In previous work, Jull had determined that the carbonates in ALH84001 are substantially enriched in the isotope carbon-13 compared to those on Earth. He and his colleagues interpret this as an indication that the carbon dioxide in the early Martian atmosphere was also enriched in carbon-13. If that is the case, then the tissue of Martian organisms would also have elevated levels of carbon-13. When the team analyzed the ratio of carbon isotopes in the organic carbon, however, it found that fully four-fifths of the material had the same isotopic signature as terrestrial carbon. The other 20 percent appears to have a preterrestrial origin, they found.
"It looks like regular terrestrial organic material, with the exception of one small component in ALH84001," Jull said in a University of Arizona news release.
The analysis "indicates a much greater degree of terrestrial contamination in the meteorite than I suspected was present two years ago," Zare said. "In that sense, Jull's study does cast new doubt on our hypothesis that the meteorite contains evidence of past Martian life."