Mars has captivated scientists and the public alike for centuries, not only for the remote possibility of alien life, but also the planet’s reddish hue. But what exactly gives the planet its iconic color?
Results from a new study published by researchers from Brown University and the University of Bern suggest that the water-rich iron mineral ferrihydrite may be the main pigment behind Mars’ reddish dust. Their theory — which they reached by analyzing data from Martian orbiters, rovers and laboratory simulations — runs counter to the prevailing theory that the iron-oxide mineral hematite is the reason for the planet’s color.
“The fundamental question of why Mars is red has been thought of for hundreds if not for thousands of years,” says first author Adomas (Adam) Valantinas, a postdoctoral fellow at Brown who started this work as a Ph.D. student at the University of Bern.
The researchers analyzed data from multiple Mars missions, combining orbital observations from NASA’s Mars Reconnaissance Orbiter and the European Space Agency’s Mars Express with ground-level measurements from rovers like Curiosity, Pathfinder and Opportunity.
Instruments on the orbiters and rovers provided detailed spectral data of the planet’s dusty surface, revealing the chemical composition. These findings were then compared to laboratory experiments, where the team tested how light interacts with different minerals with the same chemical composition under simulated Martian conditions.
“From our analysis, we believe ferrihydrite is everywhere in the dust and also probably in the rock formations, as well. We’re not the first to consider ferrihydrite as the reason for why Mars is red, but it has never been proven the way we proved it now using observational data and novel laboratory methods to essentially make a Martian dust in the lab.”
“Martian dust is very small in size, so to conduct realistic and accurate measurements we simulated the particle sizes of our mixtures to fit the ones on Mars,” continues Valantinas. “We used an advanced grinder machine which reduced the size of our ferrihydrite and basalt to submicron sizes. The final size was 1/100th of a human hair and the reflected light spectra of these mixtures provide a good match to the observations from orbit and red surface on Mars.”
Hematite was suggested as pigment giving Mars its distinct color because it can form under dry conditions. In this model, the intense UV-radiation as experienced on Mars forces charged iron particles to react with oxygen, forming iron-oxide without the need of water.
Ferrihydrite is an iron oxide mineral that forms in water-rich environments. On Earth, it is commonly associated with processes like the weathering of volcanic rocks and ash. Until now, its role in Mars’ surface composition was not well understood, but this new research suggests that it could be an important part of the dust that blankets the planet’s surface.
The finding offers a tantalizing clue to Mars’ wetter and potentially more habitable past because unlike hematite, which typically forms under warmer, drier conditions, ferrihydrite forms in the presence of water. This suggests that Mars may have had an environment capable of sustaining liquid water — an essential ingredient for life — and that it transitioned from a wet to a dry environment billions of years ago.
The study, “Detection of ferrihydrite in Martian red dust records ancient cold and wet conditions on Mars,” was published in the journal Nature Communications.
Additional material and interviews provided by Brown University and ESA.
