Multiple star explosions may have driven mass extinction on Earth about 359M years ago: Study
One or more explosions, about 65 light-years away from Earth, could have been responsible for the protracted loss of ozone and the resulting mass extinction
About 359M years ago, the Earth witnessed a mass extinction event but the circumstances that led up to it are unclear. A new study has pinpointed a probable cause: one or a series of explosions of stars or supernova.
The team focused on the boundary between Devonian and the Carboniferous periods. About 75% of animal species were wiped out of existence in the late Devonian period, which is one of the five major extinction events. Studies have suggested the extinction event coincided with a drop in the levels of ozone — a chemical that blocks harmful UV rays from entering Earth. Experts have since proposed that depleted ozone layers were the culprits.
"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," Brian Fields, a professor at the university who led the study, said in a statement. "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. Betelgeuse or the red supergiant has been in the news for mysteriously going dim in 2019. Several theories — from an impending supernova to cosmic dust and starspots — have been proposed to explain its strange behavior. Though it returned to its former sheen, reports have suggested that it is dimming again.
In this study, researchers from the University of Illinois, Urbana-Champaign, think cosmic rays emanating from a supernova could have driven ozone depletion. They arrived at this after ruling out other possible causes: meteorite impacts, solar eruptions and gamma-ray bursts. "But these events end quickly and are unlikely to cause the long-lasting ozone depletion that happened at the end of the Devonian period," said graduate student and study co-author Jesse Miller.
But rays from a supernova explosion can cause long-lasting damage, according to the researchers. When one or more stars about 65 light-years detonate, it can send harmful UV, X-rays and gamma rays towards Earth. The result: the destruction of our planet and its protective ozone layer that can persist for up to 100,000 years.
The supernova is just a probable culprit at the moment. The researchers will need to find evidence to confirm their theory. More specifically, they will have to look for two chemical signatures: Pu-244 or Sm-146 in rocks from the Devonian-Carboniferous boundary. These chemicals have their origins in cosmic explosions and are not naturally found on Earth. But if the team manages to detect them, it could implicate the involvement of supernova.
Giving an example, 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."
"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."
The study is published in PNAS.