NASA SCIENTISTS FIND HISTORY OF ASTEROID IMPACTS IN EARTH ROCKS

WASHINGTON -- Research by NASA and international scientists concludes
giant asteroids, similar or larger than the one believed to have
killed the dinosaurs, hit Earth billions of years ago with more
frequency than previously thought.

To cause the dinosaur extinction, the killer asteroid that impacted
Earth 65 million years ago would have been almost 6 miles (10
kilometers) in diameter. By studying ancient rocks in Australia and
using computer models, researchers estimate that approximately 70
asteroids the same size or larger impacted Earth 1.8 to 3.8 billion
years ago. During the same period, approximately four similarly-sized
objects hit the moon.

"This work demonstrates the power of combining sophisticated computer
models with physical evidence from the past, further opening an
important window to Earth's history," said Yvonne Pendleton, director
of NASA's Lunar Science Institute (NLSI) at NASA's Ames Research
Center at Moffett Field, Calif.

Evidence for these impacts on Earth comes from thin rock layers that
contain debris of nearly spherical, sand-sized droplets called
spherules. These millimeter-scale clues were formerly molten droplets
ejected into space within the huge plumes created by mega-impacts on
Earth. The hardened droplets then fell back to Earth, creating thin
but widespread sedimentary layers known as spherule beds.
The new findings are published today in the journal Nature.

"The beds speak to an intense period of bombardment of Earth," said
William Bottke principal investigator of the impact study team at the
Southwest Research Institute (SwRI) in Boulder, Colo. "Their source
long has been a mystery."

The team's findings support the theory Jupiter, Saturn, Uranus and
Neptune formed in different orbits nearly 4.5 billion years ago,
migrating to their current orbits about 4 billion years ago from the
interplay of gravitational forces in the young solar system. This
event triggered a solar system-wide bombardment of comets and
asteroids called the "Late Heavy Bombardment." In the paper, the team
created a model of the ancient main asteroid belt and tracked what
would have happened when the orbits of the giant planets changed.
They discovered the innermost portion of the belt became destabilized
and could have delivered numerous big impacts to Earth and the moon
over long time periods.

At least 12 mega-impacts produced spherule beds during the so-called
Archean period 2.5 to 3.7 billion years ago, a formative time for
life on Earth. Ancient spherule beds are rare finds, rarer than rocks
of any other age. Most of the beds have been preserved amid mud
deposited on the sea floor below the reach of waves.

The impact believed to have killed the dinosaurs was the only known
collision over the past half-billion years that made a spherule layer
as deep as those of the Archean period. The relative abundance of the
beds supports the hypothesis for many giant asteroid impacts during
Earth's early history.

The frequency of the impacts indicated in the computer models matches
the number of spherule beds found in terrains with ages that are well
understood. The data also hint at the possibility that the last
impacts of the Late Heavy Bombardment on Earth made South Africa's
Vredefort crater and Canada's Sudbury crater, both of which formed
about 2 billion years ago.

"The Archean beds contain enough extraterrestrial material to rule out
alternative sources for the spherules, such as volcanoes," said Bruce
Simonson, a geologist from Oberlin College in Oberlin, Ohio.

The research was funded by NLSI and conducted by members or associates
of NLSI's Center of Lunar Origin and Evolution, based at SwRI.

The impact study team also includes scientists from Purdue University
in West Lafayette, Ind.; Charles University in Prague, Czech
Republic; Observatorie de la Cote d'Azur in Nice, France; and
Academia Sinica in Taipei, Taiwan.