New laboratory studies suggest that Mars' red dust is formed by the grinding of surface rocks, and not through rusting by oceans that once flooded the planet's surface.

It is widely believed that Mars is red because ancient rivers and oceans 'rusted' the rocks. A new study presented today at the European Planetary Science Congress by Jonathan Merrison of the Aarhus Mars Simulation Laboratory, Denmark, suggests that Mars' signature red dust may be a result of the ongoing grinding of surface rocks and that liquid water need not have played any significant role. The finding opens up fresh debate about the function of water in Mars' history.

“Mars should really look blackish, between its white polar caps, because most of the rocks at mid-latitudes are basalt,” says Merrison. “For decades we assumed that the reddish regions on Mars are related to the water-rich early history of the planet and that, at least in some areas, water-bearing heavily oxidized iron minerals are present.”

Fine red dust covers the surface of Mars and is even present in its atmosphere, sometimes shrouding large areas of the planet in vast dust storms. Understanding the properties of this dust, and indeed of the mineralogy of Mars as a whole, is crucial in the quest for understanding the past and present Martian environment and whether habitable locations could still exist today.

Merrison and colleagues pioneered a novel technique to simulate sand transport on Mars by sealing quartz sand samples in glass flasks and 'tumbling' them for several months – equating to some ten million revolutions. After seven months almost ten percent of the sand had been reduced to dust, and when powdered magnetite – an oxide present in Martian basalt – was added, the dust became redder the more the flasks were tumbled.

“Reddish-orange material deposits, which resemble mineral mantles known as desert varnish, started appearing on the tumbled flasks,” says Merrison. “Subsequent analysis of the flask material and dust has shown that the magnetite was transformed into the red mineral hematite, through a completely mechanical process without the presence of water at any stage of this process.”

The same effect also occurred in a dried carbon dioxide atmosphere – conditions that resemble those occurring on Mars today, which could have implications for a geologically recent occurrence of the red dust. But it is early days, and new results need to be further investigated through more experimental and analytical work. “By simulating the conditions and developing accurate analogues of the Martian environment, we will certainly gain a deeper understanding of its dusty nature,” says Merrison. “In particular, developing better analogues of the Martian surface and atmosphere is vital in interpreting observations made on Mars by landers as well as pioneering the next generation of experiments to be flown.”

The research has been accepted for publication in the journal Icarus.