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Reflections on the Tagish Lake meteorite

Remember the meteorite that fell on Tagish Lake in Canada in January 2000? Some 70 people witnessed the meteor streak across the sky and explode, raining debris into the remote Yukon Territory. Since that time, scientists have clamored to study the meteorite, hailing it the most pristine carbonaceous chondrite to date. "The meteorite was so exciting, because it basically fell in a ready-made freezer," says chemist Sandra Pizarello at Arizona State University. That's why she and her colleagues were eager to analyze the specimen. Their study is one of two published in the Sept. 21 Science that take a closer look at the chemistry and origin of the Tagish Lake meteorite.

[University of Calgary scientist Alan Hildebrand holds
the ice-encased meteorite fragment. Courtesy of
University of Western Ontario and University of Calgary]

Pizarello and her colleagues analyzed the organic compounds in a 4.5-gram sample of the Tagish meteorite. What they found was a unique chemical composition, unlike any observed in other meteorites. "The meteorite shows a unique chemical evolution pathway," Pizarello says.

Compared with the Murchison meteorite, a well-known specimen found in Australia in 1969, the Tagish specimen is very simple. It seems to contain preserved organics that accumulated in the early history of the Solar System, including bubbles of carbon, with helium and argon in similar ratios as in the gas and dust cloud that formed the planets. And while Murchison contains amino acids, the Tagish specimen contains virtually none, perhaps representing an entirely separate line of chemical evolution. Pizarello says the Tagish meteorite could untangle the complexity surrounding most carbonaceous chondrites. She hopes it will bring scientists one step closer to understanding the connection between chemical prebiotic compounds and the biomolecules of the origin of life, she says.

Appearing in the same issue of Science is a very different kind of study on the Tagish Lake meteorite. Led by Takahiro Hiroi at Brown University, the research team analyzed the reflectance spectrum of the Tagish specimen. Hiroi says that the Tagish specimen displayed a reflectance pattern showing it came from a D-type asteroid, which would have originated in the cold, outer region of the asteroid belt. "We concluded that this must be the first [carbonaceous chondrite] identified to come from [a] D-type [asteroid]," he says. Knowing where the approximately 4.5 billion-year-old meteorite came from, Hiroi adds, will allow scientists to learn how the solar system formed, what materials were there and how the materials accreted.

Lisa M. Pinsker

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