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Experts: Little Evidence of Life on Mars

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Aug. 8, 2006 — Ten years ago this summer, NASA announced the discovery of life on Mars.

At a Washington, D.C., news conference, scientists showed magnified pictures of a four-pound Martian meteorite riddled with wormy blobs that looked like bacterial colonies.

The researchers explained how they had pried numerous clues from the rock, all strongly supporting their contention that microscopic creatures once occupied its nooks and crannies.

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Ten years later, the results have not been verified. Skeptics have found non-biological explanations for every piece of evidence that was presented on Aug. 6, 1996. And though they still vigorously defend their claim, the NASA scientists who advanced it now stand alone in their belief.

"We certainly have not convinced the community, and that's been a little bit disappointing," said David McKay, a NASA biochemist and leader of the team that started the scientific episode.

But even though the majority of his colleagues don't buy his "life on Mars" theory — McKay's own brother, also a NASA scientist, is one of his most prominent critics — many say they respect him and greatly appreciate his efforts.

The announcement and the technical paper that followed it practically created exobiology, the scientific field that investigates the potential for life on other planets.

Debating the claim has helped researchers develop standards that will eventually prove useful for evaluating the presence of life in other Martian meteorites or a sample from the red planet.

It has given the scientific community ideas about exactly where on the planet they would most like to scoop up a sample, should they ever get to retrieve one.

And it is undeniable that McKay and his colleagues have drawn attention to what is — whether it contains evidence of life or not — a very interesting rock.

The rock in question, named ALH84001, was discovered in Antarctica, where rocks that fall from the heavens are easy to spot on the icy glacial plains.

At first ALH84001 was misclassified, so it wasn't until 1993 that researchers even realized the rock came from Mars.

At 4.5 billion years old, it dates from a period of Martian history when liquid water — a requirement for the presence of life — probably existed at the now barren planet's surface.

It made sense to ask: Could there be fossils of ancient Martian microbes, or maybe traces of them, preserved in the cracks and pore spaces of ALH84001?

The NASA scientists proffered four reasons to support their view that the answer to that question is "Yes."

First, chemical analysis showed that the meteorite contained a variety of organic molecules that can be produced by biological processes.

But they are also commonly found in asteroids, comets and meteorites, not to mention the Antarctic ice where ALH84001 is estimated to have lain for 13,000 years. For that reason, skeptics immediately dismissed the importance of those molecules in the Martian meteorite.

A second line of evidence — that the elongated blobs in the electron microscope images could be fossils of ancient Martian bacteria — was also rejected pretty quickly by most scientists.

The two other lines of evidence survived longer. Both revolved around minerals sprinkled through the meteorite that could have been produced by microbes.

The first mineral, carbonate, is typically formed on earth by the remains of living organisms that make shells and other skeletal parts out of minerals they extract from seawater. Some of those organisms can be quite tiny. So finding carbonate in ALH84001 could indicate the presence of ancient microbes in the rock.

The story is similar for magnetite, the other mineral of interest in ALH84001. Some bacteria produce extraordinarily small and pure magnetite crystals, then align the magnetic grains to make a microscopic compass needle that helps them navigate.

 

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