What Makes the Red Planet Red?

The short time that Spirit and now Opportunity have been exploring their new home on Mars, they have returned stunning images. We are all getting used to a reddish landscape with the pink sky, so different and yet familiar. But why is everything so red on Mars?

The short answer to what makes the red planet red is that Mars' soil is rich in iron oxide, or rust, but that raises questions such as why iron oxide is so prevalent?


Larger image of Mars Horizon

Mars Spirit Mission Gallery of images

It had been believed that the oxidation began very early in Mars evolution. This would have been at a time when warm water flowed on the planet. It is likely this water carved out the long now-barren channels that we have observed in multiple places on Mars. Iron-rich rocks would have been slowly worn away by such flowing water and the oxygen in the water would have combined with the iron in the rocks to form iron oxide. More simply put: rust would have formed. Flecks of this reddish rust would have dispersed all over the planet, carried by raindrops.

That theory was before Pathfinder visited Mars in 1997. Scientists at JPL found that Mars' soil contained far more iron that its rocks. This seemed to suggest that at least some of Mars' iron came from meteorites. Evidence for meteor strikes is all over Mars with many impact craters. Then, another JPL scientist suggested that it would not have to have been necessary for water to be present for the iron from meteorites to be converted into iron oxide.

In 2000, an experiment was conducted to test this theory using a chunk of labradorite, a mineral very commonly found in Martian soil. The labradorite was put in a test tube which was filled with gases composed very similar to the atmosphere on Mars. To mimic temperature, the tube was chilled to a typical Mars-like minus 76 degrees Fahrenheit. Then the scientists exposed the tube and labradorite to ultraviolet light to mimic the effects of sunlight on Mars. There isn't an ozone layer on Mars like ours, the Martian ozone layer is very thin. Because of this, a lot of ultraviolet radiation gets through the Martain atmosphere to the surface. After the experiment had run for a week the sample was analyzed. The scientists were looking for negatively charged oxygen molecules that are capable of causing iron oxidation even with no water present.

Sure enough, the molecules were there, proving the theory. That struck a major blow long held by astrobiologists who attributed the oxidation on Mars as evidence of water in the planet's past.

There was a bright side of this for the astrobiologists though, because the presence of those negatively charged oxygen molecules also gave an explanation for why the Viking landers of 1976 found no evidence of organic material on Mars. Those same molecules would also break down all organic compounds.

There are still challenges to the theory though. Some scientists allege that the processes which cause oxidation without water work so slowly that it could not possibly account for all the rust on Mars.

More studies are needed. Hopefully the new data provided by Spirit and Opportunity will provide clues.


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