New research makes strongest case yet for why Mars is red

FAYETTEVILLE — If you know one thing about Mars it’s probably this: it’s red. The distant planet’s inaccessibility has long made the reason for its hue a matter of conjecture. The prevailing theory has been that hematite, an iron-oxide mineral, is the most likely contributor to Mars’ terrestrial color.
New research by an international team of researchers, including Vincent Chevrier, an associate research professor at the University of Arkansas’ Center for Space and Planetary Science, now argues that a different iron oxide mineral is in fact responsible for the Mars’ color: ferrihydrite.
The team’s research was published in Nature Communications. The researchers combined observational data from a range of orbital and ground-level measurements by rovers with novel laboratory experiments that synthesized Martian dust. In so doing, they were able to reverse engineer Martian dust that conformed to known spectral data.
Does it matter which kind of iron oxide is coating Mars? It does if you want to gain insight into what the conditions on Mars were in the distant past.
“What we want to understand is the ancient Martian climate, the chemical processes on Mars — not only ancient — but also present,” explained first author Adomas Valantinas, a postdoctoral fellow at Brown University. “Then there’s the habitability question: Was there ever life? To understand that, you need to understand the conditions that were present during the time of this mineral formation.”
Chevrier’s biggest contribution to the study was a collection of natural and synthetic Martians soils he created and assembled for spectroscopic analysis. He actually developed these iron oxide-based soils more than two decades ago as part of his Ph.D. work on a thesis subtitled… “Why is Mars Red?”

Chevrier sent the samples to his colleagues at Brown so they could measure their spectra and compare them to data that had already been returned from Mars, including data collected by the Curiosity, Pathfinder and Opportunity rovers.
Ultimately, laboratory experimenters determined that a mixture of submicron-sized ferrihydrite and basalt dust best matched the observational data.
If the ferrihydrite is the basis of Martain soil, then it must also mean that at some point a more liquid or humid environment existed on Mars, which hydrated the iron oxide. But the existence of ferrihydrite also means that the presence of water was transient or the ferrihydrite would have likely transformed into a more crystalline structure, such as hematite or goethite, due to longer contact with water. This supports the argument that at some point conditions on Mars were profoundly different than the extremely cold and dry environment that exists there today – leading to a veneer of red dust blown across its surface.
Unfortunately, the researchers will not be able confirm this finding until regolith samples are brought back from Mars. Rovers are collecting samples and leaving them around the planet, but to the best of Chevrier’s knowledge, there are no current plans underway to retrieve them and bring them back to Earth.
Still, the researchers are another step closer to confirming Mars’ watery past and determining its potential for habitability – future or past.