Life on Mars Update
(From the May/June 1997 issue of StarDate magazine)
Evidence of ancient microscopic life on Mars may or may not hold up, but a scientific committee says there is no reason to take chances with future samples from the planet: Since they may contain living microbes, they should be treated as potential threats to life on Earth.
A National Research Council committee recommended that Martian rocks and soil brought back to Earth by robotic spacecraft early in the next century should be isolated until scientists can determine whether they contain living organisms. Since they evolved in different environments, it is unlikely that Martian microbes could harm life on Earth, but the risk is great enough that scientists should be careful, the committee wrote. Similar precautions were used for Moon rocks picked up by Apollo astronauts, but no signs of life were found.
The NRC report was released at about the same time that two teams of scientists reported opposite conclusions from their test of a Martian meteorite that contains evidence of microscopic life. The rock, which was blasted off the Martian surface by a large meteorite impact and landed in Antarctica 13,000 years ago, contains possible microfossils and organic chemicals.
Tiny globules of carbonates (minerals rich in carbon and oxygen) in the meteorite probably formed at temperatures below 200 degrees Fahrenheit, which makes it more likely they were produced by biological processes, according to a team led by John W. Valley, a geochemist at the University of Wisconsin-Madison.
Some scientists had suggested that the globules formed through chemical processes at temperatures of more than 1,200 degrees, which would kill any known organism. But Valley and his colleagues, analyzing the ratio of different forms of carbon and oxygen within the globules, found that they formed at much lower temperatures. The finding does not prove that the globules were the byproducts of living organisms, but it removes one challenge to the theory.
And then again, maybe it doesn't.
A few days after Valley's team released its findings, a second group -- led by UCLA geochemist Laurie Leshin -- reported that the globules almost certainly formed as the result of chemical processes, not biological ones. Leshin's group used the same techniques as Valley's to study pieces of the meteorite that were much smaller than a grain of sand.
The UCLA scientists found that the ratios of different forms of oxygen found inside the carbonate globules don't fit the biological models proposed by other scientists. Conditions when they formed were either too hot or too dry for life, according to the UCLA team. Leshin and her colleagues did identify one possible scenario that involves living organisms, but they consider it unlikely.
Several other scientific teams are studying the meteorite, so additional evidence should be added to the debate within the next few months. -- — Damond Benningfield
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