SPACE/COSMOS: MARS SPECIAL
Why is Mars red? Scientists may finally have the answer
A new study led by Brown University researchers shows a water-rich mineral could explain the planet’s color — and hint at its wetter, more habitable past.
Brown University
PROVIDENCE, R.I. [Brown University] — Mars has captivated scientists and the public alike for centuries. One of the biggest reasons is the planet’s reddish hue, earning the third rock from the sun one its most popular nicknames — the “Red Planet.” But what exactly gives the planet its iconic color? Scientists have wondered this for as long as they’ve studied the planet. Today, they may finally have a concrete answer, one that ties into Mar’s watery past.
Results from a new study published in the journal Nature Communications and led by researchers from Brown University and the University of Bern suggest that the water-rich iron mineral ferrihydrite may be the main culprit 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 a dry, rust-like mineral called 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 1000s of years,” said Adomas (Adam) Valantinas, a postdoctoral fellow at Brown who started this work as a Ph.D. student at the University of Bern. “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.”
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 cool 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.
“What we want to understand is the ancient Martian climate, the chemical processes on Mars — not only ancient — but also present,” said Valantinas, who is working in the lab of Brown planetary scientist Jack Mustard, a senior author on the study. “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 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 could 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.”
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 and Trace Gas Orbiter 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. These findings were then compared to laboratory experiments, where the team tested how light interacts with ferrihydrite particles and other minerals under simulated Martian conditions.
“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,” Valantinas said. “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.”
As exciting as these new findings are, the researchers are well aware none of it can be confirmed until samples from Mars are brought back to Earth, leaving the mystery of the Red Planet’s past just out of reach.
“The study is a door opening opportunity,” Mustard said. “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.”
Journal
Nature Communications
Subject of Research
Not applicable
Article Title
Detection of ferrihydrite in Martian red dust records ancient cold and wet conditions on Mars
Article Publication Date
25-Feb-2025
Have we been wrong about why Mars is red?
European Space Agency
image:
Mars is immediately recognisable in the night sky by its red hue, but where does its iconic colour come from?
Martian dust is mostly rust!
Mars’s famed colour has captivated humankind for centuries, earning its nickname of the ‘Red Planet’. Romans named Mars for their god of war because its colour was reminiscent of blood, while Egyptians called it ‘Her Desher’, meaning ‘the red one’.
Thanks to the fleet of spacecraft that have studied Mars over the last decades we know that the red colour is due to iron minerals in the soil rusting. That is, iron bound up in the chemistry of Mars’s rocks has at some point reacted with water and oxygen in some form, just like everyday rust forms on Earth. Over billions of years this rusty material – iron oxide – has been eroded down into dust and spread all around the planet by winds, a process that continues today.
Exciting new research, published in February 2025, has shown that this rusty dust has a much wetter history than previously thought.
Because of the absence of liquid water on Mars’s surface today, its rusty red minerals were thought to arise from dry iron oxides present in the dust, such as hematite.
However, new analysis of spacecraft observations in combination with novel laboratory techniques suggests that Mars’s red colour is better matched by iron oxides containing water, known as ferrihydrite. Ferrihydrite typically forms quickly in the presence of cool water, and so must have formed early on ancient Mars when the planet was still wet. It has remained stable under present day conditions on Mars.
The stunning image of Mars featured here shows off the Red Planet’s renowned colour from the viewpoint of ESA’s Rosetta mission as it flew past on 24 February 2007, en route to Comet 67P/Churyumov-Gerasimenko. It is a composite image created by combining near-infrared, green and near-ultraviolet colour information obtained by the OSIRIS Narrow Angle Camera. The polar ice cap at the south pole is particularly bright, and wispy clouds are seen most clearly around the planet’s curved horizons.
Read more about how Mars got its iconic colour in Have we been wrong about why Mars is red?
[Image description: the full disc of Mars is seen with the polar ice caps slightly off centre to the top left and bottom right. Clouds wrap around the planet’s curved horizons. Dark surface markings are clearly seen against the characteristic red tones of the dusty martian surface.]
view moreCredit: ESA & MPS for OSIRIS Team MPS/UPD/LAM/IAA/RSSD/INTA/UPM/DASP/IDA, 2007
Mars is easily identifiable in the night sky by its prominent red hue. Thanks to the fleet of spacecraft that have studied the planet over the last decades, we know that this red colour is due to rusted iron minerals in the dust. That is, iron bound up in Mars’s rocks has at some point reacted with liquid water, or water and oxygen in the air, similar to how rust forms on Earth.
