|Jan8-04, 06:13 PM||#1|
Was extinction caused by UV rays from sun?
ATLANTA -- The second-largest extinction in the Earth's history, the killing of two-thirds of all species, may have been caused by ultraviolet radiation from the sun after gamma rays destroyed the Earth's ozone layer.
Astronomers are proposing that a supernova exploded within 10,000 light years of the Earth, destroying the chemistry of the atmosphere and allowing the sun's ultraviolet rays to cook fragile, unprotected life forms.
All this happened some 440 million years ago and led to what is known as the Ordovician extinction, the second-most severe of the planet's five great periods of extinction.
"The prevailing theory for that extinction has been an ice age," said Adrian L. Melott, a University of Kansas astronomer. "We think there is very good circumstantial evidence for a gamma-ray burst."
Melott is the leader of a team, which includes some astronomers from the National Aeronautics and Space Administration, that presented the theory Wednesday at the national meeting of the American Astronomical Society.
Fossil records for the Ordovician extinction show an abrupt disappearance of two-thirds of all species on the planet. Those records also show that an ice age that lasted more than a half million years started during the same period.
Melott said a gamma-ray burst would explain both phenomena.
He said a gamma-ray beam striking the Earth would break up molecules in the stratosphere, causing the formation of nitrous oxide and other chemicals that would destroy the ozone layer and shroud the planet in a brown smog.
"The sky would get brown, but there would be intense ultraviolet radiation from the sun striking the surface." he said. The radiation would be at least 50 times above normal, powerful enough to kill exposed life.
In a second effect, the brown smog would cause the Earth to cool, triggering an ice age, Melott said.
The extinction "could have been a one-two punch," said Bruce S. Lieberman, a paleontologist at the University of Kansas and a co-author of the theory.
"Our theory builds on earlier theories" that included an ice age.
Before the extinction, the Earth was unusually warm. Melott said climate experts have been unable to find a model that would explain the sudden onset of massive glaciers.
"They need something to jump-start the ice age," he said. "The gamma-ray burst could have done it."
Jere H. Lipps, a paleobiologist at the University of California, Berkeley, said gamma rays as a source of the Ordovician extinction should be regarded as only one of several theories. "It is a hypothesis that should be tested," Lipps said.
He said the widely accepted idea that the dinosaurs were wiped out by an asteroid 65 million years ago started out as a "wild idea" but that it gained wide support after other research.
Most of the life killed in the Ordovician extinction were primitive sea creatures. Those that lived at or near the surface would be at greatest risk from the ultraviolet radiation.
Melott said the species killed lived in shallow waters or reproduced with larvae that spent part of their lives near the water surface. Animals living in deep water were not harmed.
There were only primitive plants living on land, but they, too, would have been affected, he said.
Melott said it is almost certain that Earth has been zapped by gamma rays several times in its 4.5 billion-year history.
"You can expect a dangerous gamma-ray burst every few hundred million years," he said. "It could happen tomorrow or it could be millions of years."
Supernovae, the source of gamma rays, usually leave behind remnant clouds of dust, shock waves and black holes that can be detected for millions of years.
Melott said there is no known evidence of such a nearby supernova, but that in 440 million years the Milky Way would have rotated almost twice and traces of the explosion could have been moved during that time.
The Ordovician was the first of five great extinctions in history.
The Devonian, 360 million years ago, killed 60 percent of all species; the Permian-Triassic, 250 million years ago, killed 90 percent of all life; the late Triassic, 220 million years ago, killed half of all species; and the Cretacious-Tertiary event destroyed the dinosaurs and half of all other species about 65 million years ago.
|Jan10-04, 12:33 AM||#2|
I don't know if you can answer this, but wouldn't the smog caused by the gamma burst be able to be detected in the Earth's strata?
|Jan10-04, 08:39 AM||#3|
IIRC, the composition of 'old atmospheres' can be determined from air trapped in ice; similarly, particulates may precipitate out with snow and also be found in ice cores.
AFAIK, the oldest ice - from Greenland and Antarctic ice cores - is way less than 1 million years old, so it wouldn't help much for testing for smog at the time of any of the major extinctions.
Perhaps there are some subtle chemical traces in sedimentary rocks, which reflect the concentration of minor constituents of the atmosphere?
|Jan10-04, 08:48 PM||#4|
Gamma-ray burst linked to mass extinction
Some 440 million years ago, a nearby gamma-ray burst may have extinguished much of life on Earth, say US astronomers.
