Multiple star explosions may have contributed to a mass extinction on Earth

Several global extinction events occurred during Devonian period, which lasted from 359 million to 419.2 million years ago, and ended it as well. The collective events resulted in the loss of 70 to 80 percent of all animal species present during the Devonian, largely affecting marine life.

The beginning of the Carboniferous period, occurred after, lasting between 299 million to 359.2 million years ago.

No single cause has previously been associated with the Devonian extinctions, but new research suggests that multiple supernovae could have caused the drop in ozone levels with the final extinction event that ended the Devonian period.

The research team focused on the boundary between the Devonian and Carboniferous periods because rocks from this time reveal something intriguing. The plant spores contained in them appear as though they were burnt by ultraviolet light. Ozone levels in Earth's stratosphere also dropped at this time.

"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," said Brian Fields, study author and University of Illinois at Urbana-Champaign astronomy and physics professor, 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."

The "kill distance" for supernovae is about 25 light-years away. A supernova, or multiple explosions, from 65 light-years away wouldn't decimate life on Earth, but it could hit our planet with enough radiation to damage the ozone layer and expose Earth to ultraviolet rays and cosmic rays.

The study published this week in the journal Proceedings of the National Academy of Sciences.

In striking distance

Gamma ray bursts, eruptions from the sun and the impact of meteorites striking Earth could also deplete ozone, but they don't seem likely in this case.

"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 Jesse Miller, study coauthor and University of Illinois at Urbana-Champaign graduate student, in a statement.

An exploding star, or multiple exploding stars, would immediately cause damage by hitting Earth with UV, gamma and X-rays. And about a thousand years later, the actual supernova debris would hit our solar system, irradiating Earth with high levels of intense cosmic rays.

This kind of damage could persist for 100,000 years, according to Adrian L. Melott, study coauthor and physicist at the University of Kansas.

"The cosmic rays from such a supernova will produce muons in the atmosphere, which are a very penetrating kind of radiation," Melott said. "They could cause internal damage in large animals and in organisms up to a half-mile down in the ocean."

There was also likely an excess of lightning, which could cause wildfires and lead to climate change.

Leading up to the mass extinction event 359 million years ago, evidence in the fossil record suggests that Earth's biodiversity declined over 300,000 years -- the other extinction events of the Devonian period.

This is why researchers believe multiple supernovae are responsible for what happened on Earth, which is very likely.

"Very massive stars are typically born in large associations, and they live short lives, so they are likely to go off around the same time," Melott said.

Proving that supernovae were the cause of this mass extinction will require finding radioactive isotopes in the rock record from this time period, like plutonium-244 and samarium-146. Supernovae would have left traces of these in rocks and fossils on Earth.

"Neither of these isotopes occurs naturally on Earth today, and the only way they can get here is via cosmic explosions," said Zhenghai Liu, study coauthor and University of Illinois, Urbana-Champaign undergraduate student, in a statement.

"When you see green bananas in Illinois, you know they are fresh, and you know they did not grow here," Fields said. "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."

It's the latest in a series of studies authored by members of this research team in recent years, which also suggest that supernovae potentially motivated early humans to walk upright by causing a dirth of wildfires and killed off large marine animals at the dawn of the Pleistocene 2.6 million years ago.

"For more than a decade, my colleagues and I have been interested in the possibility of ionizing radiation events causing extinction events on Earth," Melott said.

"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."