Over billions of years this rusty material – iron oxide – has been broken down into dust and spread all around the planet by winds, a process that continues today.
But iron oxides come in many flavours, and the exact chemistry of martian rust has been intensely debated because how it formed is a window into the planet’s environmental conditions at the time. And closely linked to that is the question of whether Mars has ever been habitable.
Previous studies of the iron oxide component of the martian dust based on spacecraft observations alone did not find evidence of water contained within it. Researchers had therefore concluded that this particular type of iron oxide must be hematite, formed under dry surface conditions through reactions with the martian atmosphere over billions of years – after Mars’s early wet period.
However, new analysis of spacecraft observations in combination with novel laboratory techniques shows that Mars’s red colour is better matched by iron oxides containing water, known as ferrihydrite. Ferrihydrite typically forms quickly in the presence of cool water, and so must have formed when Mars still had water on its surface. The ferrihydrite has kept its watery signature to the present day, despite being ground down and spread around the planet since its formation.
“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,” says lead author Adomas Valantinas, a postdoc at Brown University in the US, formerly at the University of Bern in Switzerland where he started his work with ESA’s Trace Gas Orbiter (TGO) data.
“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.”
Other studies have also suggested ferrihydrite might be present in martian dust, but Adomas and colleagues have provided the first comprehensive proof through the unique combination of space mission data and novel laboratory experiments.
They created the replica martian dust using an advanced grinder machine to achieve the realistic dust grain size equivalent to 1/100th of a human hair. They then analysed their samples using the same techniques as orbiting spacecraft in order to make a direct comparison, finally identifying ferrihydrite as the best match.
“This study is the result of the complementary datasets from the fleet of international missions exploring Mars from orbit and at ground level,” says Colin Wilson, ESA’s TGO and Mars Express project scientist.
Mars Express’s analysis of the dust’s mineralogy helped show that even highly dusty regions of the planet contain water-rich minerals. And thanks to TGO’s unique orbit that allows it to see the same region under different illumination conditions and angles, the team could disentangle particle size and composition, essential for recreating the correct dust size in the lab.
Data from NASA’s Mars Reconnaissance Orbiter, together with ground-based measurements from NASA Mars rovers Curiosity, Pathfinder and Opportunity, also helped make the case for ferrihydrite.
“We eagerly await the results from upcoming missions like ESA’s Rosalind Franklin rover and the NASA-ESA Mars Sample Return, which will allow us to probe deeper into what makes Mars red,” adds Colin.
“Some of the samples already collected by NASA’s Perseverance rover and awaiting return to Earth include dust; once we get these precious samples into the lab, we’ll be able to measure exactly how much ferrihydrite the dust contains, and what this means for our understanding of the history of water – and the possibility for life – on Mars.”
For a little while longer, though, Mars’s red hue will continue to be admired and puzzled over from afar.
Notes for editors
‘Detection of ferrihydrite in Martian red dust records ancient cold and wet conditions on Mars’ by A. Valantinas et al is published today in Nature Communications.
The Red Planet Mars got its iconic colour from a combination of rusting and erosion over its 4.6-billion-year history.
Mars was a once wetter place than the dry, barren world we know today. During its early history, iron in its rocks reacted with oxygen and water to create rust. The rust was washed into the rivers, lakes and seas that used to cover Mars, becoming incorporated into the underlying rocks. Volcanic activity could also have triggered ice-melting events, further contributing to this process.
Once Mars dried out, this rusty rock was broken down into dust over billions of years. Strong winds blew this dust all over the planet’s surface, gradually turning Mars red.
Signatures of the water-rich environment in which the rust formed are still preserved in the dust, as analysed by spacecraft studying Mars today.
[Image description: Graphic showing how Mars turned from a grey, wet planet into a dusty red planet. From left to right, four steps are illustrated in a single image. First, iron in the planet’s rocks react with oxygen and water to create rust. Then the rust is washed into rivers, lakes and seas, and becomes incorporated in the underlying rocks. A volcano is also shown to represent a heat source that may have melted ice, further washing the rust into pools. Over billions of years the rusty rock is broken down into dust. Finally, winds blow the dust around the planet. A rover is shown on the surface, representing the direct analyses of this rusty dust. An orbiting spacecraft surveys the scene from above]
Credit
ESA
Martian dust is mostly rust!
