More than four years after researchers at the US space agency NASA reported signs of fossil life in a martian rock, another team has produced what may be the strongest evidence yet to support the controversial finding. But many scientists are still unconvinced by the claims.

Microbiologist Imre Friedmann, an emeritus professor at Florida State University and a specialist on life in extreme environments, says that high-power electron-microscope images of magnetite crystals in the meteorite suggest that they are of biological origin.

Is the similarity between terrestrial magnetite (top) and that on the martian meteorite evidence for bacterial activity on Mars? Credit: NASA

Writing in Proceedings of the National Academy of Sciences (98, 2176–2181; 2001), Friedmann and his colleagues describe chains of magnetite, which some terrestrial bacteria use to orientate themselves in Earth's magnetic field. They say the chains are too orderly to have been produced without the action of living organisms. The images also reveal possible remnants of membranes around the crystals, which are of uniform size and shape.

Meanwhile, Kathie Thomas-Keprta, a planetary scientist at NASA's Johnson Space Center in Houston, Texas, and co-author of the original paper on fossils in the meteorite, has examined individual magnetite grains. Some 25% of the magnetites in the meteorite show characteristics of crystals produced by living organisms, she and her colleagues write in the same issue of the journal.

But most experts remain sceptical. Allan Treiman, a researcher at the Lunar and Planetary Institute in Houston, will claim in a talk next week at the institute's annual science conference that the grains were created by ordinary decomposition of iron-rich carbonate minerals.