Scientists had shown an interest in the sulfur isotope ratios in Martian meteorites, particularly ALH 84001, because similar variations seen in terrestrial rocks have been linked to biochemical reactions.
Organisms are very good at separating isotopes and choosing one over the other.
So when you see big changes in isotope ratios, it often means biochemistry. To test this, Thiemens, lead researcher James Farquhar, and colleagues studied samples from five Martian meteorites to better characterize the variations. Those studies, combined with the known composition of the Martian atmosphere, led them to conclude that inorganic chemical reactions, and not biochemistry linked to native Martian life forms, caused the sulfur variations. Biology can't accommodate what we see, but the photochemistry in the Martian atmosphere does. The results are the latest round in the scientific debate regarding whether Martian meteorites show evidence of ancient primitive life on Mars.