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 & Planetary Laboratory and Barbara Cohen, formerly at the UA and now with the University of Hawaii.
Their findings appear in the Feb. 28 edition of the Journal of Geophysical Research 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. Which, Kring says, is the reason why the oldest rocks found are less than 3.9 billion years old.
Additionally, they argue, impact-generated hydrothermal systems would have been excellent incubators for pre-biotic chemistry and the early evolution of life, consistent with previous work that shows life originated in hot water systems around or slightly before 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 were produced, including about 40 impact basins with diameters of about 1,000 kilometers in diameter. Several impact craters of about 5,000 kilometers 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 a globally-significant environmental change at an average rate of once per 100 years.
Also, the event is recorded in the asteroid belt, as witnessed by the meteoritic fragments which have survived to fall to Earth today.
Kring has been involved in the research and measurements of the Chicxulub impact crater located near Merida, Yucatan, Mexico. He has collaborated on 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 dinosaur extinction.
Earlier this month, Kring returned from a drilling operation at the impact site where crews worked around the clock to recover core samples to determine what the impactor was and details of the catastrophic event that wiped out more than 75 percent of all plant and animal species on Earth.