Nov. 16, 2000
Vol. 20 No. 5

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    Latest meteorite find is the new standard

    Steve Koppes
    News Office

    A new meteorite from Canada, called the Tagish Lake meteorite, appears to be the most primitive sample of the solar system on Earth and may become the new chemical standard against which celestial bodies are measured. University and other scientists reported the finding in the Oct. 13 issue of the journal Science.

    “This is clearly a meteorite that doesn’t fit any of our previous classifications. It’s a new kind of meteorite,” said study co-author Robert Clayton, Enrico Fermi Distinguished Service Professor in Chemistry and Geophysical Sciences.

    Today, scientists regard the Orgueil (pronounced OR-gay) meteorite from France as the best sample of the solar system’s chemical composition. Orgueil is the touchstone against which they compare the composition of the Earth, the moon and other celestial bodies.

    “Tagish Lake is not very much different from Orgueil, but it might become the new standard meteorite for chemical abundances,” said Clayton, who also is director of the Enrico Fermi Institute.

    Clayton and Toshiko Mayeda, Senior Research Associate in the Enrico Fermi Institute, contributed the oxygen isotopic measurement to the Science study. The oxygen isotopic measurement was a key piece of data in revealing the meteorite’s unusual composition. The Science study was led by the University of Western Ontario’s Peter Brown.

    Isotopes are the siblings of the elemental world. They are atoms of an element that have different numbers of neutrons, and hence they have slightly different chemical properties. Oxygen has three isotopes, which Clayton uses to trace meteorites to their points of origin.

    “We’re sort of a clearinghouse for strange meteorites,” Clayton said.

    Broadly speaking, the Tagish Lake meteorite is a carbonaceous chondrite. This type of meteorite has changed little since the birth of the solar system approximately 4.5 billion years ago. These rocks resemble charcoal but are actually the crown jewels of meteorite research. They contain the interstellar dust grains and high carbon content that scientists prize for the clues they hold regarding the origin of the sun and planets and of life itself.

    Tagish Lake, in fact, contains 5.4 percent carbon by weight, more than any other carbonaceous chondrite, reported Monica Grady of the Natural History Museum in London, in the Science paper.

    But the oxygen isotopic characteristics of the Tagish Lake meteorite do not fit the existing categories of carbonaceous chondrites.

    Each classification represents a different parent body from which the meteorites sprang, usually an asteroid. Tagish Lake comes closest in isotopic composition to the CM and CI classifications for carbonaceous chondrites. C stands for carbonaceous, meaning high in carbon content. M stands for the Mighei meteorite from Ukraine, and I for the Ivuna meteorite from Tanzania, for which the classifications are named.

    “We can make a pretty big case that the parent bodies of these carbonaceous chondrites had water on them. They probably originated as mixtures of rock and ice and the ice melted due to radioactive heating,” Clayton said. “The oxygen isotopes serve nicely to give us some idea of both how wet the body was and also the temperature at which the alteration took place.”

    Clayton’s analysis showed that Tagish Lake is wetter than the CMs, but that it was chemically altered at much cooler temperatures than the CIs.

    “I think it needs a new label of some sort.” But in keeping with scientific convention, that will have to await the discovery of four more meteorites similar to Tagish Lake.

    “Until you get more samples, it just becomes an anomalous, unclassified meteorite,” he said.