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MARS鈥檚 distinctive red colour may be the result of thousands of years of wind-borne sand particles smashing together, not rust as was thought.

It is generally agreed that Mars鈥檚 colour is the result of a black form of iron called magnetite oxidising into the reddish-orange form called haematite. How the transformation came about is a matter of debate, however. The favoured idea is that water is responsible, but some argue that hydrogen peroxide and ozone, which might be created when ultraviolet light breaks down the carbon dioxide and oxygen in the Martian atmosphere, could be involved.

Now Jonathan Merrison of Aarhus University in Denmark and colleagues say the trigger may be wind. They sealed particles of magnetite and quartz 鈥� minerals present on both Earth and Mars 鈥� in a glass flask filled with CO₂. After tumbling the flask for several months to simulate wind transport, they found that its contents became redder over time. The team suspects that the constant collisions split the quartz grains, exposing chemically reactive surfaces that oxidised the magnetite into haematite.

On Mars, quartz and magnetite particles could collide while being blown about by winds in dust devils and global dust storms, says Merrison. Even without water, the wind on early Mars might have transformed it from a charcoal colour to red in just a few hundred thousand years (Icarus, ).

鈥淲ind might have transformed Mars from charcoal to red in just a few hundred thousand years鈥�

Not everyone agrees, though. Quartz is a relatively hard mineral that may not be easily broken down into sand by the Martian wind, says Joel Hurowitz at NASA鈥檚 Jet Propulsion Laboratory in California. And while quartz has been detected in the bedrock in some regions, NASA鈥檚 twin Mars rovers have found little in the sand and dust.