After 10 years, few believe life on Mars

Of all the lines of evidence presented by the NASA scientists, it was the magnetite grains that proved most provocative. They were embedded in the carbonate along with other iron-containing minerals in such an unusual arrangement that something out of the ordinary must have put them there -- could it have been alive?

"The shape of the magnetite grains is still rather distinctive," McKay said. "If it were found on Earth it would be a very strong biosignature."

For years McKay and his detractors argued about how distinctive the magnetite grains in ALH84001 are, and whether a non-biological process could have produced them. Certainly nobody had ever produced similar magnetite grains in the laboratory.

Then somebody did. In 2001 a second team of NASA scientists, including McKay's brother Gordon and a consultant to the space agency named D.C. Golden, managed to cook up a batch of magnetite grains very similar to the ones in ALH84001. Golden and Gordon McKay were also able to incorporate the magnetite grains into balls of carbonate like the ones David McKay and his colleagues described in 1996.

"He got a little testy about the results we were getting," said Gordon McKay, whose office is down the hall from his brother's. "What we have shown is that it is possible to form these things inorganically."

What's more, their laboratory method simulated conditions ALH84001 is known to have experienced during its time on Mars.

Yet David McKay insists his brother's team has not accurately described the synthetic crystals' shape, and that they aren't sufficiently similar to the ones found in ALH84001. He also suggests that the purity of the magnetite crystals stems not from the lab process itself, but from using unrealistically pure raw materials as a starting point.

How to tell Earthlings that Martian life is here

After any Mars rocks are returned to Earth, preliminary NASA plans call for them to be isolated and quarantined in a specially constructed laboratory. They would be tested there to determine whether they pose any threat to life on Earth and, if deemed safe, released to other laboratories for further study.

The communications strategy should emphasise the scientific benefits of the sample return mission, while not ignoring public concerns about a potential biohazard, Rummel says.

A lobbying group has already formed to prevent such a mission from happening, citing biohazard concerns.