Barring any one-eyed, multi-limbed green monsters, the search for life on Mars is shaping up to be a painstaking process of definitions and details. What are we looking for? And how do we know when we've found it?

A new study of Martian meteorites adds one more complication to how scientists will study rocks that may one day be scooped up on the Red Planet and, possibly, returned to Earth.

Only a handful of meteorites thought to have come from Mars have been found. James Farquhar and colleagues at the University of California at San Diego studied compounds of sulfur from these rocks, finding that the sulfur originated in the Martian atmosphere, not from biological processes.

Sulfur, abundant on the surface of Mars, can exist in different forms, known as isotopes. Some scientists have previously suggested that the relative abundance of sulfur isotopes could provide signals of biological activity.

But Farquhar's group, reporting in the March 2 issue of the journal Nature, show that the signs can just as easily be produced by non-biological processes in the atmosphere.

Harry McSween, Jr. of the University of Tennessee said the sulfur isotope patterns could only result from reactions of sulfur in the Martian atmosphere.

"This implies some kind of cycling that we don't really understand," said McSween, who was not involved in the research. "This finding just means that scientists must be cautious in interpreting the sulfur isotopic compositions that will be measured in returned Martian samples as indicators of life."