Gamma-ray bursts not fun for ancient Earth life?

[LARRY KLAES]

INTERESTING SPECULATION: GAMMA-RAY BURST MAY HAVE CAUSED GLOBAL COOLING AND MASS EXTINCTION ABC Science Online, 29 April 2004 http://abc.net.au/science/news/stories/s1097756.htm Anna Salleh   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. Professor Adrian Melott of the University of Kansas and team argued their case in a paper accepted for publication in the International Journal of Astrobiology. 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. ================ (2) GAMMA-RAY BURST LINKED TO MASS EXTINCTION Nature Online, 24 September 2003 http://www.naturecom/nsu/030922/030922-7.html PHILIP BALL   Some 440 million years ago, a nearby gamma-ray burst may have extinguished much of life on Earth, say US astronomers1. 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 could 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.   References Melott, A. L. et al. Did a gamma-ray burst initiate the late Ordovician mass extinction?. Preprint, http://xxx.arxiv.org/abs/astro-ph/0309415, (2003). |Article|