Mars was not always red, according to a new theory for how the planet took on its characteristic ruddy hue.
Until recently, Mars' color was thought to be a product of liquid water, which scientists think flowed over the planet's surface billions of years ago, rusting rocks. But after the Mars Exploration Rovers Spirit and Opportunity landed on the planet in 2004, they found evidence of certain minerals that would have been destroyed by water, suggesting that the red dust on Mars never came into contact with flowing water.
"That was a surprise to everybody," said Jonathan Merrison of the Aarhus Mars Simulation Laboratory in Denmark.
Now new research has found a possible mechanism to explain Mars' rusty color without liquid water. In fact, the study implies that the red tones on the planet are a relatively recent development. A simple grinding down of rocks from erosion could produce a red mineral that stains the dust on Mars, the new thinking goes.
In the lab
To test the idea, Merrison and colleagues sealed samples of quartz sand in glass flasks and used a machine to tumble them over and over. They found that the gentle process, which approximates the mild wind flowing over the Martian surface, is enough to cause erosion, reducing about 10 percent of the sand grains to fine dust particles over seven months.
The scientists then added powdered magnetite, an iron oxide present on Mars, to the flasks.
As the researchers continued to tumble the samples, they observed the sand getting redder and redder.
"We think we have a process that explains how the dust became red without liquid water, which doesn't seem to fit in with the data," Merrison told SPACE.com.
As the sand grains turned over in the flasks and hit each other, they fractured, breaking apart some chemical bonds at the newly-exposed surfaces. When these surfaces came into contact with the magnetite, an oxygen atom could be transferred from quartz to magnetite, forming a new mineral, hematite.
Hematite is an iron oxide that is deep red in color. It only takes a little hematite, Merrison said, to stain all the dust a reddish hue.
"When we finished we could see red stuff on the side of the bottle," he said.
Same on Mars?
Though they can't yet prove that this is what happened on Mars, it seems like a plausible explanation, and doesn't require water for the reddening process.
In fact, since the process can occur relatively quickly, it could be that the thin red layer of dust on Mars is somewhat new.
"I think it means that Mars wasn't always red," Merrison said. "Before this work, I think most people in the field kind of thought the Martian surface was billions of years old and had always been red. This work seems to imply that it could be quite recent – millions of years instead of billions of years."
Merrison presented the results last week at the European Planetary Science Congress in Germany.
- Image Gallery: Seeing Red: A Tour of Mars
- Video - Mars: Red Dust Dangers
- Rover Video: 5 Years on Mars