Asteroid: dinosaurs' nemesis?

Sixty-five million years ago half the life forms on Earth suddenly became extinct. It's one of several puzzling "sudden extinctions" in the fossil record. Could the dinosaurs and their contemporaries have been done in by asteroid?

Last year, Luis and Walter Alvarez, Frank Asaro, and Helen V. Michel of the University of California thought they might have evidence that radiation from a nearby supernova was responsible. They now think the evidence points to an asteroid.

It would have been one of the larger Apollo objects (asteroids that cross Earth's orbit) 10 kilometers in diameter, give or take 3 km. Hitting Earth, it would have blasted a crater 175 km in diameter, throwing enough dust into the stratosphere to effectively block out the sunlight for several years.

During this perpetual night, microscopic plants in the sea would have died out, along with higher organisms depending on them. While land plants would have stopped growing, they could regenerate from seeds and roots when the air finally cleared. Nevertheless, there would have been little food for herbivors and their predators, although small animals (including the mammals) that could eat seeds, nuts, and decaying vegetation could have survived. This pattern of extinction fits fairly well with that suggested by the fossil record.

The California team was led to suspect an asteroid in following a clue in sedimentary rocks laid down at the time of the extinction. A thin mud layer has an unusually high concentration of the rare element iridium. At first, the scientists thought this might be the signature of a supernova, since these giant star explosions synthesize such elements.But if that were the case, the rock should also have traces of plutonium 244. Subsequent analysis by Asaro and Michael has shown no sign of that element. For this and other reasons, the scientists now doubt there was a supernova.

However, asteroids should have enough iridium to account for the anomaly. Also, the mud layer is unlike other sedimentary rocks above and below it, suggesting it could be the dust fallout from an asteroid impact.

The scientists stress that they have not proved their hypothesis. But they do marshall reasons and evidence showing it to be consistent with the fossil record. Furthermore, they note that the expected frequency of large asteroid impacts -- about one in 100 million years -- fits the frequency of major extinctions.

If this idea is correct, they expect to find worldwide evidence of the iridium anomaly, having already found it in Italy and Denmark. At the very least, their idea gives scientists a notion of what did in the dinosaurs -- a notion they can follow up in the field and not just speculate about.