Mars has been called the Red Planet for centuries, but a 2025 study suggests its colour comes largely from ferrihydrite—an iron mineral that typically forms rapidly in cool liquid water. If the identification is correct, Mars did not rust only after becoming a desert: much of its famous red dust formed while the ancient planet was still wet.

From Earth, Mars looks red because a veil of extraordinarily fine dust sits on its surface and hangs in its atmosphere. Iron in that material has been oxidised, so the comparison with rust is useful. But “rust” names a family resemblance, not one mineral or one history.

A 2025 study in Nature Communications offers the strongest integrated case yet that much of the colour comes from ferrihydrite. This poorly crystalline iron oxide contains hydroxyl and water in its structure and commonly forms rapidly in cool liquid water.

If that identification holds, the Red Planet’s dust carries a memory of a wetter Mars. The planet did not necessarily acquire its familiar colour only through slow, dry oxidation after its rivers and lakes disappeared. Some of the material may have formed while water was still moving through the ancient landscape, then survived and spread after Mars became a desert.

The old answer was right, but not specific enough

Humans associated Mars with red long before anyone knew what its surface contained. Modern telescopes and spacecraft eventually supplied the broad chemical answer: iron-bearing minerals in Martian rock, regolith and dust became oxidised. NASA’s current Mars overview still explains the colour in those sensible terms.

The harder question is which oxidised iron phase dominates the dust. For decades, many interpretations emphasised extremely small particles of hematite, an iron oxide whose formula contains no water. Hematite can form in several ways, including pathways that do not require persistent liquid water at the surface.

Ferrihydrite is different. It is disordered, poorly crystalline and hydrated. On Earth it often precipitates quickly when dissolved ferrous iron is oxidised in cool water, particularly around roughly neutral pH. Identifying it across Mars would therefore connect the planet’s colour to chemical weathering under conditions unlike the dry, cold surface of today.

That does not make the older explanation foolish. Both hematite and ferrihydrite are oxidised iron minerals, and earlier researchers had considered ferrihydrite among the possibilities. The advance lies in bringing many spacecraft spectra and purpose-built laboratory mixtures into a single test.

The team tried to rebuild the colour grain by grain

The researchers began with observations from several generations of missions. Orbital spectra came from CRISM aboard NASA’s Mars Reconnaissance Orbiter and OMEGA aboard ESA’s Mars Express. They also used measurements from the ExoMars Trace Gas Orbiter and the Sojourner, Opportunity and Curiosity rovers.

Each instrument sees a different part of the problem. A spectrometer does not collect a neat mineral specimen. It records how a surface absorbs and reflects many wavelengths of light, producing a pattern that researchers compare with materials measured in a laboratory.

CRISM, for example, was designed to search for minerals associated with past water by reading hundreds of colours in reflected sunlight. NASA’s instrument description says its range extends from visible light to wavelengths near four micrometres, covering several diagnostically useful mineral features.

The team then ground basalt and candidate iron minerals into exceptionally small particles and measured their spectra. A hyperfine ferrihydrite-basalt mixture matched the Martian dust more closely than comparable mixtures made with hematite, goethite, akaganeite or schwertmannite.

This was not just a judgement that two powders looked similarly red. The ferrihydrite mixture reproduced the locations and shapes of absorption features across the visible and near-infrared range. In the authors’ quantitative comparison, it also produced much smaller spectral errors than the alternatives.

Particle size solves an apparent contradiction

Ferrihydrite contains water, yet some of its familiar hydration bands are difficult to see in Martian dust. That might appear to argue against it. The laboratory work showed why the absence is not decisive when grains are tiny and minerals are intimately mixed.

The most successful analogues contained particles smaller than one micrometre, about one hundredth the width of a human hair. At this scale, light bounces among grains in complicated ways. A mixture’s spectrum is not simply the spectrum of ingredient A added to ingredient B.

Dark basalt can mask ferrihydrite’s water-related bands near 1.4 and 1.9 micrometres while leaving other features that control the visible colour. Adding magnesium sulfates improved the match around the broad three-micrometre hydration feature observed in Martian dust.

The preferred laboratory mixtures required substantial ferrihydrite, not a trace contaminant. The paper reports that at least ten percent by weight was needed to produce the characteristic steep red slope, while mixtures containing roughly 20 to 33 percent fitted the dust spectra more closely.

Those are analogue proportions, not a direct global census of Martian soil. Grain shape, other minerals and local mixtures can affect the answer. Still, the need for a sizeable ferrihydrite component is part of why the authors describe it as the dominant oxidised iron-bearing phase rather than a minor curiosity.

One dust system links distant landing sites

Mars is geographically diverse, but its brightest airborne dust is unusually well mixed. Spectra taken at five widely separated sites are similar in visible wavelengths. Planet-wide winds lift the finest grains, carry them across immense distances and let them settle over terrains with very different underlying rocks.

That global mixing gives the study leverage. Agreement among orbital measurements and several rover sites is harder to dismiss as a local deposit. ESA’s archive of OMEGA mineralogical maps shows how spectroscopy can place local compositions within a planetary context.

