years.

In any case, definitive answers regarding the cause of the planet's red color will come from soil samples collected by the Perseverance rover. These will be brought back to Earth after 2030, and analysis will reveal the precise composition of the dust.

The scientific work of Valantinas' team has been published in the journal Nature Communications.