[meteorite-list] New Lunar Rock Ages Indicate Cataclysmic Meteorite Bombardment of Moon, Earth

[Ron Baalke]

News and Communications
Oregon State University

Media Contact:
Mark Floyd, 541-737-0788

Source:
Bob Duncan, 541-737-5189

Media Release: 04-12-06

New Lunar Rock Ages Indicate Cataclysmic Meteorite Bombardment of Moon, 
Earth

CORVALLIS, Ore. -- New age measurements of lunar rocks returned by the 
Apollo space missions have revealed that a surprising number of the rocks 
show signs of melting about 3.9 billion years ago, suggesting that the 
moon -- and its nearby neighbor Earth -- were bombarded by a series of 
large meteorites at that time.

The idea that meteorites have hammered the moon's surface isn't news to 
scientists. The lunar surface is pock-marked with large craters carved out 
by the impact of crashing asteroids and meteorites, said Robert Duncan, a 
professor and associate dean in the College of Oceanic and Atmospheric 
Sciences at Oregon State University.

But the narrow range of the impact dates suggests to researchers that a 
large spike in meteorite activity took place during a 100-million year 
interval -- possibly the result of collisions in the asteroid belt with 
comets coming from just beyond our solar system.

Results of the study are being published in Geochimica et Cosmochimica 
Acta, the journal of the international Meteoritical Society. Co-authors 
with Duncan are Marc Norman of the Australian National University and John 
Huard, also an oceanographer at OSU. The study was funded by NASA.

Tiny melted fragments from the lunar rocks were dated at the noble gas 
geochronology laboratory at Oregon State. Duncan and Huard were able to 
use radiometric dating techniques to determine when the rocks had melted 
after being struck by meteorites. What is particularly intriguing, Duncan 
says, is that this apparent spike in meteorite activity took place about 
3.8 to 4 billion years ago -- an era that roughly coincides with when 
scientists believe life first began on Earth, as evidenced by the fossil 
record of primitive one-cell bacteria.

It is possible that life was introduced to Earth from one of these 
meteorites, Duncan said. Or it could have developed spontaneously once the 
bombardment subsided, or developed beneath the ocean near life-nurturing 
hydrothermal vents. The lack of evidence on Earth makes the analysis of 
moon rocks much more compelling. The meteorite activity that bombarded the 
moon likely struck our planet as well.

"Unfortunately, we haven't found many very old rocks on Earth because of 
our planet's surface is constantly renewed by plate tectonics, coupled 
with erosion," Duncan said. "By comparison, the moon is dead, has no 
atmosphere and provides a record of meteorite bombardment that we can only 
assume is similar to that on Earth."

When the solar system was formed, scientists say, it spun away from the 
sun like a huge, hot disk that subsequently condensed into planets. At 
least nine planets survived, sucking in loose space matter from around 
them. Those planets closer to the sun were more solid, while those farther 
away were comprised primarily of gases.

Over time, the space debris has lessened, either being gravitationally 
collected into the planets, or smashed into cosmic dust through collisions 
with other objects. The discovery of this apparent spike in meteorite 
activity suggests to the authors that a major event took place.

"We may have had a 10th and 11th planet that collided," Duncan said, "or 
it's possible that the outward migration of Neptune may have scattered 
comets and small planet bodies, inducing collisions in the asteroid belt. 
The close passing of a neighboring star could have had a similar effect."

Duncan and his colleagues examined about 50 different rock samples scooped 
up by astronauts on the Apollo missions. All but a few of them produced 
ages close to 3.9 billion years and they exhibited different chemical 
"fingerprints," indicating that they had melted from different meteorites 
and lunar surface rocks.

"The evidence is clear that there was repeated bombardment by meteorites," 
Duncan said.

When meteorites collide with the moon, the surface rock and the meteorites 
partially melt, and then turn to glass. After the glasses quenched, they 
slowly began to accumulate argon gas that scientists can measure and 
calculate from the known isotopic decay rate (from potassium) to determine 
age.

"The formation of glass from the melting is like starting a clock," Duncan 
said. "It resets the time for us to determine billions of years later."

Duncan and his colleagues say the intense bombardment ended about 3.85 
billion years ago, and there has been a slowly declining pattern of 
meteorite activity since. Many of the prominent craters found on the moon 
date back to that era, including Imbrium, at 3.84 billion years; 
Serenitatis, 3.89 billion years; and Nectaris, 3.92 billion years.

Many of the moon's craters are 10 to 100 kilometers across and scientists 
say that meteorites of that size or larger may have struck the Earth in 
the past. Such meteorites impacts may have been responsible for the 
extinction of dinosaurs 65 million years ago, and a mass extinction that 
wiped out an estimated 75 percent of the Earth's plant and animal species 
250 million years ago.

However, Duncan said, these mass extinctions could also be linked to 
climate, disease and volcanism -- or a combination of such factors.

"It is clear that there was a spike of meteorite activity on the moon 
about 3.9 billion years ago, and that it lasted for roughly 100 million 
years," Duncan said. "The moon provides important information about the 
early history of our solar system that is missing from the Earth's 
geologic record."

About the OSU College of Oceanic and Atmospheric Sciences

COAS is internationally recognized for its faculty, research and 
facilities, including state-of-the-art computing infrastructure to support 
real-time ocean/atmosphere observation and prediction. The college is a 
leader in the study of the Earth as an integrated system, providing 
scientific understanding to address complex environmental challenges.