Scientists in the USA have announced that they can find no evidence for life in microscopic structures observed on a meteorite from Mars.

In the new edition of the journal, Nature, they write that geological processes and laboratory procedures could account for all the supposed micro-fossils.

When scientists from the US space agency NASA announced evidence of fossil life in a meteorite from Mars last year, it caused a sensation.

Most exciting of all were tiny structures, far thinner than a human hair, resembling microscopic 'nanobacteria' from Earth. Some were segmented, as if they had been multiplying.

The scientists agreed at the time that such extraordinary claims required extraordinary evidence, and there have always been sceptics.

Now, Ralph Harvey, leader of the team that found the meteorite in Antarctica, together with colleagues, has published evidence suggesting the structures may not be fossil life at all.

Dr Harvey copied the techniques used by the original research team and showed that structures resembling the nanobacteria were really tiny whiskers of minerals, and that the segmented structures could be produced by techniques used to prepare samples for the electron microscope.

But the NASA scientists stand by their claims. They point out they have stronger, though only circumstantial, evidence for life from Mars, in the shape of chemicals which are produced by living organisms on Earth, and magnetic grains like those found only in bacteria on Earth.

The fossils are different from the mineral whiskers and segmented structures turned up in samples that had been prepares differently.

The Martian bacteria may be dead, but the controversy surrounding them lives on.

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