Old Martian questions may have new answer.

A mystery surrounding the 1976 Viking mission to Mars has lingered for a quarter-century, but a recent experiment may have revealed the solution. The new results may guide future Mars missions, particularly those in search of life.

The Viking mission appeared to find that Martian soil can destroy organic molecules. Twenty-five years of trying to identify chemicals that might cause this breakdown hadn't found a convincing explanation, says Albert S. Yen of NASA's Jet Propulsion Laboratory in Pasadena, Calif. So, he and his colleagues simulated the Martian surface in their laboratory. They theorized that a chemical agent, superoxide radical ions, could arise and destroy organics on Mars.

In test-tube experiments, the researchers exposed mineral grains resembling those in Martian soil to a simulated Martian atmosphere and then irradiated the samples with ultraviolet (UV) light like that reaching Mars.

Using spectroscopy, Yen and his team. observed that superoxide radicals form on the mineral surfaces under these conditions, they report in the Sept. 15 SCIENCE. Such radicals would survive at Martian temperatures, they add.

The researchers report that the superoxide radicals are reactive enough to decompose organic molecules at the Martian surface. These radicals could also migrate through the Martian soil, the team claims. That is consistent with the Viking findings of soil capable of destroying organic matter both at the surface and 10 centimeters below, where superoxide-forming UV light wouldn't penetrate.

Although the superoxides could explain why Martian soil breaks down organic molecules, there still are other potential culprits, comments Christopher P. McKay of NASA's Ames Research Center in Mountain View, Calif. The most compelling aspect of the work by Yen's group is its simplicity, he says, adding that he'd like to know how far the superoxides could percolate through Mars' surface. Such information would be useful for any missions that might search for superoxides or organic chemicals and other potential signs of life.

Yet Gilbert Levin, the leader of one of the other Viking experiments, insists that the fundamental idea that Mars' surface lacks organics is wrong. He says that the technology used to detect organic molecules wasn't sensitive enough. He holds that the results of his 1976 experiment were consistent with microbial life on Mars and says that the new report doesn't adequately explain those results.

Others are skeptical of Levin's interpretation of the Viking data. McKay says that Yen's superoxide theory can account for them more convincingly.

Superoxides would provide "a simple way" of destroying any organics that reach Mars' surface in dust and meteorites, adds Allan H. Treiman of the Lunar and Planetary Institute in Houston. Like Yen, Treiman suspects that robots searching for Martian life will have to drill into rocks or dig deep into the soil.