The surface of the Earth was immersed in life-damaging radiation from nearby supernovae on several different occasions over the past nine million years. That is the claim of an international team of astronomers, which has created a computer model that suggests that high-energy particles from the supernovae created ionizing radiation in Earth’s atmosphere that reached ground level. This influx of radiation, the astronomers say, potentially changed the course of the Earth’s climate and the evolution of life.
Earlier this year, two independent teams of astronomers published evidence that several supernovae had exploded some 330 light-years from Earth. Each event showered the solar system in iron-60, an overabundance of which has been found in core samples from the bottom of the Atlantic, Pacific and Indian oceans. A discovery of the same element ‘iron-60’ was found on the moon.
Iron-60 is not all that supernovae produce – they also produce cosmic rays, which are composed of high-energy electrons and atomic nuclei. Previous work by Neil Gehrels of NASA’s Goddard Space Flight Center, was found to be incorrect as he indicated that a supernova would have to explode within 25 light-years of Earth to give our planet a radiation dose strong enough to cause a major mass extinction.
Now, a team led by Brian Thomas of Washburn University, and Adrian Melott of the University of Kansas argues that this conclusion is incorrect. The researchers looked at what would happen if a supernova exploded at a distance of 325 light-years and worked-out how its radiation would affect Earth. They found that cosmic rays accelerated towards Earth by the supernova are a different story. These have energies in the teraelectronvolt (TeV) region and are able to “pass right through the solar wind and Earth’s magnetic field and propagate much further into the atmosphere than cosmic rays normally do.”, says Melott.
When a cosmic ray strikes an air molecule, it produces a shower of secondary particles that is filled with the likes of protons, neutrons and a strong flux of muons. Ordinarily this takes place in the upper atmosphere and can be responsible for ionizing and destroying ozone in the stratosphere. However, the supernova cosmic rays are so energetic that they will pass straight through the stratosphere, lower atmosphere, and down to the surface and deep into the oceans and mantle.
Today, muons contribute a sixth of our annual radiation dose, however, the team calculated a supernova hit would result in a 20-fold increase in the muon flux that would triple the annual radiation dose of life forms on the planet.
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