Mars’s famed colour has captivated humankind for centuries, earning its nickname of the ‘Red Planet’. Romans named Mars for their god of war because its colour was reminiscent of blood, while Egyptians called it ‘Her Desher’, meaning ‘the red one’.
Thanks to the fleet of spacecraft that have studied Mars over the last decades we know that the red colour is due to iron minerals in the soil rusting. That is, iron bound up in the chemistry of Mars’s rocks has at some point reacted with water and oxygen in some form, just like everyday rust forms on Earth. Over billions of years this rusty material – iron oxide – has been eroded down into dust and spread all around the planet by winds, a process that continues today.
Exciting new research, published in February 2025, has shown that this rusty dust has a much wetter history than previously thought.
Because of the absence of liquid water on Mars’s surface today, its rusty red minerals were thought to arise from dry iron oxides present in the dust, such as hematite.
However, new analysis of spacecraft observations in combination with novel laboratory techniques suggests that Mars’s red colour is better matched by iron oxides containing water, known as ferrihydrite. This photo shows a mixture of ferrihydrite and basalt made in the lab as part of the research; the mixture was found to best match spacecraft observations of real martian dust.
Ferrihydrite typically forms quickly in the presence of cool water, and so must have formed early on ancient Mars when the planet was still wet. It has remained stable under present day conditions on Mars.
Read more about how Mars got its iconic colour in Have we been wrong about why Mars is red?
[Image description: A disc of orange-terracotta coloured dust fills the frame. The dust looks quite soft and fine, more like flour than sand.]
Credit
A.Valantinas
Mars’s famed colour has captivated humankind for centuries, earning its nickname of the ‘Red Planet’. Romans named Mars for their god of war because its colour was reminiscent of blood, while Egyptians called it ‘Her Desher’, meaning ‘the red one’.
Thanks to the fleet of spacecraft that have studied Mars over the last decades we know that the red colour is due to iron minerals in the soil rusting. That is, iron bound up in the chemistry of Mars’s rocks has at some point reacted with water and oxygen in some form, just like everyday rust forms on Earth. Over billions of years this rusty material – iron oxide – has been eroded down into dust and spread all around the planet by winds, a process that continues today.
Exciting new research, published in February 2025, has shown that this rusty dust has a much wetter history than previously thought.
Because of the absence of liquid water on Mars’s surface today, its rusty red minerals were thought to arise from dry iron oxides present in the dust, such as hematite.
However, new analysis of spacecraft observations in combination with novel laboratory techniques suggests that Mars’s red colour is better matched by iron oxides containing water, known as ferrihydrite.
The team of researchers created mixtures of ferrihydrite and hematite in the lab and measured how much they reflected light over a wavelength range of 500–840 nm. These plots show that the reflection of light by the ferrihydrite mixture (left graph) matches spacecraft observations of real martian dust much closer the hematite mixture (right graph) does.
Ferrihydrite typically forms quickly in the presence of cool water, and so must have formed early on ancient Mars when the planet was still wet. It has remained stable under present day conditions on Mars.
Read more about how Mars got its iconic colour in Have we been wrong about why Mars is red?
[Image description: Two graphs side by side comparing the lab-made ferrihydrite and hematite dust mixtures with measurements of real martian dust by spacecraft. The left graph compares the ferrihydrite mixture with spacecraft data, showing that this type of dust closely matches. The right graph compares the hematite mixture with spacecraft data, showing that this type of dust does not match so closely.]
Credit
A.Valantinas/ESA
Journal
Nature Communications
Method of Research
Experimental study
Subject of Research
People
Article Title
Detection of ferrihydrite in Martian red dust records ancient cold and wet conditions on Mars
Article Publication Date
25-Feb-2025
New research makes strongest case yet for why Mars is red
Experimental research conducted by an international team argues that the reason Mars is red is due to a water-rich mineral known as ferrihydrite.
University of Arkansas
image:
Vincent Chevrier
view moreCredit: University Relations
If you know one thing about Mars it’s probably this: it’s red. The distant planet’s inaccessibility has long made the reason for its hue a matter of conjecture. The prevailing theory has been that hematite, an iron-oxide mineral, is the most likely contributor to Mars’ terrestrial color.
New research by an international team of researchers, including Vincent Chevrier, an associate research professor at the University of Arkansas’ Center for Space and Planetary Science, now argues that a different iron oxide mineral is in fact responsible for the Mars’ color: ferrihydrite.
