The famed ALH84001 Martian meteorite. NASA Detailed measurements of sulfur isotopes in five Martian meteorites have enabled researchers at the University of California, San Diego to determine that the abundant sulfur on the surface of Mars is due largely to chemical reactions in the Red Planet's atmosphere that are similar to those that occur in Earth's atmosphere. Their conclusions, which are detailed in a paper in the March 2 issue of Nature , also suggest that the variations in sulfur isotopes found on ALH84001, the Martian meteorite thought by some scientists to contain evidence of ancient Martian life, are not due to biological processes.
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. On Earth, such changes are often produced by terrestrial bacteria that derive their energy solely from the conversion of sulfur compounds from one form to another. In so doing, they selectively break the chemical bonds of the lighter isotopes of sulfur, producing large variations in the normal sulfur-isotope ratio. In their laboratory, Thiemens and UCSD researchers James Farquhar, Joel Savarino and Terri L. Jackson sought to find out whether some of this sulfur may have been produced by organisms. They also examined the sulfur in the Martian meteorites to find clues to the evolution of the Martian atmosphere, a major puzzle for planetary scientists.