A team of geologists has determined the age of the oldest known
meteorite impact on Earth – a catastrophic event that generated
massive shockwaves across the planet billions of years before a
similar event helped wipe out the dinosaurs.
In a study published in the Aug. 23 issue of the journal Science, the
research team reports that an ancient meteorite slammed into Earth
3.47 billion years ago. Scientists have yet to locate any trace of
the extraterrestrial object itself or the gigantic crater it
produced, but other geological evidence collected on two continents
suggests that the meteorite was approximately 12 miles (20
kilometers) wide – roughly twice as big as the one that contributed
to the demise of the dinosaurs some 65 million years ago.
“We are reporting on a single meteorite impact that has left
deposits in both South Africa and Australia,” said Donald R. Lowe, a
Stanford professor of geological and environmental sciences who
co-authored the Science study. “We have no idea where the actual
impact might have been.”
To pinpoint when the huge meteorite collided with Earth, Lowe and his
colleagues performed highly sensitive geochemical analyses of rock
samples collected from two ancient formations well known to
geologists: South Africa`s Barberton greenstone belt and Australia’s
Pilbara block. The two sites include rocks that formed during the
Archean eon more than 3 billion years ago – when Earth was “only” a
billion years old and single-celled bacteria were the only living
things on the planet.
“In our study, we’re looking at the oldest well-preserved
sedimentary and volcanic rocks on Earth,” Lowe noted. “They are
still quite pristine and give us the oldest window that we have on
the formative period in Earth’s history. There are older rocks
elsewhere, but they’ve been cooked, heated, twisted and folded, so
they don’t tell us very much about what the surface of the early
Earth was really like.”
Controversial findings
Lowe and Louisiana State University geologist Gary R. Byerly – lead
author of the Science study – began collecting samples from the South
African and Australian formations more than 20 years ago. Although
thousands of miles apart, both sites contain 3.5-billion-year-old
layers of rock embedded with “spherules” – tiny spherical particles
that are a frequent byproduct of meteorite collisions.
“A meteor passes through the atmosphere in about one second, leaving
a hole – a vacuum – behind it, but air can’t move in fast enough to
fill that hole,” Lowe explained. “When the meteor hits the surface,
it instantaneously melts and vaporizes rock, and that rock vapor is
sucked right back up the hole into the atmosphere. It spreads around
the Earth as a rock vapor cloud that eventually condenses and forms
droplets that solidify into spherules, which rain back down onto the
surface.”
The meteorite that led to the dinosaur extinction produced spherule
deposits around the world that are less than 2 centimeters deep. But
the spherule beds in South Africa and Australia are much bigger –
some 20 to 30 centimeters thick. A chemical analysis of the rocks
also has revealed high concentrations of rare metals such as iridium
– rare in terrestrial rocks but common in meteorites.
In the mid-1980s, when Lowe and Byerly first suggested that these
iridium- and spherule-rich rock layers were produced by fallout from
a meteorite, they were greeted with some skepticism – primarily from
geochemists, who argued that the spherules probably did not come from
space but were more likely to have been formed through some kind of
volcanic activity on Earth.
Doubts remained until two years ago, when isotopic studies confirmed
that much of the chromium buried in the rock samples came from an
extraterrestrial source.
“That pretty well laid to rest any lingering doubts of their impact
origin,” Lowe recalled.
SHRIMP technology
To narrow down the timeframe when the meteorite impact occurred, Lowe
and Byerly turned to a powerful analytic instrument at Stanford
called the Sensitive High-Resolution Ion MicroProbe Reverse Geometry
– or SHRIMP RG.
Operated jointly by Stanford and the U.S. Geological Survey (USGS),
the SHRIMP RG rapidly can determine the age of minute grains of
zircon – one of nature’s most durable minerals.
“Of all the minerals on Earth, zircons are the most resistant to all
the things that can happen to rocks,” said USGS scientist Joseph L.
Wooden, co-director of the SHRIMP RG and consulting professor in
Stanford’s Department of Geological and Environmental Sciences.
Zircons often contain ancient isotopes of radioactive uranium that
have been trapped for billions of years.
“The SHRIMP RG makes it possible to work with an individual zircon
and quickly determine its age by measuring how much radioactive decay
has occurred,” noted Wooden, co-author of the Science paper. “To
dissolve and prepare individual zircon grains for analysis in a
standard lab could take months.”
