EARTH SCIENCE:
Comet Shower Hit, But Life Didn't Blink
Richard A. Kerr
SAN FRANCISCO--Back in the 1980s, hints that mass extinctions like the one that swept away the dinosaurs might have recurred every 30 million years or so led researchers to propose that regular swarms of comets have battered Earth, devastating life. But little evidence has emerged for even a single comet shower, much less periodic ones. Now, researchers tapping a new sort of geologic record find solid evidence that at least one comet shower did pelt Earth--but without apparent effects on life. Last month at the annual fall meeting of the American Geophysical Union here, geochemists reported finding a high level of comet dust in 35-million-year-old ocean sediments--evidence that the inner solar system was aswarm with comets for some 2 million years. The work supplies more evidence that there was a flood of comets into the solar system at a time when at least two major impacts struck the planet. But unlike the comet showers envisioned in the 1980s, this one left life unscathed and apparently was not part of a series.
