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."
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