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

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 13 13:25:26 2006
Message-ID: <200604131640.JAA21506_at_zagami.jpl.nasa.gov>

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.
Received on Thu 13 Apr 2006 12:40:21 PM PDT