The grains in the Mars rock claimed to be identical to a terrestrial ones

"We argue that existing evidence is inadequate to support the inference of former life on Mars," they say in the Proceedings of the National Academy of Sciences (PNAS).

But advocates of the life on Mars theory say that the critics suppose that terrestrial and Martian bacteria were identical. Martian bacteria could have come in types unknown on Earth today, they argue.

Stand-off

The new doubts come from researchers led by Peter Buseck of Arizona State University, US. This team says that earlier findings of signs of past life in a Mars rock were based on flawed reasoning and problematic measurements, particularly those performed with a transmission electron microscope (Tem).

The new analysis of the shapes of the magnetite grains

Tem images of thin sections of a rock from Mars revealed lines of bright objects assumed to be magnetite crystals, which are also found in terrestrial bacteria.

The international team says the research which claimed that some of the magnetite grains in a Mars rock were indistinguishable from Earth bacteria could not differentiate between faces, rough surfaces, or rounded edges unrelated to internal symmetry.

In addition, when the researchers compared the crystals from the meteorite with crystals from three bacterial strains from Earth, none matched.

Scientific stand-off

"Although the similarities are intriguing, we believe that they do not provide evidence that the crystals are 'Martian magnetofossils' or that they constitute evidence of the oldest life yet found," says the research team.

But the advocates of the life on Mars theory say that although they have claimed that magnetite grains in the meteorite are indistinguishable from those produced by terrestrial bacteria, it need not be the case that the simple lifeforms on Mars would have produced, in all cases, magnetite grains similar to those produced by microbes on Earth today.

In a sense the stand-off continues. One researcher told BBC News Online: "Perhaps the only way to sort it out would be to bring rocks directly back from Mars."