- Mars is known as the red planet. But why is it so red, and how did it get that way?
- Iron oxides are involved, but scientists have long debated exactly which one. Previous studies suggested hematite. But a new study says that ferrihydrite is actually the culprit.
- This also supports other evidence for a much wetter and potentially habitable ancient Mars.
Why Mars is red and what it means for ancient Martian life
Mars is popularly known as the red planet. But why is it so reddish in color? We know that the surface is basically rusted due to iron oxides. But there is a variety of iron oxides, and scientists are still debating the exact one responsible. Previous studies have said hematite is the most likely form of iron oxide. But on February 25, 2025, an international team of researchers said that ferrihydrite might be the main culprit. Intriguingly, this also adds even more evidence for a much wetter past on Mars, since the mineral forms in cool water.
The researchers published their peer-reviewed results in Nature Communications on February 25, 2025.
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Mars is red … but why?
Mars has a distinctive reddish hue when seen in the night sky. And indeed, rovers and landers on the surface also see a vast reddish-brown landscape. Scientists had determined that iron oxides were the cause. They are everywhere on Mars, in the dust, sand and rocks.
But exactly which iron oxide has been a matter of debate. Scientists have largely thought it was hematite. Previous studies of Martian dust showed no evidence of water. Therefore, hematite seemed the most likely culprit since it forms under dry conditions.
But that might not be the case after all.
Have we been wrong about why #Mars is red??The Red Planet’s iconic colour has long been attributed to a dry rusty dust, but new research points to water-rich rust ? www.esa.int/Science_Expl…?? #planetaryscience #planetsci
— ESA Space Science (@esascience.esa.int) 2025-02-25T10:00:53.723Z
Water-rich ferrihydrite
Instead of hematite, the new study points to another type of iron oxide: ferrihydrite. And unlike hematite, it requires water to form.
The researchers conducted a new analysis of orbiting spacecraft data and tried to replicate Martian dust in the laboratory. They used an advanced grinder machine to create the replica Martian dust. In fact, they achieved a realistic dust grain size equivalent to only 1/100th the width of a human hair. They then analyzed their samples using the same techniques as orbiting spacecraft in order to make a direct comparison.
With the improved techniques, they found evidence of ferrihydrite in the Martian dust. Interestingly, on Earth, it forms in cool water. Lead author Adomas Valantinas, a postdoc at Brown University in Providence, Rhode Island, said:
We were trying to create a replica Martian dust in the laboratory using different types of iron oxide. We found that ferrihydrite mixed with basalt, a volcanic rock, best fits the minerals seen by spacecraft at Mars.
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.
Colin Wilson is the project scientist for ESA’s TGO and Mars Express. He added:
This study is the result of the complementary datasets from the fleet of international missions exploring Mars from orbit and at ground level.
Observational data and new laboratory methods
Valantinas said:
The fundamental question of why Mars is red has been considered for hundreds if not for thousands of years. 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 we can now better test this using observational data and novel laboratory methods to essentially make a Martian dust in the lab.
Other studies had hinted at ferrihydrite in the dust, but this is the first hard evidence for its presence.
Implications for past potential habitability and life
The findings add yet another layer of evidence for a much wetter Mars in the past. This is because ferrihydrite forms in the presence of cool water, and at lower temperatures than other previously considered iron minerals, including hematite. And this, of course, has direct implications for past potential habitability and possible life. Valantinas said:
What we want to understand is the ancient Martian climate, the chemical processes on Mars – not only ancient – but also present. 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’s formation. What we know from this study is the evidence points to ferrihydrite forming and for that to happen there must have been conditions where oxygen from air or other sources and water can react with iron. Those conditions were very different from today’s dry, cold environment. As Martian winds spread this dust everywhere, it created the planet’s iconic red appearance.
Further confirmation needed
Scientists will be able to further confirm – or not – the ferrihydrite in the dust when samples from the Perseverance rover are eventually returned to Earth. As co-author John Mustard at Brown University noted:
The study really is a door-opening opportunity. It gives us a better chance to apply principles of mineral formation and conditions to tap back in time. What’s even more important though is the return of the samples from Mars that are being collected right now by the Perseverance rover. When we get those back, we can actually check and see if this is right.
Bottom line: Are we wrong about why Mars is red? An iron oxide called ferrihydrite might be responsible. If so, this also further supports ancient Mars being habitable.
Source: Detection of ferrihydrite in Martian red dust records ancient cold and wet conditions on Mars
Via NASA
Via ESA
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