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.
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