Adapted by:
Matías S. Zavia
WriterAerospace and energy industries journalist at Xataka.
Alba Mora
WriterAn established tech journalist, I entered the world of consumer tech by chance in 2018. In my writing and translating career, I've also covered a diverse range of topics, including entertainment, travel, science, and the economy.
Mars is known for its reddish color, which can be seen with the naked eye. This association with the color red has led ancient cultures, like the Babylonians, to link the planet with their god of war, Nergal. Similarly, the Greeks and Romans associated it with their gods Ares and Mars. However, the reason behind Mars’ distinctive red hue is due to something completely different.
Mars is rusty. Scientists have long known that the characteristic reddish hue of Mars is due to the presence of iron oxides in the dust on the planet’s surface.
A recent international study combined NASA and the European Space Agency (ESA) data with laboratory experiments. The findings indicate that these iron oxides formed in the presence of water much earlier than scientists previously believed.
A surprisingly wet history. Until recently, experts thought hematite was the predominant iron oxide on Mars. Hematite can form with little or no liquid water, provided some oxidizing agent is present in the atmosphere.
However, recent observations and laboratory analyses have revealed clear signs of water in the spectral signature of Martian dust. These findings suggest that ferrihydrite, an iron oxide that contains water molecules within its structure, is responsible for Mars’ unmistakable red color. This evidence points to a wetter history for Mars than was previously known.
What is ferrihydrite and why it matters. Ferrihydrite is a poorly crystallized, iron-rich mineral that forms rapidly in the presence of freshwater and moderate levels of oxidation.
Its structure can retain water molecules or other hydroxyl groups, which means it leaves a water footprint even in drier conditions. This characteristic differs from that of hematite, which is anhydrous and much more stable when liquid water is absent. As such, the presence of ferrihydrite suggests that Mars experienced periods with relatively abundant liquid water in its past.
Methodology. To reach this conclusion, researchers combined data from several NASA missions (Mars Reconnaissance Orbiter, Curiosity, Opportunity, and Pathfinder) and ESA missions (Mars Express and the Trace Gas Orbiter) with laboratory experiments.
The experiments involved creating simulated Martian dust by mixing ferrihydrite with basalt ground into particles 100 times finer than human hair. Researchers then measured the optical properties of these samples similarly to how the instruments used on Mars work. The team confirmed that ferrihydrite best explained the spectral signatures observed on the Red Planet.
A Red Planet full of secrets. On Earth, ferrihydrite typically transforms over time into more crystalline phases, such as goethite or hematite, when conditions allow, including adequate temperature and water availability. However, Mars is extremely cold and dry and has a very thin atmosphere, making this transformation difficult. As a result, ferrihydrite on Mars tends to remain in its original form, preserving its “water seal” even after billions of years.
While the image of Mars as an arid, reddish planet remains unchanged, the study reveals a new explanation of its past. The confirmation of an oxidation phase occurring in the presence of water suggests that the planet experienced wetter periods, which could have had the potential to support living organisms.
Image | NASA
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