Asteroid Watch: An Idea Dinosaurs Could Have Used

EVIDENCE has built up over 15 years to suggest strongly that an asteroid did in the dinosaurs. Now research reported this summer would seem to clinch the matter.

One study undercuts skeptics' claims that volcanoes could counterfeit the impact's telltale signs. Another pins down the age of what appears to be the crater that impact left. It dates precisely to the end of the Cretaceous and beginning of the Tertiary periods - the so-called K/T boundary - 65 million years ago, when dinosaurs and many other plant and animal species disappeared.

University of California, Berkeley, geologist Walter Alvarez and his physicist father, the late Luis Alvarez, and colleagues first fingered the cosmic culprit when they identified the element iridium in K/T boundary sediments. Rare in Earth's crust, iridium is characteristic of meteorites. Scientists have found iridium in many places around the world. K/T deposits have also yielded quartz grains scarred by extreme shock pressures.

Doubters claimed that volcanoes could have produced the iridium and shocked quartz. But when Andrew J. Gratz and colleagues at the Lawrence Livermore (Calif.) National Laboratory tried to shock quartz in the laboratory, they found volcanoes are too wimpy. They reported in Geophysical Research Letters in July that it took pressures 10 times greater than the most powerful volcanic events produce to shock the grains. They conclude that "the microstructural features seen at the K/T boundary ... must have bee n caused by impact of a large meteorite."

The favored site for that impact is the 180-kilometer (112.5-mile) wide Chicxulub crater in Yucatan. But is it old enough? Carl C. Swisher III at the Institute for Human Origins in Berkeley, Calif., and 11 colleagues have settled the question. Their radioactive dating, reported in Nature last month, yields an age of 64.98 million years give or take 50,000 years.

There's much less doubt now that an asteroid helped change the course of organic evolution 65 million years ago. Whether it did it by veiling Earth in sun-blocking debris, creating a rain of acid, or by other means is an open question. Whether it was the main cause or only a contributing cause of the K/T boundary extinctions is also undecided.

It now seems likely, however, that, even though they are rare events, asteroid impacts of various sizes have played a role in earthly life's evolution. And they could continue to do so. But humans are not dinosaurs. As the United States Congress noted in the 1990 NASA Multiyear Authorization Act, "We have the technology to detect such asteroids and to prevent their collision with the Earth."

The law directed the National Aeronautics and Space Administration to hold two workshops to look into this. These found that the prospect for asteroid deflection - perhaps with rocket-borne explosives - needs much more study. But the report to Congress concluded that a global detection network using modest-size telescopes can be set up now. It would likely cost under $100 million and could catalog something like 90 percent of potentially dangerous asteroids in about 15 years.

Congress should fund the network. The asteroid watchers would add to astronomical knowledge while lowering the risk that a possibly avoidable natural calamity would take us by surprise.

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