Mars, the red planet, owes its iconic color to a very specific chemical process. But new research suggests that this rusty hue hides a more complex history than previously thought.
For decades, scientists have attributed Mars' red color to the oxidation of iron, a phenomenon similar to rust on Earth. However, a recent study proposes that Martian dust contains a water-rich mineral called ferrihydrite, which may have played a key role in this coloration. This discovery opens new perspectives on Mars' climatic history and its past potential habitability.
The composition of Martian dust
The dust covering Mars is primarily composed of iron-rich minerals, including iron oxide. Until now, scientists believed that hematite, a dry iron oxide, was responsible for the planet's red color. However, new analyses reveal that ferrihydrite, a mineral formed in the presence of cold water, better matches the observations. This finding challenges previous theories about the formation of Martian rust.
a) Ochre hue of Mars' bright regions observed on August 14, 2021, by the Emirates Exploration Imager (R = 635 nm, G = 546 nm, B = 437 nm).
b) Fine-grained (< 1 µm) ferrihydrite-basalt mixture (1:2 ratio) in the laboratory, acquired under ambient conditions.
c) Comparison of an orbital spectrum of Martian dust (CRISM image FRT00009591) with the spectrum of the ferrihydrite-basalt mixture. The increase in reflectance near 0.5 µm is due to ferric iron and its electron transition absorption, dominating the UV and blue spectrum. The NIR spectral bands at 1.41 and 1.93 µm, associated with water bound in ferrihydrite, are not detected in these samples. The characteristic increase in reflectance between 1 and 2.5 µm for pure ferrihydrite also does not appear, likely due to the non-linear mixing with basalt powder. The band at 3 µm could be due to chemically bound water in Martian dust and the laboratory sample. To reach this conclusion, researchers recreated Martian dust in the laboratory by grinding samples of basalt and ferrihydrite. These samples were then compared to spectral data collected by Martian orbiters and rovers. The results show that ferrihydrite is the best candidate to explain Mars' red hue. This innovative method allowed for the recreation of conditions close to those on the red planet.
Data from space missions, such as ESA's Trace Gas Orbiter and NASA's Curiosity rover, played a key role in this discovery. By combining these observations with laboratory experiments, scientists were able to identify the spectral signature of ferrihydrite in Martian dust. This breakthrough paves the way for a better understanding of the chemical processes that shaped Mars.
A history linked to water
The presence of ferrihydrite suggests that Mars experienced wet conditions billions of years ago. This mineral forms quickly in the presence of cold water, indicating that liquid water was once abundant on the planet's surface. Unlike hematite, ferrihydrite retains its aqueous signature even after billions of years of erosion and dispersal by Martian winds.
This discovery reinforces the idea that Mars could have hosted environments conducive to life. The conditions necessary for the formation of ferrihydrite, namely liquid water and oxygen, suggest that the red planet experienced a more temperate period than its current cold and dry state. This opens new perspectives on Mars' past habitability and the possibility that it could have supported microbial life forms.
The left panel depicts ancient Mars during a phase of active chemical weathering, where hydration and oxidation of the basaltic crust produce ferrihydrite-rich waters. Meltwater runoff, triggered by volcanic activity, transports insoluble ferric iron to crater lakes and basins, forming sedimentary deposits.
The right panel illustrates modern Mars, where erosion redistributes sedimentary layers and disperses fine materials, giving the planet its characteristic ochre appearance. Diagram not to scale. Future missions, such as ESA's Rosalind Franklin rover and NASA's Mars Sample Return program, could provide samples to confirm this hypothesis. Direct analysis of Martian dust and rocks on Earth would allow for precise determination of the amount of ferrihydrite present and a better understanding of the environmental conditions that prevailed on Mars during its formation. These samples could also reveal other clues about the history of water and life on the red planet.
To go further: What is ferrihydrite?
Ferrihydrite is an iron- and water-rich mineral, often associated with wet and cold environments. On Earth, it is found in soils and lake sediments, where it forms quickly in the presence of liquid water and oxygen. Its unique chemical structure allows it to retain water molecules, making it a valuable indicator of past aqueous conditions.
On Mars, the presence of ferrihydrite would suggest that the planet experienced wet episodes, even though its current environment is extremely dry. This mineral forms at relatively low temperatures, indicating that liquid water on Mars must have been cold and likely abundant for a limited period. Unlike other iron oxides, such as hematite, ferrihydrite is less stable in the long term, but it can persist under specific conditions, such as those observed on Mars.
The discovery of ferrihydrite in Martian dust would open new perspectives for understanding the history of water on the red planet. By studying this mineral, scientists can trace the environmental conditions that prevailed on Mars billions of years ago. This could also help identify regions where life might have emerged, increasing the interest of future exploration missions.
Article author: Cédric DEPOND