Scientists from NASA's Jet Propulsion Laboratory and the California Institute of Technology have now figured out precisely why Martian soil lacks organic molecules, the building blocks of life. Shortly after the Viking landers went looking for alien life in the 1970s, scientists realized that oxygen free radicals capable of destroying organic molecules could be found on the Red Planet's surface. But they didn't know what those chemicals were or how they formed.

To find out, Albert S. Yen and his colleagues recently simulated the Martian surface in the lab, exposing mineral grains like those found on the planet to the kind of ultraviolet radiation the sun would deliver there. "We found that the combination of ultraviolent radiation, mineral grain surfaces, atmospheric oxygen and extremely dry conditions produce superoxide ions," Yen says. "This is all that is necessary to make the reactive component of soil." The complete report appears in the September 15th issue of Science.

Now knowing the role ultraviolet radiation plays in forming the superoxide ions, Yen suggests that researchers can narrow their search for organic matter to below the planet's surface and within rocks. How far they will need to dig is, of course, a mystery. Notes co-author Bruce Murray, "Determining how deep that oxidizing layer is on Mars is the most important next step in searching for life there."