Supernova Explosions May Have Caused Mass Extinction on Earth 350 Million Years Ago

One or more massive supernova explosions that occurred around 65 light-years from Earth may have contributed to a mass extinction event on our planet around 359 million years ago, scientists have proposed.

Around this period, our planet suffered a huge, rapid decline in biodiversity, with recent evidence suggesting that the extinction event may have coincided with a dramatic drop in the amount of ozone—a gas composed of three oxygen molecules—in the atmosphere, possibly due to a global rise in temperatures.

Now, for a study published in the journal Proceedings of the National Academy of Sciences, a team of scientists examined an alternative possible cause for this drop in ozone: a nearby supernova explosion that could have showered the Earth in damaging cosmic radiation at the end of the Devonian period (around 419 million to 359 million years ago).

Supernovae are cataclysmic explosions that occur during the last evolutionary stages in the life of massive stars. They shoot out vast amounts of radiation.

"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, an author of the study from University of Illinois, Urbana-Champaign, 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," a co-author of the study, Adrienne Ertel from Urbana-Champaign, said in the statement.

The scientists examined other phenomena that could have led to the depletion of the ozone layer, such as meteorite impacts, solar eruptions and gamma-ray bursts.

However, the team rejected these factors because they are short-lived and, thus, unlikely to cause the long span of ozone depletion thought to have occurred at the end of the Devonian period.

Meanwhile, a supernova can damage Earth and its ozone layer over a period of up to 100,000 years. Immediately after the explosion, the damage is caused by ultraviolet radiation, X-rays and gamma rays emitted by the event. Subsequently, the planet is bombarded by debris from the explosion.

Evidence from the fossil record suggests the decline in biodiversity before the end-Devonian extinction event took place over a period of around 300,000 years. This indicates that multiple catastrophes occurred—possibly several supernovae explosions in the thin galactic disk where our sun resides.

"This is entirely possible," Urbana-Champaign graduate student and study co-author Jesse Miller said in the statement. "Massive stars usually occur in clusters with other massive stars, and other supernovae are likely to occur soon after the first explosion."

Evidence for the impact of a supernova explosion—or multiple explosions—on Earth comes from rocks dated to this time period that contain plant spores with characteristics consistent with "severely deteriorating environmental conditions, and UV-B damage following destruction of the ozone layer," the authors wrote in the study.

However, the scientists say that to confirm their hypothesis, studies would need to detect the radioactive isotopes plutonium-244 and samarium-146 in rocks and fossils from the time of the mass extinction event.

"Neither of these isotopes occurs naturally on Earth today, and the only way they can get here is via cosmic explosions," Urbana-Champaign undergraduate student and another co-author of the study Zhenghai Liu said.