Adrian Melott, of the University of Kansas in Lawrence, and colleagues reckon that the fossil record of the end of the Ordovician period fits with how such a cosmic explosion a few thousand light years away could have altered the environment. At that time, more than 100 families of marine invertebrates died out; it was the second most devastating mass extinction in our planet's history.
The possibility of life on Earth being affected by cosmic events has been long recognized. Giant asteroid impacts have been proposed as a cause of global wildfires and climate cooling that culd have been behind events such as the extinction of the dinosaurs 65 million years ago.
Researchers have also suggested that supernovae - explosions of old stars - could flood our planet with deadly radiation if they happen within around 100 light years of us (our galaxy is 150,000 light years across). This has been put forward as the cause of the mass extinction two million years ago.
Compared to gamma-ray bursts (GRBs), supernovae are just firecrackers. Most GRBs come from beyond our galaxy. They are visible across such immense distances because they are extraordinarily bright and powerful, despite lasting just seconds.
It seems increasingly likely that they are linked to supernovae. Jets or blobs of material thrown out from a collapsing star could produce a flash of gamma rays when they collide with the gas between stars.
Flash in the past
Water would protect marine organisms from the heat of a GRB, but not from its other effects, argues Melott's team. Its gamma-rays would convert some nitrogen and oxygen in the atmosphere into nitrogen dioxide, the brownish gas present in urban smog.
Nitrogen dioxide would filter out sunlight, turning the skies dark. The cooling effect could trigger an ice age - there is evidence of widespread glaciation 440 million years ago. Nitrogen oxides also cause acid rain and destroy the ozone layer, exposing Earth to more of the Sun's harmful ultraviolet rays.
Ultraviolet radiation can penetrate tens of metres of water, so it could harm marine organisms at these depths. Indeed shallow-dwelling species, or those that spend their early lives in shallow water, seem to have suffered more than deep species in the Ordovician extinction.
In short, a nearby GRB might first have showered harmful radiation onto the exposed face of the planet, killing more or less indiscriminately, and may then have exposed the other hemisphere to increased ultraviolet radiation, damaging marine life decreasingly with increasing depth.
The fingerprint of such an event might be revealed by gathering more information about the geographical pattern of the Ordovician extinctions, the researchers conclude.
Over 100 families of marine invertebrates, including trilobites, became extinct at the Ordovician Extinction.
At least five times in the history of life, the Earth experienced mass extinctions that eliminated a large percentage of the biota. Many possible causes have been documented, and gamma-ray bursts (GRB) may also have contributed. GRB produce a flux of radiation detectable across the observable Universe. A GRB within our own galaxy could do considerable damage to the Earth's biosphere. Rate estimates suggest that a number of such GRB may lie within the fossil record. The late Ordovician mass extinction shows a water-depth dependent extinction pattern that is a natural result of the attenuation of the strong ultraviolet radiation expected to result from a nearby GRB. In addition, a GRB would trigger global cooling which is associated with this mass extinction.
The Ordovician period was an era of extensive diversification and expansion of numerous marine clades. Although organisms also present in the Cambrian were numerous in the Ordovician, a variety of new types including cephalopods, corals (including rugose and tabulate forms), bryozoans, crinoids, graptolites, gastropods, and bivalves flourished. Ordovican communities typically displayed a higher ecological complexity than Cambrian communities due to the greater diversity of organisms. However, as in the Cambrian, life in the Ordovician continued to be restricted to the seas.
The Ordovician extinction occurred at the end of the Ordovician period, about 440-450 million years ago. This extinction, cited as the second most devastating extinction to marine communities in earth history, caused the disappearance of one third of all brachiopod and bryozoan families, as well as numerous groups of conodonts, trilobites, and graptolites. Much of the reef-building fauna was also decimated. In total, more than one hundred families of marine invertebrates perished in this extinction.
Ordovician period (510- 438 million years ago)
Ordovician extinction (440-450 million years ago)
Ordovician extinction was second most devastating in earth history
|Jan19-04, 03:51 PM||#5|
Orion1 - Welcome to Physics Forums.
helpful note - it's against policy to post other people's articles in full (copyright violation). In the future, please post just a relevant paragraph and then provide the URL for people to link over to the full article.
|Similar discussions for: Ultraviolet Extinction...|
|Ultraviolet Image||Introductory Physics Homework||3|
|metal invisible in the ultraviolet?||Atomic, Solid State, Comp. Physics||1|
|ultraviolet light||Classical Physics||4|
|Ultraviolet spectroscopy - need help!||Introductory Physics Homework||1|
|Ultraviolet catastrope||Quantum Physics||2|