Dinosaur destroyer
About 65 million years ago, at the end of the Cretaceous Period, a wayward Apollo-type asteroid ploughed into our planet and was responsible for wiping out the dinosaurs. This hypothesis is proposed by Luis W. Alvarez and colleagues from the University of California, Berkeley. Neutron activation analysis of the elements in a thin layer of clay from Italy and Denmark has revealed a 25-fold increase in the amount of iridium present, compared with that in the limestone rocks above and below. The layer coincides precisely with the time of the Cretaceous-Tertiary extinctions, when 75 per cent of all plant and animal species on Earth suddenly disappeared. Its relative abundance in space compared to Earth makes iridium a good indicator of an influx of extraterrestrial material. Alvarez estimates the size of the asteroid to be about 7 to 10 km in diameter. This would make a 175-km diameter dent in the crust and material 100 times the object鈥檚 mass would be thrown skyward in a gigantic 100 million megaton explosion. Around 20 per cent would stay, as a thick layer of dust, in the stratosphere for about 3 to 5 years. The resulting darkness would suppress photosynthesis and give rise to the pattern of extinctions observed in the fossil record. Distributed uniformly about the globe, the dust would eventually fall out, forming the 1-centimetre-thick clay layer all over the Earth鈥檚 surface. Alvarez further suggests that statistically the Earth would be hit by an asteroid every 100 million years, and that fact conveniently fits roughly with the interval between major extinctions.
From New 杏吧原创, 10 January 1980
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