By Katie Hunt, CNN
(CNN) — Violent supernovas may have caused two of Earth’s largest mass extinctions that have never been completely explained, according to a theory put forward in new research.
During the final stages in the life of a gargantuan star, its death throes culminate in a powerful thermonuclear blast — a supernova — that typically destroys the celestial object, unleashing material and radiation.
A research team linked nearby stellar explosions to at least one, possibly two, mass die-offs after calculating the supernova rate of stars closest to the sun — within 65 light-years — in the past 1 billion years.
The work was part of a wider survey in the Milky Way galaxy of rare, massive O- and B-type stars that are relatively short-lived, using data from the European Space Agency’s Gaia space telescope.
The calculations suggested 2.5 supernovas might affect Earth in some way every 1 billion years, equating to one or two in the past 500 million years during which life evolved on the planet.
The rate is lower than previously thought, said Nick Wright, coauthor of the study published Tuesday in the journal Monthly Notices of the Royal Astronomical Society. This realization led Wright and his coauthors to connect the cosmic phenomenon with mass extinctions on Earth. Cataclysmic events have taken place five times in the past 500 million years, killing off most species from water and land over a relatively short geological interval.
“It’s a lot more feasible to think that this could be an effect that could affect extinction events,” said Wright, a lecturer in physics and astrophysics and Ernest Rutherford Fellow at Keele University in the United Kingdom.
The findings highlight how colossal stars can both create and destroy life, lead study author Alexis Quintana said.
“Supernova explosions bring heavy chemical elements into the interstellar medium, which are then used to form new stars and planets,” said Quintana, formerly a postdoctoral researcher at Keele and currently at Spain’s University of Alicante, in a statement. “But if a planet — including the Earth — is located too close to this kind of event, this can have devastating effects.”
Unexplained mass extinctions on Earth
In the study, the researchers provided no evidence that a supernova caused mass extinctions. Instead, the team hypothesized a stellar explosion may have been a potential factor in the Late Devonian extinction event 372 million years ago and one at the end of the Late Ordovician 445 million years ago. The team suggested a supernova may have stripped the ozone layer that shields the Earth from damaging radiation, resulting in a chain of events that could cause a mass extinction.
During the Devonian geological era, life thrived on land for the first time, but early land plants and animals making the transition from water to land were wiped out, along with armored fish and other marine species. A cataclysmic shift at the end of the Ordovician led to the disappearance of about 85% of species at a time when life was mostly limited to the seas.
“Their link to those mass extinctions, especially the Late Ordovician, is because a suggested consequence of such an explosion close to Earth would be glaciation, which we know did happen then. So, it’s an open hypothesis, but lacking evidence,” said Mike Benton, a professor of vertebrate paleontology at the UK’s University of Bristol, who was not involved in the research.
“I’d like to see a calibration of such historical events to show that one actually occurred at the same time as the mass extinction in question — we have those geological events reasonably well dated, but we need some way of dating supernova explosions of the deep past,” Benton said via email. He is the author of “Extinctions: How Life Survives, Adapts and Evolves.”
Paul Wignall, a professor of paleoenvironments at the UK’s University of Leeds, called the research interesting and said it was not the first time that the concept of a supernova-driven extinction had arisen. What is needed, he said, is tangible evidence that the extinctions coincided with supernovas.
“This could come from the exotic elements sourced from the explosion and present in trace amounts in the sedimentary record.”
Dino-killing asteroid
Celestial events have triggered at least one mass extinction, according to scientific evidence. A city-size asteroid slammed into Earth off the coast of what’s now Mexico one fateful day 66 million years ago, dooming the dinosaurs and many other species to extinction.
Researchers first identified the cause of the end-Cretaceous extinction by the discovery of the “iridium anomaly” — a 1-centimeter-thick (0.4-inch-thick) layer of sedimentary rock rich in iridium, an element rare on Earth’s surface but common in meteorites. A study describing the finding was published in June 1980.
First met with skepticism, the iridium anomaly eventually was spotted in more and more places around the world. A decade later, researchers identified a 200-kilometer-wide (125-mile-wide) crater off the coast of Mexico’s Yucatan Peninsula.
“It was the enrichment of iridium in Cretaceous/Palaeogene boundary sediments that was the highly convincing ‘smoking gun’ for the dinosaur extinction when (the) idea was first published in 1980. The supernova idea needs its iridium equivalent, iron-60 or plutonium maybe?” Wignall said via email, referring to elements that could be a marker of a supernova.
Iron-60 is a radioactive variant of iron that isn’t abundant on Earth but is produced in large quantities in supernova explosions. Wright also said it might be possible to measure ozone depletion in rocks and sediments.
Recent studies on mass extinction events have shown it was typically a series of consequential events, often triggered by large-scale volcanic eruptions, that got progressively worse, Wignall added.
“It’s hard to see how a supernova would fit into such a scenario,” he said. “At the start, before things got too bad or at the peak when things were already going wrong?”
Wright said the goal of his team’s work was to draw attention to the new supernova timescale the researchers had identified. “I think there were a lot of people who will rightfully say, you don’t know what caused these extinction events. And then it might be some that say we’re speculating too much. What we just want to do is draw attention to the numbers.”
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