The Oldest Meteorite Collision On Earth
Posted: Friday, August 23, 2002
Source: Stanford University (http://www.stanford.edu)
Scientists Confirm Age Of The Oldest Meteorite Collision On Earth
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."
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."
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.
Note: The original news release can be found at
Send page by E-Mail Mystery Of Large Change In Earth's Gravity Field
Posted: Friday, August 2, 2002
Source: NASA/Goddard Space Flight Center (http://www.gsfc.nasa.gov)
Satellites Reveal A Mystery Of Large Change In Earth's Gravity Field
Satellite data since 1998 indicates the bulge in the Earth's gravity field at the equator is growing, and scientists think that the ocean may hold the answer to the mystery of how the changes in the trend of Earth's gravity are occurring.
Before 1998, Earth's equatorial bulge in the gravity field was getting smaller because of the post-glacial rebound, or PGR, that occurred as a result of the melting of the ice sheets after the last Ice Age. When the ice sheets melted, land that was underneath the ice started rising. As the ground rebounded in this fashion, the gravity field changed.
"The Earth behaved much like putting your finger into a sponge ball and watching it slowly bounce back," said Christopher Cox, a research scientist supporting the Space Geodesy Branch at NASA's Goddard Space Flight Center, Greenbelt, Md.
Currently, the Earth has a significant upward bulge at the equator, and a downward bulge at the poles. "Observations of the Earth's gravity field show that some phenomena are counteracting the gravitational effects of PGR. Whereas PGR has been decreasing the bulge in the Earth's gravity field at the equator, this recent phenomena is causing the bulge to increase," Cox said. Such changes in the gravity field can be sensed using ultra precise laser tracking of satellites to observe tiny changes in the orbits of those satellites and by tracking changes in the length of day or rotation of the Earth.
Scientists believe movements of mass cause this recent change from the high latitudes to the equator. Such large changes may be caused by climate change, but could also be part of normal long-period climatic variation. "The three areas that can trigger large changes in the Earth's gravitational field are oceans, polar and glacial ice, and atmosphere," Cox said.
Cox and colleague Dr. Benjamin Chao have ruled out the atmosphere as the cause. Instead, they suggest a significant amount of Ice or water must be moving from high latitude regions to the equator, and oceans could be the vehicles of this movement.
Estimates of today's glacier and polar ice melting are too small to explain the recent changes in the gravity field. If melting ice were the cause of the recent changes in the gravitational field, it would require melting a block of ice 10 km (6.2 miles) on each side by 5 km (3.1 miles) high every year since 1997 and pouring it into the oceans.
"The recent reports of large icebergs calving in Antarctica can't explain this, because they were already floating in the ocean," Cox said.
Further, radar altimeter observations of the average sea level rise provided by the TOPEX/POSEIDON satellite show no corresponding change in the rate of the global sea level increase.
Consequently mass must have been redistributed within the oceans. That's where the ocean circulation theory comes in. Ocean currents can redistribute mass quickly, such as the 5-year time frame that these changes were first observed. The TOPEX/POSEIDON observations of sea level height do show an increase in the equatorial bulge of the oceans corresponding to the observed gravity changes, but the data are not yet conclusive. One critical factor is the temperature of the world's oceans, and its salinity, for which detailed data are not yet available.
In 2002 NASA also launched the GRACE and JASON missions, missions that will help to more precisely track these sorts of changes in Earth's geodesy, and will launch the ICESAT mission this winter.
An article on this NASA-funded study appears in the August 2 issue of the journal Science.
For more information and images, go to: http://www.gsfc.nasa.gov/topstory/20020801gravityfield.html
The web site for the International Laser Ranging Service can be found at: http://ilrs.gsfc.nasa.gov
For more about the TOPEX/Poseidon mission, go to: http://sealevel.jpl.nasa.gov/mission/mission.html
Editor's Note: The original news release can be found at http://www.gsfc.nasa.gov/topstory/20020801gravityfield.html
Send page by E-Mail Human-Specific Variety Developed When Humans, Chimps Diverged
Posted: Friday, August 2, 2002
Source: University Of Georgia (http://www.uga.edu/)
Retroviruses Shows That Human-Specific Variety Developed When Humans, Chimps Diverged
Scientists in the past decade have discovered that remnants of ancient germ line infections called human endogenous retroviruses make up a substantial part of the human genome. Once thought to be merely "junk" DNA and inactive, many of these elements, in fact, perform functions in human cells.
