Have Scientists Been Wrong About Why Mars Is Red?

For decades, scientists have thought Mars’s iconic red hue to be the result of iron-rich dust reacting with the planet’s thin atmosphere. Now, new research suggests that this long-accepted explanation may be incorrect. A combination of spacecraft observations and advanced laboratory experiments has provided fresh insight into how Mars became covered in rust-like dust, and the findings suggest that the planet rusted far earlier than previously thought.

The Red Planet’s Watery Past

The widely accepted explanation for Mars’s iron-rich dust has been that it formed under dry conditions, long after the planet’s ancient lakes and rivers had disappeared. The belief was that iron in Martian rocks slowly oxidised over billions of years through chemical reactions with the planet’s atmosphere, forming hematite. This is an iron oxide that doesn’t contain water. 

This process was thought to have taken place long after liquid water was abundant on Mars, meaning the planet’s red dust was a product of its dry, barren era rather than its once-wet past.

New research using ESA and NASA spacecraft data has found evidence that the red dust is not just hematite but also ferrihydrite, an iron oxide that forms quickly in the presence of cool water. This suggests that Mars’s rusting process happened much earlier when water was still present on the surface.

Lab Tests Confirm a Different Type of Rust

To investigate the true nature of Mars’s dust, researchers at Brown University and the University of Bern recreated a realistic Martian dust sample in a lab. Using an advanced grinding machine, they produced dust particles just 1/100th the width of a human hair, mimicking the fine texture observed on Mars. The dust was then analysed using the same techniques as orbiting spacecraft, allowing for a direct comparison with the Red Planet’s surface.

The results revealed that ferrihydrite mixed with basalt (a volcanic rock) best matched the Martian dust composition seen in spacecraft data. This was a crucial finding because ferrihydrite only forms in the presence of water. As lead researcher Adomas Valantinas explains, “Mars is still the Red Planet. It’s just that our understanding of why Mars is red has been transformed.”

“The major implication is that because ferrihydrite could only have formed when water was still present on the surface, Mars rusted earlier than we previously thought. Moreover, the ferrihydrite remains stable under present-day conditions on Mars.”

A Peek Into Mars’s History

The presence of ferrihydrite in Martian dust suggests that the planet’s rusting process began during its wetter past, not in its present. This finding challenges the previous timeline of Mars’s climatic evolution, suggesting that liquid water was still actively shaping the planet’s surface while this rust was forming.

The ESA’s Mars Express mission also contributed valuable insights. Its analysis showed that even highly dusty regions on Mars contain minerals that still hold traces of water, further supporting the idea that ferrihydrite played a key role in Mars’s past. Additionally, the Trace Gas Orbiter (TGO) helped scientists differentiate between particle size and composition, essential for accurately recreating Martian dust in the lab.

The case for ferrihydrite is further supported by data from NASA’s Mars Reconnaissance Orbiter and the Curiosity, Pathfinder, and Opportunity rovers. These missions have collected mineralogical data showing that certain areas of Mars contain hydrated iron oxides, which furthers the idea that water was involved in the formation of its famous red dust.

Future missions such as ESA’s Rosalind Franklin “ExoMars” rover and the NASA-ESA Mars Sample Return will allow scientists to study actual samples of Martian dust in Earth-based laboratories. Some of these samples have already been collected by NASA’s Perseverance rover, and once returned to Earth, they will be examined to determine exactly how much ferrihydrite is present.

A New Perspective on Mars

For decades, scientists believed that Mars turned red long after its water disappeared, but these new findings suggest otherwise. If ferrihydrite formed during the planet’s wetter era, then Mars rusted much earlier than previously assumed. This not only reshapes our understanding of its climate history but also raises further questions about Mars’s potential for past habitability.