Spirit, NASA's scrappy exploration robot, has been snapping some breathtaking photos of Mars. More portraits of red-tinged landscapes should emerge shortly, as Spirit ventures forth from its lander. But why does every nook and cranny on Mars invariably look red?
The simple explanation is that the planet's soil is rich in iron oxide, but there's much debate as to why the mineral is so ubiquitous in the Martian environment. The old theory is that the oxidization process began early in Mars' life cycle, when warm water flowed on the planet—water that may have carved out the long, now-barren channels that snake through portions of the planet. Rocks containing iron would have slowly been worn away by rivers and seas, and the oxygen in the water would have combined with the iron to create iron oxide—or, in lay terms, the iron would have rusted into red dust. Flecks of the reddish mineral would then have been dispersed all over the planet via raindrops. Scientists who believe that Mars was once flooded with water have pointed to the abundance of iron oxide as proof of their claims. And where there's water, of course, there may also be life.
But data culled from Pathfinder's visit to Mars in 1997 hints at an alternative explanation, championed by Albert Yen of NASA's Jet Propulsion Laboratory. Pathfinder found that Mars' soil contained far more iron than its rocks, which suggests that at least some of the planet's iron came from meteorites—a pretty easy contention to support since Mars' surface is pockmarked with impact craters. Yen has gone a step further, however, in arguing that water needn't have been present for the meteorite-borne iron to be converted into iron oxide.
In 2000, he conducted an experiment with a 100-milligram chunk of labradorite, a mineral commonly found in Martian soil. The sample was placed in a test tube filled with gases common to the Martian atmosphere and chilled to a Mars-like minus 76 degrees Fahrenheit. Finally, Yen and his cohorts pelted the tube with ultraviolet light, to mimic the effects of sunlight. (UV light on Mars is particularly harsh since the planet's ozone layer is so thin.) After a week, they analyzed the sample for evidence of superoxide ions, negatively charged oxygen molecules that are capable of causing iron oxidization even when there's no water present. Sure enough, Yen found the superoxides as he'd predicted—a blow to the astrobiologists who'd long believed that Mars' red hue indicated that water, and perhaps life, must have once been abundant on the planet.
The presence of superoxides in the Martian soil would also explain why the Viking landers of 1976 found no evidence of organic material on the planet—superoxides break down all organic compounds, including those carried on meteorites. But they also work extremely slowly, perhaps too slowly to fully explain the prevalence of reddish hues on the Red Planet. It's possible that Mars' coloration, then, is due to the combined effects of ancient water and superoxides.