The team’s research was published in Nature Communications. The researchers combined observational data from a range of orbital and ground-level measurements by rovers with novel laboratory experiments that synthesized Martian dust. In so doing, they were able to reverse engineer Martian dust that conformed to known spectral data.
Does it matter which kind of iron oxide is coating Mars? It does if you want to gain insight into what the conditions on Mars were in the distant past.
“What we want to understand is the ancient Martian climate, the chemical processes on Mars — not only ancient — but also present,” explained first author Adomas Valantinas, a postdoctoral fellow at Brown University. “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 formation.”
Chevrier’s biggest contribution to the study was a collection of natural and synthetic Martian soils he created and assembled for spectroscopic analysis. He actually developed these iron oxide-based soils more than two decades ago as part of his Ph.D. work on a thesis subtitled… “Why is Mars Red?”
Chevrier sent the samples to his colleagues at Brown so they could measure their spectra and compare them to data that had already been returned from Mars, including data collected by the Curiosity, Pathfinder and Opportunity rovers. Ultimately, laboratory experimenters determined that a mixture of submicron-sized ferrihydrite and basalt dust best matched the observational data.
If the ferrihydrite is the basis of Martain soil, then it must also mean that at some point a more liquid or humid environment existed on Mars, which hydrated the iron oxide. But the existence of ferrihydrite also means that the presence of water was transient or the ferrihydrite would have likely transformed into a more crystalline structure, such as hematite or goethite, due to longer contact with water. This supports the argument that at some point conditions on Mars were profoundly different than the extremely cold and dry environment that exists there today – leading to a veneer of red dust blown across its surface.
Unfortunately, the researchers will not be able confirm this finding until regolith samples are brought back from Mars. Rovers are collecting samples and leaving them around the planet, but to the best of Chevrier’s knowledge, there are no current plans underway to retrieve them and bring them back to Earth.
Still, the researchers are another step closer to confirming Mars’ watery past and determining its potential for habitability – future or past.
Journal
Nature Communications
Method of Research
Imaging analysis
Subject of Research
Not applicable
Article Title
Detection of ferrihydrite in Martian red dust records ancient cold and wet conditions on Mars
Article Publication Date
25-Feb-2025
What is it like in the core of Mars?
European Synchrotron Radiation Facility
image:
Leonid Dubrovinsky, Lianjie Man, Ilya Kupenko, Xiang Li at ESRF's ID15B beamline
view moreCredit: ESRF
What is it like in the core of Mars?
Researchers from the University of Bayreuth and the ESRF, the European Synchrotron, have subjected a blend of iron and sulphur to extreme conditions resembling the deep interior of Mars. They observed the formation of a crystal phase, Fe4+xS3, under high pressures and temperatures – raising the possibility that the Red Planet has a solid inner core.
As a young planet, Mars was home to flowing rivers and lakes, but its fate soon diverged from Earth’s as the Martian surface transitioned into the cold, arid world we see today. To better understand how Mars evolved, scientists must explore its deep interior – the focus of a new study at the ESRF.
In recent years, NASA’s InSight Mars Lander has returned Marsquake data, providing clues about the internal structure of Mars. This seismic data indicates the planet has a molten core with a 1,650–1,800 km radius, accounting for over half of Mars’ total radius. However, unlike Earth, there is insufficient data to establish whether the inner core region is solid.
Many geoscientists assumed the iron-rich heart of Mars was too hot to solidify. That’s partly because the Mars core is richer in light elements compared to the Earth’s. One of those elements is likely sulphur, which is abundant in rocks at the Martian surface and seen in meteorites of the material that coalesced to form Mars.
Breakthrough at the ESRF
In a new study, researchers first identified an iron-sulphide phase, Fe4+xS3, which could theoretically crystallise under Martian core conditions.
“Since the late 1990s, scientists have known this phase could exist, but they didn’t have the experimental techniques to identify the structure and stability,” explains Lianjie Man of Bayerisches Geoinstitut, Germany, lead author of the new study conducted at ESRF beamline ID14 and ID15B.
Using diamond anvil cells, the team subjected iron-sulphur samples to the pressures estimated at the Martian core. Simultaneously, laser heating established high temperatures in the sample. And single-crystal diffraction revealed the crystal structure and density of this novel phase.
Complementary experiments confirmed this iron sulphide could crystallise from liquid when temperatures drop below 1960 (±105) K – which falls within the temperature range in geophysical models for the Mars core. In other words, if the Martian core is on the cooler side of predictions, then a solid core seems perfectly feasible.
