A Chip off the Doomsday Rock

By LEON JAROFF

What killed the dinosaurs? Scientists have been debating that one for a long time. They know that 65 million years ago, a large object, five or six miles across, blasted a 120-mile-wide crater at the tip of what today is Mexico's Yucatan peninsula. They also know that the impact, or more accurately, the worldwide, sunlight-blocking shroud of dust it kicked up, wiped out some 70% of the earth's plant and animal species--including the dinosaurs. But what, precisely, was the object that sealed their fate?

UCLA geochemist Frank Kyte thinks he may have found not just the answer but also a piece of the thing itself: a tiny meteorite fragment, a tenth of an inch across, that was extracted from a 65-million-year-old geological layer under more than 50 yds. of sediment at the bottom of the Northern Pacific. In a report in the current issue of Nature, Kyte notes that the little chunk contains concentrations of metals (such as iridium and nickel) and mineral textures that clearly show that it is extraterrestrial and that it probably was once part of a much larger asteroid.

The fact that the meteorite exists at all, says Kyte, also strongly suggests that it came from an asteroid, not from a comet, as many scientists still believe. He notes that comets strike the earth at such high speeds--many of them well over 100,000 m.p.h.--that they are usually completely melted and vaporized. But a typical asteroid hits at less than half that speed, and some fragments often survive. So why did this one turn up 5,400 miles away from the Yucatan impact site? Kyte believes it was flung by the explosion high above the atmosphere in a ballistic path that plopped it into the Pacific.

--By Leon Jaroff