March 14, 2000
Analysis of Lunar Debris Offers Clues to Life on Earth
By JOHN NOBLE WILFORD
In popular lore, the Apollo 14 astronauts may be best remembered for
things they were not sent to the Moon in 1971 to do. Alan Shepard drove
a golf ball in the low lunar gravity to a distance Tiger Woods would
die for. Edgar Mitchell on his own conducted what he said were extrasensory
perception experiments.
Yet the astronauts kept their eyes on the prize and took time to collect
plenty of rocks and soil and bring them back in airtight containers. One
gram of that soil -- or regolith, as lunar dirt is called -- has only
now been put to a rigorous analysis, which has revealed what may be a
surprising new chapter in the history of the solar system.
The findings have even inspired
speculation about another possible extraterrestrial influence on the course
of life on Earth. Could an upsurge of
meteorite bombardment have contributed to the emergence of complex
lifeforms with diverse anatomical designs? The speculation is already
drawing the fire of critics.
By dating minute glass beads in
lunar soil, melted debris from the
heat of asteroid and comet impacts,
scientists at the University of California at Berkeley and the Berkeley Geochronology Center have made two discoveries, one expected and the other a
big surprise.
The ages of many beads, the size of
tiny peas or smaller, confirmed the
heavy meteorite bombardment of the
Moon from 4 billion to 3.5 billion years
ago. This was a violent epoch in the
young solar system, which formed 4.5
billion years ago. Impacts of asteroids
and comets gouged out many of the
larger lunar craters and basins that
remain virtually unchanged on the
waterless, airless Moon.
How the Moon Might Tell Earth's Story
On the Moon debris from impacts of comets or meteors is not disturbed by wind or water. So it creates a record of lunar bombardment. Scientists who have analyzed this debris say it suggests a period of heavy impact activity occurred about the same time complex life forms emerged on Earth.
|
USGS/NASA
|
Edgar Mitchell on a moonwalk during the Apollo 14 mission.
|
The scientists had not expected to
find so many of the beads with ages
less than 400 million years old. After a
relatively quiet period of 3 billion
years, they said, the inner solar system appeared to erupt with another
meteorite blitz, which continues today. The Earth, being larger than the
Moon, would presumably have suffered even more impacts, though erosion and shifting crustal plates would
have erased most of the traces.
Describing the findings in the current issue of the journal Science, the
Berkeley researchers concluded that
the age data from 155 glass beads
"show that the crater production rate
has not been constant over the last 3
billion years, as had been generally
assumed." The data, they said, also
provided evidence for a striking increase in the cratering rate over the
last 400 million years.
The research was conducted by
Timothy S. Culler, a graduate student
at the University of California; his
faculty adviser, Dr. Richard A. Muller, a physicist, and Dr. Paul R. Penne,
director of the Berkeley Geochronology Center and specialist in dating
technology. The ages of the impact-formed beads were determined by a
method of dating the decay of radioactive potassium to argon.
Other scientists said they had confidence in the analytical techniques
used to date the beads. Dr. Graham
Ryder of the Lunar and Planetary
Institute in Houston said the Berkeley
researchers "persuasively argue that
there has been a substantial increase
in impacting in the Earth-Moon system" in recent times. Some geologists
were concerned, however, that the soil
sample, coming from near the surface, might not be representative of
impact frequencies.
"If the sample is biased, then you
could be drawing a totally erroneous
conclusion," Dr. Paul D. Spudis, a
geologist at the Lunar and Planetary
Institute, said in an interview.
Dr. Renne said in response: "We
understand those things well. There
really is no uncertainty about the
chronology we've put together."
But the sharpest criticism by geologists and paleontologists was aimed
at provocative remarks by the Berkeley scientists in the last paragraph of
their report. The data, they said,
"show an increase in the cratering
rate roughly coincident with the
'Cambrian explosion' of complex life
on Earth."
While conceding that the bead
ages were not specific enough to
support claims of a relationship to
terrestrial life, the scientists said
"the correlation is permissible evidence of a causal relation." Their
last sentence read: "It is possible
that increased debris influx had a net
stimulating effect on biotic diversity,
despite the occasional occurrence of
large impacts with destructive consequences."
A planetary geologist who spoke
on condition of anonymity said the
speculation "really came out of left
field." For one thing, the Cambrian
explosion, the sudden rise of more
diverse organisms as preserved
most famously in the Burgess shale
of British Columbia, started more
than 500 million years ago. Nor is it
clear, paleontologists said, what life
forms in what ecological niches were
destroyed by impacts at this time to
clear the way for a new epoch.
In an interview, Dr. Muller said he
was not arguing that there was a
connection, "just noting that the impact increase happens to coincide
with the explosion of life."
The scientists said the more recent
period of bombardment came from
relatively small objects, which have
left few young large craters on the
Moon. Their dating was not specific
enough, they said, to detect any upsurge in impacts at the time the
dinosaurs perished, presumably by
an asteroid or comet impact 65 million years ago.
Dr. Muller had the dinosaur extinction in mind when he proposed
the current research. In the 1980's,
he proposed a controversial theory
that the Sun has an unseen companion star, which he called Nemesis,
that orbits the Sun every 26 million
years and periodically knocks comets out of their orbits, sending them
hurtling through the inner solar system. This might, he said, explain the
apparent 26-million-year cycles of
widespread extinctions, including the
one that doomed the dinosaurs and
cleared the way for mammals.
Looking for evidence of increased
impacts coinciding with those extinctions, Dr. Muller had Mr. Culler examine glass beads collected by Apollo 14 and other lunar missions to see
if they reflected spurts of objects
crashing into the Moon.
In a statement issued by the University of California, Dr. Muller
said: "The 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."
Writing an accompanying article
in Science, Dr. Ryder of the Lunar
and Planetary Institute said there
was now "some fairly hard evidence
for the earlier tentative conclusion
that cratering in the Earth-Moon
system has not been constant over
the past 3 billion years."
In that case, he concluded, there
may be "considerable uncertainties
in the absolute dating of other planetary surfaces, such as Mars." Geologists have usually estimated these
ages on the abundance of craters left
there by the steady bombardment of
asteroids and comets over time.