"But about 3.8 billion years ago the inner solar system was torn up by some cataclysmic event. The evidence is found in the crater basins on the moon and Mars and Venus." The Earth was probably heavily battered at that time too, but since the oldest known surface rocks on Earth are about 3.5 billion years old, no evidence of that exists, Byerly said.
A second battering took place 3.5 billion years ago, and it is this event that left the record in the rocks Byerly is studying. This was a smaller series of impacts with a gradual dropoff rate, he said. That the 3.8 billion-year-old event occurred is accepted by most scientists, and Byerly's work is substantiating the existence of the second, which, up to now, has not been as well accepted.
Byerly and his team have been able to date the event very accurately using an instrument at Stanford that measures the decay of uranium into lead. The uranium was found in zircons at both the Australian and South African sites and was dated to within 2 million years of 3.47 billion years ago. The fact that zircons of identical ages were found in impact strata on two continents shows the worldwide effects of the impact, Byerly said.
The zircons were not created by the impact but probably by volcanic action and deposited in the impact layers when the tsunami washed over the land.
These massive, early impacts were similar to the one that killed the dinosaurs 65 million years ago. But, they were hundreds to thousands of times more powerful. Probabilities for a similar impact today are predicted to be about one such strike every 100 million years.
"What that means is, eventually there will be another such event. We know that large asteroids get disturbed by interactions with Jupiter and fall into Earth's orbit. When that happens they will strike the Earth. We can't say when it will happen but we can say for certain that it will happen," Byerly said.
The evidence is on a small, grayish slab of rock on his desk.
Copyright 2002 Newswise
National Geographic Resources on Comets and Asteroids
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