It also sets a limit. Global dust can conceal where its ingredients were originally made. Ferrihydrite now resting in Gale Crater may have been eroded from ancient sediment elsewhere, lofted repeatedly and mixed through countless storms. Colour tells us about formation chemistry more readily than birthplace.

As SpaceDaily noted while following the divided fate of Mars’s ancient water, “lost” is an oversimplification. Water escaped to space, froze underground and became bound inside minerals. Ferrihydrite would be another mineral archive of that vanished surface cycle.

A wet mineral can outlive a wet planet

On Earth, ferrihydrite is metastable. Given sufficient time and the right conditions, it can reorganise into more crystalline minerals such as goethite or hematite. Finding it in dust billions of years after formation therefore raises an obvious question: why has it not changed?

The answer may be Mars itself. The team exposed ferrihydrite to simulated present-day conditions for 40 days, including ultraviolet radiation, a thin carbon-dioxide atmosphere and pressure near six millibars. It lost some water adsorbed on grain surfaces but retained its poorly crystalline mineral structure.

Dry solid-state conversion is extremely slow at Mars’s low average temperature. The other important pathway requires dissolution and reprecipitation in liquid water, which is scarce and unstable at the modern surface. Once the climate became cold and hyper-arid, the environment that ended widespread ferrihydrite formation may also have helped preserve what already existed.

Wind then converted ancient deposits into a planetary coating. The proposed sequence is therefore chemical weathering first, long preservation second, and global redistribution continuing into the present. Mars looks red today partly because its desert is still moving material created under earlier conditions.

Water alone is not the whole reaction

Ferrihydrite formation requires iron to become oxidised as well as hydrated. Ancient Mars did not possess an oxygen-rich atmosphere like modern Earth, so the source of the oxidising power remains an active problem rather than a solved detail.

The paper discusses several possibilities. Ultraviolet light can break atmospheric molecules or act on water to make reactive species. Iron oxidation can also proceed through chemical pathways involving carbon dioxide and water without abundant free oxygen. Hydrogen escaping to space may have gradually left the surface environment more oxidising.

Different mechanisms could have operated at different times and places. Meltwater associated with volcanism may have weathered basalt, while lakes, groundwater or transient surface flows moved dissolved material into basins. The study favours ferrihydrite forming late in Mars’s wetter era as conditions shifted toward greater acidity and aridity.

That scenario fits a planet in transition better than a single global ocean rusting evenly. It allows cold water activity, episodic runoff and local chemistry to make iron-rich deposits that erosion later blended into the dust seen almost everywhere.

What the result says about habitability

Liquid water is necessary for life as we know it, so evidence that the dust formed through aqueous alteration strengthens the broader case for an ancient Mars with potentially habitable environments. NASA’s summary of the research makes that connection while carefully stopping short of claiming life.

Ferrihydrite itself is not a biosignature. Non-living chemistry readily makes it. Nor does a wet interval prove that a particular lake had the right energy sources, duration, nutrients or stability for biology. “Potentially habitable” describes environmental possibility, not occupation.

The distinction resembles the one required when interpreting organic matter. SpaceDaily’s recent account of Curiosity’s preserved organic molecules noted that carbon chemistry can be biological or non-biological. Minerals and organics reconstruct conditions; neither becomes proof of organisms by association.

Still, colour is an unexpectedly large archive. If ferrihydrite supplies much of Mars’s ochre dust, evidence of water-rock chemistry is not confined to a few valley floors or clay beds. It has been ground fine, carried around the planet and placed in plain sight.

The study is strong evidence, not a returned sample

The title’s conditional matters. The team identified ferrihydrite through converging spectra, laboratory analogues, stability experiments and chemical calculations. No instrument has yet picked up a Martian grain on Earth and mapped its atoms closely enough to settle the mineralogy beyond dispute.

The work also cannot date every stage of the process. It does not show that all red material formed at once, that every iron grain passed through cool water, or that dry oxidation contributed nothing after Mars became arid. The authors describe a dominant component and a plausible planetary history, not an exclusive mechanism.

A returned sample could test crystal structure, grain textures and isotopic ratios with instruments too large and sensitive to send to Mars. The samples cached by Perseverance are mainly selected rocks rather than a guaranteed representative scoop of global dust, but laboratory examination could still reveal how hydrated iron phases formed and changed.

Until that happens, ferrihydrite is best treated as a well-supported interpretation with testable consequences. It explains the visible spectrum, the bound water detected in dust and the survival of a poorly crystalline phase under present conditions unusually well.

Mars may be red because it once had water

The most interesting change is conceptual. The old shorthand made Mars red because it is rusty and dry. The new model makes its redness partly a relic of the opposite state: a time when cool liquid water could react with iron-bearing basalt before the surface water cycle collapsed.

That history is not written in a pristine layer. It has been eroded, pulverised and blown across the planet for billions of years. The red we see is a mixture, and the study does not remove every uncertainty about its minerals or chronology.

But if the ferrihydrite identification survives future tests, the planet’s most familiar feature will carry a different meaning. Mars is not simply red because it dried out. Much of it may be red because, before the desert, it was wet enough to make the dust that the desert preserved.