Scientists Find Signs Big Meteor Hit Earth 3.5 Billion Years Ago
By KENNETH CHANG

From the decay of uranium in tiny ancient crystals, geologists have dated the earliest and probably largest known meteor impact on Earth.

That asteroid was probably at least twice as wide as the meteor thought to have killed the dinosaurs 65 million years ago, and the impact probably released at least 10 times as much energy, the scientists said.

The heat would have killed all single-cell microbes, the only life on Earth at the time, on land and in the upper ocean, which would have boiled into steam. The impact appears to have sent giant tsunamis coursing around the world's oceans, scouring the early continents.

"The only thing that would have survived would have been bacteria in the deep ocean," said Dr. Gary R. Byerly, a professor of geology at Louisiana State and the lead author of the article.

Because of the scarcity of fossils from the era, scientists cannot say how the cataclysm changed the course of life.

Giant craters on the moon indicate to scientists that even earlier and larger impacts occurred on Earth. A heavy rain of meteors large enough to boil off the oceans would probably have delayed the advent of life until 3.9 billion years ago at the earliest, scientists say. But no rocks that preserve evidence of those early impacts have been found.

No crater from the crash 3.5 billion years ago remains either, but Dr. Byerly and Dr. Donald R. Lowe, a professor of geology at Stanford, found hints of the impact two decades ago: perfectly spherical sand grains about the size of BB pellets in ancient rocks from South Africa and western Australia. The grains probably condensed from the cloud of rock vapor sent up by the impact, the two scientists said.

Later research showed that the layers of rock containing the grains were also rich in iridium, a metal more abundant in asteroids and comets than in rocks on Earth. The layers of debris are 8 to 12 inches thick, compared with less than an inch for the impact that killed the dinosaurs.

Analysis of the minerals from the older impact indicated that the rock was an asteroid that had once orbited between Mars and Jupiter.

Scientists dated the impact by measuring the decay of uranium in zircon crystals in the rock. Zircon is a durable mineral formed from the force of the giant tsunamis crashing ashore.