For decades, scientists believed that Mars’s famous red hue came mainly from hematite, a dry iron oxide mineral. However, a recent study challenges this long-held view and points to a different mineral as the key to understanding Mars’s rusty surface color.
This new discovery not only sheds light on why Mars looks the way it does, but also suggests the Red Planet may have once been much wetter and colder than previously thought.
What Is Ferrihydrite?
Ferrihydrite is a water-rich iron oxide mineral that forms in the presence of cool, liquid water. Unlike hematite, which develops under dry and oxidizing conditions, ferrihydrite requires an aqueous environment to form. Its presence on the Red Planet hints at periods in the planet’s ancient history when water was abundant enough to create and preserve this mineral. This contradicts earlier ideas that Mars’s red dust was mostly shaped by dry oxidation processes alone.
According to BBC Sky Night Magazine, scientists describe ferrihydrite as a poorly crystalline iron oxide-hydroxide,“ which would be helpful to work out as that could help us determine which process produced the dust and when that occurred,” said Briony Horgan, professor of planetary science at Purdue University in West Lafayette.
This trait allows it to remain stable even under the harsh, dry, and cold conditions that now dominate Mars. The stability of ferrihydrite in today’s environment means it could have persisted on the Martian surface since those ancient wet periods.
How Researchers Identified Ferrihydrite on Mars
To reach this conclusion, researchers combined data from several sources. They used orbital measurements from Mars Express and Mars Reconnaissance Orbiter, alongside ground observations from Mars rovers and laboratory simulations on Earth. By comparing the spectral signatures of Martian dust with different mineral compositions, they found that a mix of ferrihydrite, basalt, and sulfate best explains the red color seen on Mars.
Laboratory experiments further supported this finding by demonstrating that ferrihydrite remains stable and retains its characteristic poorly crystalline form under conditions similar to those on present-day Mars. This contrasted with hematite, which does not fully match the observed spectral properties of the Martian surface dust. Such evidence points strongly to ferrihydrite as the dominant contributor to Mars’s distinctive reddish appearance.
What This Means for Mars’s Climate History
The discovery of ferrihydrite on Mars offers new insights into the planet’s climatic past. The mineral’s formation requires cold, wet, and oxidative environments, suggesting that early Mars experienced conditions very different from its current dry and arid state. According to the study, ferrihydrite likely formed during ancient wet periods before Mars transitioned into the hyper-arid desert world we know today.
“We’ve also seen evidence for ferrihydrite in the lake sediments at (Mars’) Gale crater (which is being explored by the Curiosity rover). The best way to really solve this puzzle would be to get a sample of Mars dust into our labs back on Earth,” explained Horgan.
This finding challenges previous models that assumed Mars’s surface redness came exclusively from dry oxidation of iron minerals. Instead, the presence of ferrihydrite implies that water played a significant role in shaping the planet’s surface chemistry, preserving a record of past aqueous activity. This raises important questions about the potential habitability of Mars in its early history and guides future exploration missions seeking signs of life.
