Killer cosmic rays from exploding stars may have caused a mass extinction event

Though the term “extinction event” often refers to the extinction of the dinosaurs’ Cretaceous Period or the possibility of human extinction (because of how horribly we treat the planet), Mother Earth has actually seen multiple mass extinction events. There have been a total of five mass extinction events, with the Permian-Triassic Extinction (250 million years ago) being the worst.

A new study has revealed that the late-Devonian Extinction that occurred approximately 359 million years ago may have been caused by a non-Earth, astronomical event. This extinction occurred at the border of the Devonian and Carboniferous periods. The study was led by the University of Illinois.

The team focused especially on the Late- Devonian extinction event due to evidence of rocks containing hundreds of thousands of generations of plant spores that appear to have been sunburnt by ultraviolet light. This is a sign of an ozone-depletion event.

“Earth-based catastrophes such as large-scale volcanism and global warming can destroy the ozone layer, too, but evidence for those is inconclusive for the time interval in question,” Fields said. “Instead, we propose that one or more supernova explosions, about 65 light-years away from Earth, could have been responsible for the protracted loss of ozone.”

“To put this into perspective, one of the closest supernova threats today is from the star Betelgeuse, which is over 600 light-years away and well outside of the kill distance of 25 light-years,” said graduate student and study co-author Adrienne Ertel.

The team explored different possibilities for the cause of the ozone-depletion event, including meteorite impacts, solar eruptions, and gamma-ray bursts. However, a supernova explosion that unleashes harmful radiation including UV, X-rays, and gamma rays can cause damage to the ozone layer lasting up to 100,000 years.

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The evidence found in fossils indicates a 300,000-year-old lead up to the extinction event. This leads researchers to believe that there may have been a combination of multiple natural calamities, or even multiple supernovae explosions, that may have led to the late-Devonian extinction event.

The final proof in confirming that supernovae explosions happened during that period is to find the presence of radioactive isotopes plutonium-244 and samarium-146 in the fossils from that time period. “Neither of these isotopes occurs naturally on Earth today, and the only way they can get here is via cosmic explosions,” said undergraduate student and co-author Zhenghai Liu.

The radioactive species born in the supernova are like green bananas, Fields said. “When you see green bananas in Illinois, you know they are fresh, and you know they did not grow here. Like bananas, Pu-244 and Sm-146 decay over time. So if we find these radioisotopes on Earth today, we know they are fresh and not from here — the green bananas of the isotope world — and thus the smoking guns of a nearby supernova.”

Scientists from the University of Kansas; Kings College, UK; the European Organization for Nuclear Research, Switzerland; the National Institute of Chemical Physics and Biophysics, Estonia; the United States Air Force Academy; and Washburn University, all participated in the study. The Science and Technology Facilities Council and the Estonian Research Council also supported the study.

The team have yet to search for residue of these radioactive isotopes in the rocks from the Devonian-Carboniferous border period. “The overarching message of our study is that life on Earth does not exist in isolation,” Fields said. “We are citizens of a larger cosmos, and the cosmos intervenes in our lives — often imperceptibly, but sometimes ferociously.”

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Source: Science Daily

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