 
Meteoroid
bombardment of moon has intensified in past 500 million years,
coinciding with blossoming of life on Earth
09
Mar 2000
By
Robert Sanders, Public Affairs
BERKELEY--
A new chronology of meteoroid impacts on the moon shows some
surprising correlations with major biological events on Earth.
By
dating minute glass beads thrown out by impacts over the millennia,
scientists at the University of California, Berkeley, and the
Berkeley Geochronology Center have not only confirmed expected
intense meteor activity 4 to 3.5 billion years ago, when the
large lunar seas or maria were formed, but have discovered another
peak of activity that began 500 million years ago and continues
today.
The
tapering off of the first peak of activity, which probably included
many large comets and asteroids, coincides with the earliest
know evidence of life on Earth. The second and ongoing peak,
which from the evidence seems to have been mostly smaller debris,
began around the time of the great explosion of life known as
the Cambrian.
"The
first life on Earth arose just after this real crescendo around
3.5 billion years ago," said Paul R. Renne, adjunct professor
of geology and geophysics at UC Berkeley and director of the
Berkeley Geochronology Center. "Maybe life began on Earth
many times, but the meteors only stopped wiping it out about
3 billion years ago."
The
more recent and ongoing activity is even more intriguing.
"It's
not surprising that the impacts tapered off about 3 billion
years ago. The solar system was just getting cleaned up, primarily
by Jupiter and the Sun," said Richard A. Muller, a professor
of physics at UC Berkeley and a research physicist at Lawrence
Berkeley National Laboratory. "What is surprising is the
reversion from a benign to a violent solar system about 500
million years ago.
"This
work opens up a new field that tells us something about the
history of our solar system that was totally unanticipated.
Until now we did not realize how peculiar the past 500 million
years has been."
UC
Berkeley graduate student Timothy S. Culler, along with Renne,
Muller and Timothy A. Becker, laboratory manager at the Berkeley
Geochronology Center, report their findings in the March 10
issue of the journal Science.
Though
all the Berkeley researchers agree on the new impact chronology
for the moon, they have their own ideas about its implications.
Renne,
for example, leans toward the theory that interstellar dust
seeded the Earth with organic molecules, from water to amino
acids, that were incorporated into life on Earth during the
past 500 million years.
"Life
already here would suddenly have a new stimulus, a greater need
to evolve quickly and more raw material to do it," Renne
said. "Impacts would have to be really, really big and
really, really frequent to be deleterious to life on Earth,
and it's clear that the flux over the past 500 million years
has been relatively small objects. We don't see a lot of young
large craters on the moon. We've come to accept the idea that
impacts are strictly bad news for life on Earth, but now that's
not so clear."
Culler,
the graduate student who originated the project under the supervision
of Muller and Renne, sees the intense meteor activity as evidence
that large meteor impacts played a major role in the evolution
and extinction of life.
"It
shows that large impacts may have been more frequent in the
last 500 million years, creating more extinctions, like the
comet or asteroid that wiped out the dinosaurs 65 million years
ago, " Culler said. "Even a number of smaller impacts
can have a disastrous effect on the atmosphere and cause mass
extinctions."
Muller
too emphasizes the role impacts have played in the history of
life on Earth. It's not surprising that the recent intense period
of meteor activity coincides with the rapid radiation of life
on Earth, he said.
"We're
only beginning to realize the role played by catastrophe in
the evolution of life," he said. "When it comes to
survival of the fittest, it's not only the ability to compete
with other species that counts, but also the ability to survive
occasional catastrophe. That requires complexity and flexibility."
Muller
has proposed several controversial theories about the solar
system, including that the sun has an unseen companion star,
one he calls Nemesis, that orbits the sun every 26 million years
and periodically knocks comets out of their orbits, sending
them hurtling toward the inner solar system. He also has proposed
that periodic climate changes are the result of the Earth's
orbit periodically tilting up out of the orbital plane of the
planets and intersecting a cloud of dust, debris and meteoroids.
The
current research was suggested by Muller in 1991, in part as
a way to determine whether the moon's impact record shows evidence
of a 26 million-year cycle. Muller hit upon the idea of argon-40/argon-39
dating of lunar spherules as a way to get a more precise chronology
of the intensity of bombardment of the moon and, by implication,
the Earth.
"I
realized that we didn't have to go to the individual craters
in order to determine their age, because the craters sent samples
to us," Muller said. "We could obtain samples of hundreds
of different craters from just one location, without having
the expense of going back to the moon. This idea is likely to
open up a completely new round of lunar analysis."
Spherules
are mostly basaltic glass, Culler said, created when a meteor
hits the surface and generates intense heat that melts the rock
and splatters it outward. As droplets of molten rock fall back
to the surface they quickly cool to form a glass, much like
obsidian.
Culler,
Becker and Renne analyzed 155 beads from one gram of lunar soil
picked up in 1971 by Apollo 14 from the Fra Mauro formation
- a lunar highland bordering Mare Imbrium. The mineral composition
of each bead was determined with a microprobe before it was
laser melted and the argon gas captured for isotopic analysis.
Contrary
to assumptions, they found that the cratering rate on the moon
has not been constant over its history. Approximately twice
as many impacts occurred between 4 and 3 billion years ago as
occurred between 2 and 1/2 billion years ago. About 500 million
years ago the intensity of impacts increased nearly to what
it was at the peak of activity 3.2 billion years ago.
Though
the dating method was not sensitive enough to reveal a 26 million-year
cycle in the impact record, "these findings fit in nicely
with the Nemesis theory," Muller said. "I think most
of the debris came from perturbations in the outer solar system
by Nemesis."
For
the future, Renne says, it is "critical to launch new lunar
sampling missions targeted to areas rich in potassium,"
in order to confirm the results and probe further back into
the moon's history.
The
project was funded by the Ann and Gordon Getty Foundation, through
the Berkeley Geochronology Center and Richard Muller. NASA provided
the lunar samples.
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