Ilya Kupenko, ESRF scientist and study co-author, notes that ID14’s ultra-small (1-micron diameter) X-ray beam, enabled the team to see individual Fe4+xS3 crystals within a larger matrix. “Experiments that are possible in other places are much easier to do here – it’s optimised for this type of research,” says Kupenko.
This study, published in Nature Communications, focused on the phase’s structural and chemical properties. But Kupenko and Man are already leveraging the ESRF’s unique capabilities in a follow-up study of the material’s magnetic properties. While Mars’ core temperatures likely exceed the Curie point – where permanent magnetism is lost – it is still crucial to understand the material’s magnetic order and transition temperature.
What Comes Next?
Future experiments could explore more realistic iron-sulphur mixtures, incorporating other light elements like oxygen and hydrogen. A related line of research is to determine the sound velocities in these materials – through projects like LECOR, Light Elements in the Core, an ERC project based on ESRF-EBS- which is vital for comparing candidate core materials with real-world seismological data.
This work could also help solve another puzzle: a suspected molten zone at the base of Mars’ mantle. This 150-kilometre-thick layer was predicted by two independent analyses of InSight seismic data published in Nature in 2023 (Samuel et al. and Khan et al.)
If further evidence does support a solid Martian core, then it would cast doubt on the existence of this molten zone at the mantle base. The core region would be too cold to melt the overlying mantle material. On the flip side, if new evidence supports the existence of this mantle melt zone, then it would spell bad news for the solid core theory – the region would be too hot.
For now, the question remains open. With InSight’s seismic mission complete and no follow-up missions planned, the answer may lie in deeper analysis of existing data. “If we know the temperature inside Mars, it has a lot of implications for the evolution of this planet over time,” says Kupenko.
DOI: 10.1038/s41467-025-56220-2
Journal
Nature Communications
Method of Research
Experimental study
Subject of Research
Not applicable
Article Title
The structure and stability of Fe4+xS3 and its potential to form a Martian inner core
Article Publication Date
25-Feb-2025
Adsorptive regolith on mars soaks up water, researchers reveal
image:
The schematic diagram of the model and results.
view moreCredit: ©Mirai Kobayashi
Mars, the next frontier in space exploration, still poses many questions for scientists. The planet was once more hospitable, characterized by a warm and wet climate with liquid oceans. But today Mars is cold and dry, with most water now located below the surface. Understanding how much water is stored offers critical information for energy exploration, as well as life sustainability on the planet.
A research group from Tohoku University has helped shed light on this by improving an existing Mars climate model. The enhanced model accommodates the various properties of Martian regolith, or the loose deposits of solid rock that comprise Martian soil.
Mirai Kobayashi says current models fail to account for the fact that laboratory experiments have demonstrated that the water-holding capacity of the regolith is strongly influenced by its adsorption coefficient.
"Models to date that estimate the distribution of surface and subsurface water on Mars assume that its regolith properties are uniform. This contrasts with observations made by orbiters and landers, which suggest that Martian regolith has globally non-uniform physical properties."
The model estimated Mars's subsurface water distribution down to 2 meters from the surface. Like a sponge, highly absorptive regolith in Mars's mid- and low latitudes retains substantial amounts of absorbed water. Some of this water, the findings showed, remains on the surface of the regolith as stable adsorbed water.
The study also showed that the soil on Mars could keep ice near the surface in the middle and lower areas because water vapor moves more slowly there. This means the soil helps trap water for a long time by slowing down how water vapor spreads, which is important for understanding the change in water on Mars over time.
"Our study stresses the importance of incorporating absorption and inhomogeneity of Martian regolith in forecasting Mars's surface water," says Takeshi Kuroda, who led the team alongside Kobayashi, Arihiro Kamada and Naoki Terada. "The model can also be used to study how water on Mars has changed, and how it may have moved deeper underground near the planet's mantle."
With several Mars exploration missions underway, including the Japan-led Martian Moons eXploration (MMX) and the international Mars Ice Mapper (MIM) projects, the model is expected to complement further studies that can lead to subsurface water maps of Mars.
The results were published in the Journal of Geophysical Research: Planets on February 24, 2025.
Journal
Journal of Geophysical Research Planets
Article Title
Large water inventory in a highly adsorption regolith simulated with a Mars global climate model
Article Publication Date
24-Feb-2025
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