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Shrapnel from a smoking gun

The world of the dinosaurs was brought to a shattering close by the impact of a 10-km-diameter comet or asteroid, around 65 million years ago. The impact, now usually identified with the large, buried crater of Chicxulub, on the Yucatan Peninsula of Mexico, released thousands of times more energy than the combined nuclear arsenals of all nations combined. It blasted dust and débris into the atmosphere sufficient to block out sunshine for weeks or months. It spraked firestorms that raged across the world. It may have driven the dinosaurs, and many other creatures on land and in the sea, to their extinction.

Not one intelligible scrap of the impactor has been found - until now.

In a report in Nature Frank Kyte of the University of California, Los Angeles, describes a tiny meteorite that he thinks might be a fragment of the impactor - a 2.5-millimeter shard of shrapnel from the smoking gun. If true, then this meteorite will allow researchers to get a better idea of the nature of the impactor. Detailed examination of the specimen shows it to have come from a type of asteroid called a carbonaceous chondrite. This kind of body is relatively common in the asteroid belt, and makes a plausible impactor.

By the same token, the fact that a fragment survived the impact at all suggests that the impactor was an asteroid, rather than a comet - comets fly faster, and are more fragile, so are less likely to leave recognizable fragments. To back up this claim, Kyte cites several instances of meteorites having been unearthed from the fossil record. It has usually been thought that the force of the impact would have been sufficient to vaporize the impactor itself - either that, or bounce bits of it back into space.

The boundary between the Cretaceous Period, and the succeeding Tertiary, some 65 million years ago, is known as the 'K/T' boundary. Palaeontologists have long recognized it as a benchmark in the history of life, marking a sharp transition between two entirely different assemblages of animals and plants. The 'mass extinction' at the end of the Cretaceous has long been a palaeontological puzzle.

But the idea that the K/T boundary was punctuated by an extraterrestrial impact came after the discovery that K/T boundary rocks around the world are enriched in the metal iridium. This metal, similar to platinum, is rare in the Earth's crust but more common in asteroids. The iridium evidence gave credence to the view that the Earth was struck by an asteroid, or a comet, 65 million years ago - and, until now, provided the only physical evidence of the impactor. Tiny 'nuggets' of iridium have been found in rocks at Chicxulub itself. But the mere presence of iridium could say nothing more than that the impactor might have been extraterrestrial. Kyte's meteorite changes all that - it is "the first K/T boundary sample with sufficient information from textural and chemical data to make inferences about its origin."

Kyte recovered the grain-sized meteorite from a speck of clay, collected in a deep-sea drill-core from the central Pacific Ocean, 9,000 kilometres west of the impact site. The meteorite was found in the iridium-rich layer of clay so indicative of the K/T boundary. Of course, the co-occurrence of the meteorite with the K/T boundary could have been just that - a coincidence - but it is hard to see how the meteorite could have had any other origin. The microstructure of the rock, and the disposition of the minerals within it, are consistent with it coming from a carbonaceous chondrite. However, most of the original minerals in the sample have been replaced during the meteorite's long burial - in the same way that the mineral content of bones destined to become fossils are replaced by different, yet similar, minerals. This truly is a 'fossil' meteorite.


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Gee, H. Shrapnel from a smoking gun. Nature (1998).

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