Mars is red for a mundane reason: it is rusty. The surface is coated in iron oxide dust — the same compound that forms on a neglected bicycle chain, ground into a fine powder and spread across an entire planet by billions of years of wind.
The interesting question is not why Mars is red. It is what the rust implies. Iron oxide requires oxygen, and oxygen requires an atmosphere that Mars no longer meaningfully has. The colour is a fingerprint of a very different world.
The evidence for a wetter Mars
The case that Mars was once warm and wet is now overwhelming, built from decades of orbital imagery and surface exploration.
Orbiters have mapped **branching valley networks** that look unmistakably like river systems, with tributaries feeding trunk channels in patterns that only sustained flowing water produces. There are **outflow channels** carved by catastrophic floods, some large enough to have discharged more water than any terrestrial river. There are **deltas** where sediment fanned out into what were plainly standing bodies of water.
Rovers have supplied the ground truth. Opportunity found small spherical haematite concretions — nicknamed blueberries — that form in water-saturated rock. Curiosity, exploring Gale Crater, found rounded pebbles of exactly the kind produced by tumbling in a streambed, and layered mudstones deposited in a long-lived lake. Perseverance is working in Jezero Crater, chosen specifically because it contains a well-preserved river delta.
Both rovers have found clay minerals and sulphate salts that only form through prolonged interaction between rock and water. Some of these deposits indicate water that was not strongly acidic or salty — conditions that would have been survivable for life as we understand it.
Taken together, the picture is of a Mars that, roughly three to four billion years ago, had rivers, lakes, possibly a northern ocean, and a atmosphere thick enough to keep liquid water stable at the surface.
What Mars looks like now
The contrast is stark. Atmospheric pressure at the Martian surface averages about 0.6% of Earth's — below the threshold at which liquid water can exist stably. Exposed water does not flow; it boils and freezes almost simultaneously, sublimating directly to vapour.
The remaining atmosphere is around 95% carbon dioxide, and it is far too thin to retain heat. Average surface temperature is about −60°C. Equatorial midday in summer can briefly reach a pleasant 20°C at ground level, while the same night plunges past −70°C, because there is not enough air to hold the warmth.
Substantial water remains, but it is locked up: in the polar ice caps, in vast quantities of ground ice detected across the mid-latitudes, and possibly in briny liquid reservoirs deep underground.
The magnetic field that failed
The leading explanation for the loss centres on the planet's core.
Earth generates a global magnetic field through a dynamo — convection currents in a liquid outer core of iron, kept in motion by heat escaping from the interior. That field extends far into space and deflects the solar wind, a continuous stream of charged particles flowing outward from the Sun at hundreds of kilometres per second.
Mars once had a dynamo too. We know this because the oldest regions of the Martian crust retain strong remnant magnetisation, frozen in as the rock cooled in the presence of a field. The younger crust does not. Somewhere around four billion years ago, the Martian dynamo shut down.
The reason is size. Mars is about half Earth's diameter and roughly a tenth its mass. Smaller bodies have a larger surface area relative to volume, so they radiate their internal heat away faster. Mars cooled, its core convection slowed and stopped, and the global magnetic field died with it.
Stripped by the solar wind
With no global field, the solar wind reached the upper atmosphere directly. Charged particles collided with atmospheric molecules, energised them, and carried them away — a process called sputtering, alongside several related escape mechanisms.
This is not conjecture. NASA's MAVEN orbiter was sent specifically to measure it and found atmospheric loss occurring in real time, with rates spiking sharply during solar storms. Extrapolating those measurements back across billions of years accounts for the loss of a substantial atmosphere.
Mars's low gravity, about 38% of Earth's, made escape easier at every stage. Lighter molecules, hydrogen in particular, leaked away most readily — and the isotopic composition of what remains supports this. Martian atmospheric gases are markedly enriched in heavier isotopes, exactly the signature expected when lighter atoms preferentially escape over long periods.
Some of the carbon dioxide was also removed by a different route, locked chemically into carbonate minerals in the crust. On Earth, plate tectonics recycles such deposits back into the atmosphere through volcanism. Mars has no plate tectonics, so what was buried stayed buried.
Then the oxygen went into the rocks
As water broke down under ultraviolet light and the atmosphere thinned, freed oxygen had somewhere to go: the abundant iron in Martian basalt. Iron plus oxygen produces iron oxide, and Mars has a great deal of iron.
Global dust storms — occasionally planet-encircling — ground the oxidised material into fine particles and distributed them everywhere. The result is a planet uniformly coated in rust-coloured dust, tinting even the sky, which appears butterscotch during the day and can turn blue near the setting sun, an inversion of Earth's colours caused by dust scattering.
Dig below the surface layer and the colour changes. Rover wheel tracks and drill holes routinely expose grey material underneath. The redness is largely a coating.
Why this matters beyond Mars
Mars is the clearest available demonstration that habitability is not permanent. A planet can sit in the right orbital zone, have liquid water, and still lose it — not through anything dramatic, but through slow interior cooling that switches off a magnetic field and leaves the atmosphere unprotected.
That has direct implications for how astronomers assess exoplanets. Orbital distance and planet size are the easy measurements, but the Mars example suggests that magnetic field, internal heat budget and tectonic activity may be just as decisive over billion-year timescales.
It also sharpens the search for past life. If Mars had lakes with reasonable chemistry for hundreds of millions of years, it may have had time to develop life. If it did, the evidence would be in ancient sediments — which is precisely why Perseverance is caching rock samples in Jezero Crater for a future mission to return to Earth.
Summary
Mars is red because iron in its surface rocks rusted, and it rusted because Mars once had water and a substantial atmosphere. The planet was too small to stay hot inside, its core dynamo failed roughly four billion years ago, and the resulting loss of a global magnetic field let the solar wind strip the atmosphere away over hundreds of millions of years. The dust covering the planet today is the chemical residue of that loss — a rusted record of a world that was habitable and then was not.
Be the first to share a thought.