Ray burst may have wiped out life on Earth
A massive gamma-ray burst could have helped destroy much of life on Earth 440 million years ago, say a team of U.S. scientists.
Gamma-rays bursts (GRB) are extremely high-energy bursts of gamma-ray photons that shine millions of times brighter than the Sun.
They can last from a fraction of a second to several minutes, yet scientists are not sure what causes them.
Mellot and team argued that calculations on the rate that GRBs irradiate random locations suggested that the Earth would have been blasted at some stage.
"It seems likely that a GRB has affected the Earth," the researchers wrote, "and that a GRB would have substantial effect on living organisms."
The researchers argued that a GRB from our own galaxy could have been "at least partly" responsible for a major mass extinction of coral, sponges and shelled organisms at the end of Ordovician period, about 440 million years ago.
The researchers said the blast of energy from a GRB would have punched through the Earth's atmosphere and resulted in a powerful burst of UV radiation reaching the ground.
The blast would have also triggered ozone depletion, lead to a huge increase in the Earth's exposure to solar UV, acid rain and relatively rapid global cooling.
"The result: a one, two punch for life on the planet," the researchers wrote.
Melott and team said their "credible working hypothesis" was backed up by studies involving trilobites, extinct invertebrate marine organisms.
Trilobite larvae that floated in the sea became extinct at 1.5 times the rate of trilobite species that lived at the bottom of the sea. This would make sense if a GRB caused the extinction, they argued, as UV radiation does not penetrate water very far.
They said an intense rapid cooling and glaciation that occurred at the time could have also been caused by the GRB splitting nitrogen gas, leading to the formation of oxides of nitrogen in the atmosphere.
But Australian invertebrate palaeontologist Dr John Talent of Sydney's Macquarie University, who studies mass extinctions, was not convinced.
He said there was insufficient data to support any theory about what caused the Ordovician extinction. But his "gut reaction", from looking at other extinctions, was that they were complex events with many causes.
Talent said factors the researchers hadn't included were major changes in ocean circulation and ocean chemistry at the time.
"These people have focused on only one event and have not done their homework on the spectrum of other events in the last 550 million years," he told ABC Science Online.
"It's interesting speculation, like dinosaurs dying from constipation," he said.
Australian palaeoclimatologist Dr Will Howard from the University of Tasmania in Hobart also described the work as "highly speculative". But he said this was often necessary in such fields.
"In this game, the experiment has already been run and we're trying to read the evidence of what happened," he told ABC Science Online. "I always joke that the gods leave lousy laboratory notes which is why we have such trouble in the earth sciences.
"It would be a very difficult hypothesis to actually test and the authors don't suggest tests for the geological records to distinguish between a gamma-ray burst causing this extinction versus any other mechanisms."
But he said scientists may develop such tests in the future, just as they had found geochemical evidence in the form of a high concentration of iridium in deposits at the end of the Cretaceous era to directly support the idea that an asteroid wiped out the dinosaurs.
Australian astrophysicist Dr Charles Lineweaver of Sydney's University of New South Wales welcomed the research.
"This type of research is sorely needed if we are going to understand our place in the Universe," he told ABC Science Online. He said that the longer we lived on Earth the more important it would be to understand any potential threats, like gamma-ray bursts.