By dating minute glass beads thrown out by impacts over the millennia, scientists at the University of California, Berkeley, and the Berkeley Geochronology Center have not only confirmed expected intense meteor activity 4 to 3.5 billion years ago, when the large lunar seas or maria were formed, but have discovered another peak of activity that began 500 million years ago and continues today.

The tapering off of the first peak of activity, which probably included many large comets and asteroids, coincides with the earliest known evidence of life on Earth. The second and ongoing peak, which from the evidence seems to have been mostly smaller debris, began around the time of the great explosion of life known as the Cambrian.

"The first life on Earth arose just after this real crescendo around 3.5 billion years ago," said Paul R. Renne, adjunct professor of geology and geophysics at UC Berkeley and director of the Berkeley Geochronology Center. "Maybe life began on Earth many times, but the meteors only stopped wiping it out about 3 billion years ago."

The more recent and ongoing activity is even more intriguing.

"It's not surprising that the impacts tapered off about 3 billion years ago. The solar system was just getting cleaned up, primarily by Jupiter and the Sun," said Richard A. Muller, a professor of physics at UC Berkeley and a research physicist at Lawrence Berkeley National Laboratory. "What is surprising is the reversion from a benign to a violent solar system about 500 million years ago.

"This work opens up a new field that tells us something about the history of our solar system that was totally unanticipated. Until now we did not realize how peculiar the past 500 million years has been."

UC Berkeley graduate student Timothy S. Culler, along with Renne, Muller and Timothy A. Becker, laboratory manager at the Berkeley Geochronology Center, report their findings in the March 10 issue of the journal Science.

Though all the Berkeley researchers agree on the new impact chronology for the moon, they have their own ideas about its implications.

Renne, for example, leans toward the theory that interstellar dust seeded the Earth with organic molecules, from water to amino acids, that were incorporated into life on Earth during the past 500 million years.

"Life already here would suddenly have a new stimulus, a greater need to evolve quickly and more raw material to do it," Renne said. "Impacts would have to be really, really big and really, really frequent to be deleterious to life on Earth, and it's clear that the flux over the past 500 million years has been relatively small objects. We don't see a lot of young large craters on the moon. We've come to accept the idea that impacts are strictly bad news for life on Earth, but now that's not so clear."

Culler, the graduate student who originated the project under the supervision of Muller and Renne, sees the intense meteor activity as evidence that large meteor impacts played a major role in the evolution and extinction of life.

"It shows that large impacts may have been more frequent in the last 500 million years, creating more extinctions, like the comet or asteroid that wiped out the dinosaurs 65 million years ago, " Culler said. "Even a number of smaller impacts can have a disastrous effect on the atmosphere and cause mass extinctions."

Click For Part Two Of This Report

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