Ball reported in 2003 that some 440 million years ago, a nearby gamma-ray burst may have extinguished much of life on Earth, say U.S. astronomers. Adrian Melott, of the University of Kansas in Lawrence, and colleagues inferred from the fossil record that near the end of the Ordovician period that a cosmic explosion occurred a few thousand light years away could have altered the environment. At that time, more than 100 families of marine invertebrates were impacted; it was the second most devastating mass extinction known in our planet's history; this loss included most of the trilobites, which died out at the end of the Ordovician period.
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. Geologists 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 possible 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.
[An update on Gamma-Ray Bursts (here).]
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