Now, a new study by John McDonald of the University of Georgia and King Jordan at the National Center for Biotechnology Information at the National Institutes of Health, suggests for the first time that a burst of transpositional activity occurred at the same time humans and chimps are believed to have diverged from a common ancestor - 6 million years ago. These new results implicate retroelements, a particular type of transposable elements that are abundant in the human genome, in the actual shift from more rudimentary primates to modern human beings. The research was just published in the journal Genome Letters.
"There is a growing body of evidence that transposable elements have contributed to the evolution of genome structure and function in many species," said McDonald, a molecular evolutionist and head of the genetics department at UGA. "Our results suggest that a bust of transposable element activity may well have contributed to the genetic changes that led to the emergence of the human species." Jordan received his doctoral degree at UGA working with McDonald.
There has been a molecular arms race going on between transposable elements and their host genomes for millions of years. Host genomes are continually evolving new regulatory mechanisms to silence the mutagenic effects associated with the replication of these elements which, in turn, place selective pressure on the elements to evolve mechanisms to escape these controls. The result is an internal drive mechanism to increase biological complexity.
Just as new technologies generated by the military arms races between rival countries get spun off and used for non-military purposes, so the new regulatory mechanisms resulting from the arms race between transposable elements and host genomes generate new molecular mechanisms that can be used to accelerate evolution on the organismic level.
The idea of a relatively sudden genetic change that alters evolution isn't new. Scientists, such as the late Stephen Jay Gould, proposed a mechanism called "punctuated equilibrium" more than two decades ago. This idea, not yet completely accepted by scientists, proposes that evolution has depended more often on sudden and unexpected changes in genomes rather than a simple Darwinian paradigm of gradual evolutionary change due to extremely long-term natural selection.
While Darwin's theories have been around for more than a century, it took analyses of DNA using modern tools to find that human and chimpanzee DNA are more than 95 percent identical, a clue to a mutual origin.
Finding real evidence for sudden genetic changes, however, has been slow. By using phylogenetic surveys, however, McDonald and King were able to distinguish between the youngest HERVs (human endogenous retroviruses) and more ancient lineages.
The discovery that human-specific retroviruses emerged at the same time other researchers believe humans and chimps diverged was startling. Equally interesting, however was the discovery that the oldest subfamily of HERV elements is closely related and gave rise to the youngest and most recently active group of these elements. This suggests, the authors say, that "ancient families of HERVs may be capable of retaining the potential for biological activity over long spans of evolutionary time."
Interest in retroelements, which McDonald has been studying for more than a decade, has been growing recently. In a paper published last December in Nature Genetics, two researchers from Tufts University, Jennifer Hughes and John Coffin, identified 23 new members of the HERV-K group - the assemblage thought to contain the most recently active members. They found that at least 16 percent of those elements had undergone rearrangements that resulted in large-scale "deletions, duplications, and chromosome reshuffling during the evolution of the human genome."
The widespread presence of these viral elements led Coffin to tell one science magazine that humans probably have "more viruses in our genes than genes in our genes."
Just how these retroviral elements have moved around in the human genome and possibly changed organisms at the morphological level remains speculative. But there is increasing evidence that they may have been - and may still be - a driving force between evolution at the cellular and organismal levels.
The research of Jordan and McDonald is intriguing because it suggests that rather than simply playing a role in human evolution, retroviral elements may actually be implicated in the leap from chimpanzees to humans. Until a mere 50 years ago, scientists thought all genes worked from a stable position along a chromosome. That idea, however, began to change dramatically in the 1970s, when it became clear that the elements are pervasive in plant and animal genomes and that it simply made no sense that such elements would be conserved over thousands of millennia if they had no real function.
McDonald said it is increasingly clear that organisms need the viral elements and that their apparent continual backdoor assaults on normal genes may, in truth, be more like a vast, sophisticated chess game on an enormously complex board.
This is the first evidence, however, that suggests they may have made humans what they are today.
Editor's Note: The original news release can be found at http://www.uga.edu/news/newsbureau/releases/
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