Why is Mars red?

via Astronomy Now.

As the red planet rises earlier and earlier in the Eastern skies these September mornings, it is becoming of more interest to amateur astronomers. The Martian features getting easier and easier to see as the planet approaches. But even at its present far distance, one thing immediately visible is the colour, a fiery orange-red. But why is Mars this colour?

The planet’s surface is actually mostly composed of basalt, a black rock. At the poles, there are white caps of water and carbon dioxide ices. But the entire surface is covered with something else – a thin, red iron-rich dust. The origin of the dust was thought to be related to the period more than 3.8 billion years ago, when Mars was wet and humid, or perhaps to deposits of water in the interim period, effectively leading to ‘rusting’ of the iron.

However, a study by Jonathan Merrison of the Aarhus Mars Simulation Laboratory, Denmark and colleagues suggests that the fine dust could be the result of mechanical rather than chemical action on the surface. They sealed quartz sand in flasks and tumbled them over and over to simulate the effect of the wind. After seven months of tumbling more than ten million times, ten percent of the sand had been turned to dust. In the presence of powdered magnetite (a mineral present in Martian basalt) was added, the dust turned redder and redder with each tumble. The magnetite was converted to hematite without the need for water.

The experiment is presently being repeated in a dry CO2 atmosphere, such as has existed on Mars for quite some time.

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