A team of researchers led by Professor Brian Fields hypothesizes that a supernova about 65 light-years away may have contributed to the ozone depletion and subsequent mass extinction of the late Devonian period, 359 million years ago. Pictured is a simulation of a nearby supernova collision with and compression of the solar wind. The orbit of the earth, the blue dotted circle, and the sun, red dotted, are displayed for scale. Credit: Graceful courtesy Jesse Miller
Imagine reading through the light of an exploding star, brighter than a full moon – it may be fun to think about, but this scene is the prelude to a catastrophe as the radiation destroys life as we know it. Killer cosmic rays from nearby supernovae could be the culprit in at least one event for mass extinction, researchers said, and finding certain radioactive isotopes in Earth’s rock record could confirm this scenario.
A new study led by University of Illinois, Urbana-Champaign astronomy and natural science professor Brian Fields investigates the possibility that astronomical events are responsible for an extinction event that occurred 359 million years ago, on the border between the Devonian and Carboniferous periods.
The paper is published in the Procedures of the National Academy of Sciences.
The team concentrated on the Devonian-Carboniferous border, because those rocks contain hundreds of thousands of generations of plant spores sunk by ultraviolet light – evidence of an event with long-lasting ozone depletion.
“Earth-based disasters such as large-scale volcanism and global warming can also destroy the ozone layer, but evidence for this is not one-sided for the time interval in question,” Fields said. “Instead, we suggest that one or more supernova explosions, about 65 light-years away from Earth, could have been responsible for the prolonged loss of ozone.”
“To put this in perspective, one of the closest supernova threats today is to the star Betelgeuse, which is more than 600 light-years away and well beyond the kill distance of 25 light-years,” said graduate student and co-author Adrienne Ertel.
The team investigated other astrophysical causes of ozone depletion, such as meteorite impact, solar flares and gamma-ray bursts. “But these events end quickly and are likely to cause the prolonged ozone depletion that occurred at the end of the Devonian period,” said graduate student and co-author Jesse Miller.
A supernova, on the other hand, delivers a one-two punch, the researchers said. The explosion immediately bathed the earth with harmful UV, X-rays and gamma rays. Later, the explosion of supernova punches hits the solar system, causing the planet to be subjected to long-life radiation from cosmic rays accelerated by the supernova. The damage to Earth and its ozone layer can last up to 100,000 years.
But fossil evidence points to a decline of 300,000-year-old biodiversity leading to the extinction of Devonian-Carboniferous, suggesting the possibility of multiple disasters, perhaps even multiple supernova explosions. “This is entirely possible,” Miller said. “Massive stars normally occur in clusters with other massive stars, and other supernovae are likely to occur shortly after the first explosion.”
The team said the key to proving that a supernova occurred would be to find the radioactive isotopes plutonium-244 and samarium-146 in the rocks and fossils that were deposited at the time of extinction. “None of these isotopes occur naturally on Earth today, and the only way they can get here is through cosmic explosions,” said 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’re fresh, and you know they’ve not grown here. Like bananas, Pu-244 and Sm-146 decay over time. So when we find these radioisotopes on Earth today, we know that they are fresh and not from here – the green bananas of the isotope world – and thus the smoking guns of a nearby supernova. “
Researchers have yet to search for Pu-244 or Sm-146 in rocks off the Devonian-Carboniferous border. Fields’ team said their study aims to define the patterns of evidence in the geological record that would point to supernova explosions.
“The overwhelming 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 cruelly.”
Reference: “Supernova Triggers for End-Devonian Extinctions” by Brian D. Fields, Adrian L. Melott, John Ellis, Adrienne F. Ertel, Brian J. Fry, Bruce S. Lieberman, Zhenghai Liu, Jesse A. Miller, and Brian C Thomas, August 18, 2020, Procedures of the National Academy of Sciences.
DOI: 10.1073 / pnas.2013774117
Also participating in the study were scientists from the University of Kansas; Kings College, United Kingdom; the European Organization for Nuclear Research, Switzerland; the National Institute of Chemical Physics and Biophysics, Estonia; the U.S. Air Force Academy; and Washburn University.
The Council of Science and Technology Facilities and the Estonian Research Council supported this study. Fields is also affiliated with the Illinois Center for Advanced Studies of the Universe.
