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 will appear in the Thursday, Feb. 28 edition of the
Journal of Geophysical Research published by the American Geophysical

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.