What makes Mars the 'Red' Planet? Scientists have some new ideas

Mars is widely known for its iconic rusty red color — many people even refer to it as just the "Red Planet" — but new research suggests the Martian shade isn't just lovely to look at. The chemistry behind Mars' rosy hue may actually hold important information about our cosmic neighbor.

For decades, spacecraft and rovers have gathered data pointing to a familiar explanation behind Mars' redness: the rusting of iron minerals, namely iron oxide, in the planet's dust. That's the same compound that gives your standard "rust" on Earth its red color.

Scientists already knew that on Mars, over billions of years, iron oxide has been ground into dust and carried across the planet by powerful winds, a process still shaping the Martian landscape today. However, not all iron oxides are the same, so experts have long debated the precise nature of Martian rust. Understanding how this rust formed offers a crucial glimpse into the planet’s past environment — was it once warm and wet, or always cold and dry? And, more importantly, did it ever support life?

"We were trying to create a replica Martian dust in the laboratory using different types of iron oxide," Adomas Valantinas, a postdoctoral researcher at Brown University, formerly at the University of Bern in Switzerland where he started his work with the European Space Agency's (ESA) Trace Gas Orbiter (TGO) data, said in a statement.

To recreate the Martian dust, the new study's research team used an advanced grinding machine to refine their samples such that they matched the fine, windblown particles found on Mars. The scientists then analyzed these ground-up samples using the same techniques as spacecraft orbiting Mars would, allowing for a direct comparison with real Martian data.

"This study is the result of the complementary datasets from the fleet of international missions exploring Mars from orbit and at ground level," Colin Wilson, the TGO and Mars Express project scientist, said in the statement.

What they found was that the best match for Mars' red dust is a combination of basaltic volcanic rock and a water-rich iron oxide called ferrihydrite.

This discovery is intriguing because ferrihydrite typically forms rapidly in the presence of cool water — meaning it must have originated when liquid water still existed on Mars' surface.

Even after billions of years of being ground into dust and scattered by Martian winds, ferrihydrite has retained its watery signature, offering a tantalizing clue about Mars' ancient past.

"The major implication is that because ferrihydrite could only have formed when water was still present on the surface, Mars rusted earlier than we previously thought," said Valantinas. "Moreover, the ferrihydrite remains stable under present-day conditions on Mars."

Data from NASA's Mars Reconnaissance Orbiter along with ground-based measurements from the Curiosity, Pathfinder and Opportunity rovers further support the identification of ferrihydrite. These observations provide crucial evidence that Mars’s red dust retains a signature of its watery past, reinforcing the idea that liquid water once played a key role in shaping the planet’s surface.

"We eagerly await the results from upcoming missions like ESA's Rosalind Franklin rover and the NASA-ESA Mars Sample Return, which will allow us to probe deeper into what makes Mars red," added Colin. "Some of the samples already collected by NASA's Perseverance rover and awaiting return to Earth include dust; 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 for life — on Mars."

"Mars is still the Red Planet," added Valantinas. "It’s just that our understanding of why Mars is red has been transformed."

A paper about these results was published on Feb. 25 in the journal Nature.