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More Martian Microbes?


MARCH 19, 1999


On Thursday, March 18th, NASA scientists offered new evidence that fossilized microbes could be present in at least three meteorites from Mars. Kathie L. Thomas-Keprta (NASA/Johnson) showed that the controversial stone known as Allan Hills 84001 contains many microscopic crystals of the iron-rich mineral magnetite. One-fourth of these are perfectly shaped, same-sized hexagonal prisms free of chemical impurities. Certain bacteria routinely produce such ultrapure magnetite crystals as a means of orienting themselves to Earth's magnetic field, and they cannot be formed by any known inorganic process. Of all the hints of microbial fossils seen in ALH 84001, Thomas-Keprta says the magnetite grains provide the strongest evidence.

David S. McKay (NASA/Johnson) raised the possibility that two other Martian meteorites, Nakhla and Shergotty, contain fossilized microbes. His scanning-electron-microscope views show a variety of round and oval forms found in tiny cracks within a Nakhla stone. "Are they microfossils?" McKay asked aloud. "We don't know." But he noted that the blobs are enriched in iron oxides, a common occurrence when a microbe dies and its cell becomes mineralized. Moreover, the suspect features are a few tenths of a micron across, comparable in size to many bacteria. (Many of the putative fossils in ALH 84001 were much smaller -- too tiny, microbiologists argue, to have been viable lifeforms.) The NASA team will attempt to examine the blobs' interiors for hints of cellular structure and to determine whether they resulted from terrestrial contamination. Unlike ALH 84001, which sat on the ice fields of Antarctica 16,000 years before its discovery, many pieces of Nakhla were recovered almost immediately after falling in Egypt on June 28, 1911. McKay's sample came from a fragment with an intact "fusion crust" that was opened in sterile, clean-room conditions last year.

McKay also showed suspicious features from the interior of Shergotty, though the study of those is just getting under way. Shergotty crystallized from molten rock a mere 165 million years ago, whereas Nakhla is about 1.3 billion years old and ALH 84001 is 4 billion years old. Thus, if microbial fossils really exist in all three of these meteorites, it means that life has existed on Mars throughout much of the planet's history and could be there today. NASA hopes to obtain samples of the red planet via spacecraft as early as 2008, and the agency is currently wrestling with how best to isolate and study the Martian material once it reaches Earth.

Thomas-Keprta and McKay reported their results at the 30th annual Lunar and Planetary Science Conference in Houston, Texas, which this year drew nearly 1,100 researchers -- a record attendance -- from around the world.

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