Mars is covered in a fine dust that’s rich in iron oxides. You might know these compounds better as rust, the same stuff that forms when you leave an old bicycle out in the rain too long. On Earth, rust is a sign of decay, but on Mars, it’s a defining feature. Over billions of years, the iron in the planet’s rocks has reacted with oxygen — perhaps released from water that once flowed on its surface or trapped in minerals — and turned into Mars’ reddish coating.

Iron oxide has always explained why Mars is so red. But the exact nature of these minerals — and what they reveal about Mars’ history — has remained a mystery. Now, a new study suggests that the red dust covering Mars is dominated by a mineral called ferrihydrite, a poorly crystalline iron oxide that forms in cold, wet environments.

This is yet another piece of the puzzle that shows Mars hasn’t always been a dry, barren world. Rather, it had a more complex climatic past — one once rich in liquid water.

“Mars is still the Red Planet,” says Adomas Valantinas, the study’s lead author and a postdoctoral researcher at Brown University. “It’s just that our understanding of why Mars is red has been transformed.”

A New Shade of Red

For decades, scientists believed that Mars’ red color came from hematite, a type of iron oxide that forms under dry conditions. This theory aligned with the prevailing view of Mars as a planet that lost its water billions of years ago, leaving behind a desolate, rusted landscape. But Valantinas and his team have turned this idea on its head.

Using data from the European Space Agency’s Trace Gas Orbiter (TGO) and Mars Express missions, as well as NASA’s Mars Reconnaissance Orbiter and rover missions, the researchers combined spacecraft observations with laboratory experiments to recreate Martian dust. They ground volcanic basalt rock into fine particles, mimicking the dust grain size found on Mars — about 1/100th the width of a human hair. When they analyzed these samples, they found that ferrihydrite, not hematite, best matched the spectral signatures of Martian dust.

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

Ferrihydrite typically forms in the presence of cool water. This suggests that Mars’ rusting process began when the planet still had liquid water on its surface. Its presence on Mars suggests that the planet experienced periods of aqueous activity, even if those periods were brief.

A Cold and Wet Mars

The discovery of ferrihydrite on Mars has significant implications for our understanding of the planet’s climate history. On Earth, ferrihydrite forms when iron-rich water is exposed to oxygen, often in cold environments like glaciers or icy lakes. The mineral is metastable. This means it can transform into more crystalline forms like hematite or goethite over time, especially in warmer conditions.

But on Mars, ferrihydrite has remained largely unchanged. The researchers conducted laboratory experiments to simulate Martian conditions, exposing ferrihydrite to low pressures, UV radiation, and a carbon dioxide atmosphere. They found that the mineral retained its structure, even after 40 days of exposure. This suggests that Mars’ cold, dry environment has preserved ferrihydrite for billions of years.

The researchers propose that the mineral formed during a period of cold, wet conditions, possibly during the late Hesperian epoch, around 3 billion years ago. This period was marked by intense volcanic activity, which could have melted surface ice and created temporary lakes or streams. As the water evaporated or froze, ferrihydrite would have precipitated out, eventually becoming part of the planet’s ubiquitous dust.

What This Means for Life on Mars

Water is a key ingredient for life as we know it, and the presence of ferrihydrite suggests that Mars had liquid water, at least intermittently, in its past. While the mineral itself is not a biosignature, its formation in cold, wet conditions raises the possibility that Mars could have been habitable at some point in its history.

This doesn’t mean we’ve found evidence of life. But it does mean that the conditions for life could have existed. It’s another piece of the puzzle. Recent studies have suggested that the planet may have had large oceans, rivers, and even beaches. 

But the story of Mars’ red dust is far from complete. While the new study provides compelling evidence for ferrihydrite, definitive proof will require analyzing actual Martian samples. NASA’s Perseverance rover has already collected dust and rock samples that are awaiting return to Earth as part of the Mars Sample Return mission, a joint effort by NASA and the European Space Agency.

“Once we get these precious samples into the lab, we’ll be able to measure exactly how much ferrihydrite the dust contains,” says Colin Wilson, ESA’s TGO and Mars Express project scientist. “This will help us understand the history of water — and the possibility for life — on Mars.”

The findings appeared in the journal Nature Communications.