BY MARK SHWARTZ
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.
A minute zircon particle taken from a 3.5-billion-year-old rock helped scientists determine the age of the oldest known metorite to collide with Earth. Photo courtesy of Gary Byerly / Louisiana State University
"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."
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.
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.
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."
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.
Professor Donald Lowe and Consulting Professor Joe Wooden used Stanford's SHRIMP RG instrument to confirm the age of the oldest known metorite to strike Earth. Photo: L.A. Cicero
Stanford Report, September 11, 2002