The bombardment that resurfaced the Earth 3.9 billion years ago was produced by asteroids, not comets, according to David Kring of the University of Arizona Lunar and Planetary Laboratory and Barbara Cohen, formerly at the UA and now with the University of Hawaii. Their findings appear today in the Journal of Geophysical Research - Planets, published by the American Geophysical Union.

The significance of this conclusion is that the bombardment was so severe that it destroyed older rocks on Earth. This, Kring says, is the reason that the oldest rocks ever found are less than 3.9 billion years old.

Additionally, the researchers argue, hydrothermal systems generated by the impacts would have been excellent incubators for pre-biotic chemistry and the early evolution of life, consistent with previous work that suggests life originated in hot water systems around 3.85 billion years ago.

This same bombardment, according to Kring and Cohen, affected the entire inner solar system, producing thousands of impact craters on Mercury, Venus, the Moon and Mars. Most of the craters in the southern hemisphere of Mars were produced during this event.

On Earth, at least 22,000 impact craters with diameters greater than 20 kilometers [12 miles] were produced, including about 40 impact basins with diameters of about 1,000 kilometers [600 miles] in diameter. Several impact craters of about 5,000 kilometers [3,000 miles] were created, as well, each one exceeding the dimensions of Australia, Europe, Antarctica or South America. The thousands of impacts occurred in a very short period of time, potentially producing globally significant environmental change at an average rate of once per 100 years.

Also, the event is recorded in the asteroid belt between Mars and Jupiter, as witnessed by the meteoritic fragments that have survived to fall to Earth today, the authors say.

Kring has been involved in the research and measurements of the Chicxulub impact crater located near Merida, Yucatan, Mexico. He has collaborated and led various international research teams which have drilled to unearth evidence of the Cretaceous-Tertiary (K/T) impact which is thought to have led to mass extinctions on Earth, including that of the dinosaurs. Earlier this month, he returned from a drilling operation at the impact site where crews worked around the clock to recover core samples in an effort to determine what caused the impact and other details of the catastrophic event that wiped out more than 75 percent of all plant and animal species on Earth.

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The research leading to this paper was partially supported by a grant from NASA.

Contact: Harvey Leifert, hleifert@agu.org, 202-777-7507, American Geophysical Union

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