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KNOXVILLE — A University of Tennessee geologist has uncovered new evidence that the Earth and moon underwent an intense bombardment by comets or asteroids about 3.9 billion years ago.

In a study of lunar meteorites splashed off the Moon’s surface, Dr. Barbara Cohen has found pieces of rock from seven different impacts. None of the impacts are older than 3.9 billion years. Evidence of older impacts has not been found in either the Apollo samples or in lunar meteorites.

“When we first started the research, the goal was to find something older than 3.9 billion years,” Cohen said. “We were very surprised at the evidence presented by seven different impacts, which pointed to 3.9 billion years. We can’t contradict our own data.”

The results, published in the December issue of Science, support a cataclysmic volley of debris instead of a sporadic but slowly-waning series of impacts dating from the formation of the solar system, she said.

Cohen said the age of the impacts argues for a cataclysmic bombardment that restructured the surface of the moon with the impact craters still visible today. The bombardment might have destroyed any existing life on Earth or delivered precursor molecules and conditions that gave rise to living organisms, she said.

University of Arizona geologists David Kring and Timothy Swindle collaborated with Cohen on the research. Kring said evidence of a similar bombardment on Earth has been largely erased as plate tectonic shifts reformed the planet’s crust.

“The Earth is a much bigger target than the moon and would have been bombarded by at least 10 times as many impact events,” Kring said. “We’re talking about impacts that blasted craters rim-to-rim the size of continents on Earth today.”

Cohen said the lunar meteorites represent a more random sample of moon surface than the rock specimens brought to Earth on various Apollo flights, which were restricted to near-side sites that presented good landing opportunities for spacecraft.

Kring said that asteroids are a likely source of the bombardment. The researchers speculate that the bombardment may have been the result of a catastrophic breakup in the asteroid belt between Mars and Jupiter.

The dating technique used by Cohen involves comparisons of levels of isotopic argon present in the rock. Argon is a product of the decay of potassium’s radioactive isotope, which has a half-life measured in billions of years. That fact allows astrogeologists to make judgments about the age of rocks created some 4.5 billion years ago, near the beginning of the solar system.