The Red Planet as seen by NASA's Mars Global Surveyor spacecraft. Photo: NASA/JPL/MSSS

Intense ultraviolet radiation that pierces Mars' thin atmosphere produces an abundance of oxygen ions, a common free radical, at the Martian surface that destroys organic molecules -- the building blocks of life -- according to researchers at NASA's Jet Propulsion Laboratory, Pasadena, Calif.

Scientists have been puzzled since the mid-1970s when NASA's Viking landers failed to find any organic materials, not even traces delivered to Mars by meteorites. That discovery led scientists to recognize that there were oxidants in the Martian soil capable of destroying organic molecules. It has taken until now for a team to come up with a comprehensive idea of what those oxidizing chemicals are and how they form.

"We simulated the Martian surface environment in our laboratory and found that the combination of ultraviolet radiation, mineral grain surfaces, atmospheric oxygen and extremely dry conditions produce superoxide ions. This is all that is necessary to make the reactive component of soil," said Dr. Albert Yen, a JPL planetary scientist and lead author of the study being published in Science magazine on September 15.

This combination of surface conditions exists on Mars today and the superoxides are generated during daytime exposures to ultraviolet radiation.

"Our research does not address whether life ever formed on Mars, but it does give us more information about where to look for life or evidence of past life," Yen said. "Evidence of life might exist beneath the surface or in the interiors of rocks that are protected from the superoxide ions. What we don't know is how far below the surface we would need to look."

"Determining how deep that oxidizing layer is on Mars is the most important next step in searching for life there," said Caltech Professor Bruce Murray, a co-author on the study.

The research team plans to study the movement of these oxygen radicals under simulated Martian conditions to estimate how deep they may be distributed. Future experiments to search for subsurface organic molecules could be carried out by penetrators and/or by drilling from a surface lander.  

The Martian soil experiment was conducted at JPL for NASA's Office of Space Science and the Office of Life and Microgravity Sciences and Applications, Washington, D.C. JPL is a division of the California Institute of Technology, Pasadena.