For decades, the standard explanation was simple: Mars is red because its surface is coated in iron oxide — rust. Iron in Martian rocks reacted with water and oxygen at some point in the planet's 4.5-billion-year history, the rusty material was ground into fine dust over billions of years, and winds spread that dust across the whole planet. From Earth, the result is a warm red-orange disc, and from orbit the same dust tints even the sky.

The details mattered less than the headline, and textbooks settled on hematite, a dry iron oxide, as the main culprit. Hematite is grey or black as a solid chunk but vivid red when powdered, which fits Mars perfectly: its dust is extremely fine, so a thin coating of powdered iron oxide acts like pigment over rocks, soil and even the air itself. Curiosity's measurements show iron oxides make up roughly 20 percent by weight of the dusty material in Martian soils.

In February 2025, a study in Nature Communications, led by researchers at Brown University and the University of Bern, challenged the hematite story. By combining spacecraft observations from orbiters including Mars Reconnaissance Orbiter, Mars Express and the Trace Gas Orbiter with rover data and new laboratory techniques — essentially making Martian dust in the lab — the team found that ferrihydrite fits the observed minerals better than hematite.

Ferrihydrite is an iron oxide that contains water and forms quickly in the presence of cool water. That is the crucial difference. Hematite can form under warm, dry conditions, but ferrihydrite needs water. If ferrihydrite dominates Martian dust, then the dust itself is a fossil of a wet Mars: it must have formed early, when liquid water was still present on the surface, and it has kept its watery signature ever since, despite being ground down and blown around for billions of years.

Mars today has almost no liquid water and very little free oxygen, so the obvious Earth-style rusting recipe does not apply now. Researchers point to several pathways that could have worked in the ancient past and, in modified form, still operate. Ultraviolet light from the Sun, largely unfiltered by Mars's thin atmosphere, can directly oxidise iron in a process called photooxidation, without needing free oxygen at all.

Laboratory experiments simulating roughly seven years of Martian ultraviolet exposure were enough to partially oxidise iron locked inside clay minerals. In the ancient past, the recipe was richer: water in rivers, lakes and possibly oceans reacted with iron-rich volcanic rock, oxygen released as water molecules were split by sunlight stayed behind while lighter hydrogen escaped to space, and the resulting iron oxides were washed into sediments. Mars's enriched deuterium-to-hydrogen ratio — about five times Earth's — supports this picture of massive ancient water loss.

Colour on Mars is a matter of particle size as much as chemistry. Fine hematite and ferrihydrite grains smaller than about five microns scatter longer wavelengths — reds and oranges — more effectively, reinforcing the reddish tint of both ground and sky. Even dark volcanic bedrock eventually receives a dusting of these particles, which is why Mars looks uniformly red from a distance despite having varied geology underneath.

The same dust produces one of Mars's strangest sights: blue sunsets. On Earth, gas molecules scatter blue light and let reds through at sunset. On Mars, micron-sized dust scatters blue light strongly forward, toward an observer looking near the Sun, while extinguishing red light slightly more. The result, photographed by rovers, is a pale blue glow around the setting Sun in an otherwise butterscotch sky — a reminder that Martian colour is physics, not just pigment.

If the ferrihydrite interpretation holds, Mars's colour is not just a curiosity; it is evidence. Ferrihydrite forms in cool, wet conditions of the kind that could have supported microbial life, and its survival in the dust suggests that water was widespread and persistent enough to rust a planet. That strengthens the case for studying Jezero Crater's ancient lake sediments, where Perseverance is collecting samples, and for returning those samples to Earth for the precise laboratory analysis no rover can do.

It also sets a test for future missions: look for ferrihydrite in rocks as well as dust, map where it is most abundant, and compare its chemistry with Earth's own water-formed iron minerals. The next time you see Mars glowing red in the pre-dawn sky, remember that its colour may be telling you about oceans that vanished billions of years ago — and about a planet that was once far more Earth-like than it looks today.