One of the most recognisable features of Mars is its red colour. Even the naked eye can clearly discern the Red Planet’s ruddy hue.
Humans have wondered where this red colour comes from for centuries, perhaps millennia.
Previously, it was thought to be due to the presence of hematite (from the Greek word for ‘blood’), a dry, iron-oxide mineral also commonly found on Earth.
Continuous oxidation and weathering of Martian rocks, under water-poor surface conditions during the so-called ‘Amazonian’ period (starting 3 billion years ago) could also explain Mars’s red colour.
Like hematite, the red colour of rust is also derived from the presence of various oxides and hydroxides of iron.
But recently, a team of researchers has called the hematite assumption into question and suggested an alternative explanation.
The scientists worked with data from several space probes, including NASA’s Mars Reconnaissance Orbiter and the European Space Agency’s Mars Express and ExoMars. These data were combined with measurements from NASA’s Mars rovers.
The research showed that another mineral, called ‘ferrihydrite’, a water-rich ferric oxyhydroxide, is widespread in Martian dust, and probably also in the subsurface rock layers. This may, in fact, be the main cause of the reddish colouration.
Laboratory tests on a synthetic Martian mineral mixture, consisting of basalt and ferrihydrite, gave a far better match to the observed data than hematite did.
For ferrihydrite to form on Mars, both oxygen and water capable of reacting with iron would be required. Such ingredients are sparse in the dry, cold environment of Mars today, but may have existed in the planet’s distant past.
On Earth, ferrihydrite quickly transforms into iron oxides (such as hematite), but on Mars, the researchers found, it could survive almost indefinitely once formed.
This article is an answer to the question (asked by Roy Bowman, Coventry) 'Why is Mars red?'
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