But with the SHRIMP RG, a zircon is simply mounted on a slide, then
exposed to a high-energy beam that determines its age in about 10
minutes. For the Science study, researchers analyzed about 50 zircons
extracted from South African and Australian rocks. According to
Wooden, it took about one day for the SHRIMP RG to calculate a more
precise age of the zircons – 3.47 billion years, plus or minus 2
million years.
Early Earth
What was Earth like when the ancient collision occurred? No one is
certain, but speculation abounds.
“You’ll find that the science of the Archean Earth is full of
personalities and controversies, so you can take your choice,” Lowe
observed.
He and his colleagues point to evidence showing that, 3.5 billion
years ago, Earth was mostly covered with water.
“There were probably no large continental blocks like there are
today, although there may have been microcontinents – very small
pieces of continental-type crust,” Lowe said, noting that if the
Archean ocean had the same volume of water as today, it would have
been about 2 miles (3.3 kilometers) deep.
“It would have taken only a second or two for a meteor that’s 20
kilometers in diameter to pass through the ocean and impact the rock
beneath,” Lowe said. “That would generate enormous waves kilometers
high that would spread out from the impact site, sweep across the
ocean and produce just incredible tsunamis – causing a tremendous
amount of erosion on the microcontinents and tearing up the bottom of
the ocean.”
In addition to the 3.47-billion-year-old impact, Lowe and Byerly have
found evidence of meteorite collisions in three younger rock layers
in the South African formation. According to Lowe, the force of those
collisions may have been powerful enough to cause the cracks – or
tectonic plates – that riddle the Earth’s crust today.
“In South Africa, two of the younger layers – 3.2 to 3.3 billion
years old – coincide with major tectonic changes,” he observed.
“How come? Maybe those impacts were large enough to affect tectonic
systems – to affect the dynamics of the Earth’s crust.”
Evolution and meteorites
The impact of these major catastrophes on the evolution of early life
is difficult to determine, Lowe observed.
“The most advanced organisms at the time were bacteria, so there
isn’t a big extinction event you can identify as cut-and-dry as the
extinction of the dinosaurs,” he said.
He also pointed to controversy about the fossil record, noting that
the oldest known microbial fossils have been found in rocks 3.4 to
3.5 billion years old – roughly the same age as the ancient meteorite
collision documented in the Science study. Could the meteorite
somehow have contributed to the origin of bacterial life on Earth?
Lowe has his doubts: ”It’s quite possible that life evolved as far
back as 4.3 billion years ago, shortly after the Earth had formed.”
He also pointed to uncertainty among scientists about what the
climate of the Archean Earth was really like. In a forthcoming study,
Lowe will present evidence that the average temperature of the planet
back then was very hot – perhaps 185 F (85 C).
“It’s not clear what effect a large meteorite impact would have on
an extremely hot Earth,” he explained. “We know in terms of the
present climate that if we had a very large impact, it would send
enormous amounts of dust into the atmosphere and the climate might
cool. Such a scenario may have contributed to the extinction of
dinosaurs. They’re really big guys and they’re very strong, but
they’re actually much more susceptible to environmental changes than
microbes are. Dinosaurs didn’t have anywhere to go – they couldn’t go
underground or avoid cold climates” – unlike bacteria, which have
adapted successfully to a variety of extreme conditions.
“It looks like what we are seeing is a much greater rate of the
large impacts on the early Earth, certainly than we have today, and
perhaps even a much greater rate than what was suspected,” Lowe
concluded. “I think the effort now will be to try to do studies like
this that will enhance our understanding of the impactors on early
Earth – to try to find other layers, to understand the mechanics of
impact events and how they affected early life.”
The Science study was supported by grants from the National Science
Foundation Petrology and Geochemistry Program and the NASA
Astrobiology Program. Louisiana State University graduate student
Xiaogang Xie also contributed to the study.
CONTACT: Mark Shwartz, News Service (650) 723-9296;
mshwartz@stanford.edu
COMMENT: Donald R. Lowe, Geological and Environmental Sciences
(650) 725-3040; lowe@pangea.stanford.edu
Joseph L. Wooden, Geological and Environmental Sciences &
U.S. Geological Survey (650) 725-9237; jwooden@usgs.gov
EDITORS: The Aug. 23 study, “An Archean Impact Layer from the
Pilbara and Kaapvaal Cratons,” can be obtained from Science magazine
by calling (202) 326-6440 or by e-mailing scipak@aaas.org .
Photographs can be downloaded at http://newsphotos.stanford.edu
(slug: “Impactor”).
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