Still searching for life on Mars

Some of the most evolutionarily ancient bacteria on Earth produce magnetite, McKay and his colleagues pointed out. Perhaps ancient Martian microbes did as well; at least some of the magnetite grains in ALH84001 share the shape, small size and remarkable purity of those produced by bacteria on Earth.

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.

Most of the scientific community doesn’t buy those arguments.

“Personally I don’t understand why (Gordon McKay’s and) Golden’s work hasn’t just been the final word on it,” said Treiman, the Lunar and Planetary Institute geologist.

Now David McKay has added another meteorite to the mix. At a March scientific meeting he presented microscopic images of the Nakhla meteorite, another Martian specimen. The pictures resemble pits that terrestrial bacteria create as they literally eat the volcanic rock of the sea floor.

“When I first saw it I was really struck by the similarity,” said marine geologist Fisk, who is a professor at Oregon State University.

So far the scientific community hasn’t shown much interest in David McKay’s analysis of the Nakhla meteorite, partly because it dates from a more recent period of Martian history when the planet was just as frigid and inhospitable to life as it is today. In fact all of the 30-some Martian meteorites now known to science, with the exception of ALH84001, are probably too young to have contained living organisms.

But new Martian meteorites turn up almost every year. Eventually, another 4.5 billion-year-old piece of the red planet is going to be discovered.

“Sooner or later we’re going to get another old rock,” Weiss said.

And when that happens, the talk about life on Mars will begin anew.

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