It is possible that the second largest extinction in the history of the earth, which caused the extinction of two-thirds of all species on earth, was caused by the penetration of ultraviolet radiation from the sun into the earth through the earth's ozone layer, which was destroyed by gamma rays.
It is possible that the second largest extinction in the history of the earth, which caused the extinction of two-thirds of all species on the earth, was caused by the penetration of ultraviolet radiation from the sun into the earth through the earth's ozone layer, which was destroyed by gamma rays.
Astronomers hypothesized that a supernova, which exploded 10,000 light years from Earth, destroyed the chemical composition of the atmosphere and allowed the ultraviolet rays from the sun to "cook" delicate and defenseless life forms.
All of this happened about 440 million years ago and led to what is now known as the Ordovician extinction, the second most severe of the five largest extinctions on Earth.
"The common theory to explain this extinction was an ice age," said Adrian L. Malott, an astronomer from the University of Kansas. "We think there is good circumstantial evidence for a gamma ray burst."
Malot is a team leader, which includes several astronomers from the National Aeronautics and Space Administration. The team presented its theory at the American Astronomical Society's national meeting on Wednesday.
The fossil record of the Ordovician extinction shows the sudden disappearance of two-thirds of all species on Earth. The records also show that an ice age, which lasted more than half a million years, began at that time.
Malot said that the burst of gamma rays would provide an explanation for these two phenomena.
He said that the beam of gamma rays that hits the earth causes the destruction of molecules in the stratosphere and the formation of nitrogen oxide and other chemicals, which will destroy the ozone layer and cover the planet with brown smog.
"The sky will turn brown, but intense ultraviolet radiation from the sun will hit the Earth," he said. Radiation levels will be fifty times higher than normal. This is a level of radiation strong enough to kill life forms exposed to it.
The smog will also have another effect, the cooling of the earth, which will trigger an ice age, said Malott.
The extinction may have been a double whammy, said Bruce S. Lieberman, a paleontologist from the University of Kansas and co-author of the theory. "Our theory builds on previous theories, which include an ice age."
The Earth was unusually warm before the extinction. Melot said that climate experts have not been able to find a model that would explain the sudden onset of an ice age.
"They need something to trigger the start of the ice age," he said. "The gamma ray burst may have done it."
Jer H. Lips, a paleobiologist at the University of California, Berkeley, said the gamma-ray burst explanation for the Ordovician extinction should be considered only one of many theories. "This is a hypothesis that needs to be tested," Lips said.
He added that the accepted theory, according to which the dinosaurs became extinct following an asteroid impact 65 million years ago, began as a "wild idea", but gained support after further studies.
Most of the living creatures that went extinct in the Ordovician extinction were primitive sea creatures. Those who lived close to the surface of the water were at the highest risk of damage from the ultraviolet rays. Malott said, the extinct species lived in shallow water or reproduced with the help of larvae, which spent most of their time near the surface of the water. Animals that lived in the depths of the sea were not harmed.
Only a few primitive plants lived on earth, but they too were harmed by the radiation, he said.
Mellott said that the Earth has almost certainly been hit by gamma ray bursts several times during its 4.5 million years.
"You can expect a dangerous burst of gamma rays every few hundred million years," he said. "It may happen tomorrow or in millions of years."
Supernovae, the source of gamma ray bursts, usually leave behind remnants of dust clouds, shock waves and black holes, which can be discovered over millions of years. Mellot pointed out that there is no known evidence of such a close supernova, but in 440 million years the Milky Way galaxy has completed almost two revolutions, and the remnants of the supernova could have moved during this time.
The Ordovician extinction was the first of the five largest extinctions in Earth's history.
The Devonian extinction, which occurred 360 million years ago, wiped out 60% of the species on Earth; The Permian-Triassic extinction, which occurred 250 million years ago, killed 90% of the species on Earth; The late Triassic extinction 220 million years ago wiped out 50% of all species, and the Cretaceous-Tertiary event wiped out the dinosaurs and half of the other species 65 million years ago.
(Translation: Dikla Oren)
Link to the original article on CNN
