Exploding Stars and Massive Extinction on the Earth
Some of us are familiar with the term “extinction-level event” which means a rapid decrease of biodiversity on Earth. Such an event occurs when the rate of extinction increases with respect to the rate of speciation.
The so-called “Big Five” of extinctions includes:
1. Ordovician-Silurian extinction events that took place 450–440 million years ago.
2. Late Devonian Extinction: 375–360 million years ago near the Devonian-Carboniferous transition.
3. Permian — Triassic extinction event. This event happened 252 million years ago at the Permian — Triassic transition.
4. Triassic — Jurassic event: 201.3 million years ago at the Triassic — Jurassic transition.
5. Cretaceous — Paleogene extinction event: 66 million years ago at the Cretaceous — Paleogene transition interval.
The Late Devonian was a period of low speciation which resulted in biodiversity decline, culminating in extinction events near the Devonian-Carboniferous boundary.
While the majority of the theories about the reasons of such events refers to earth-based catastrophes, a new study led by University of Illinois, Urbana-Champaign astronomy and physics professor Brian Fields explores the possibility of astronomical events being responsible for an extinction event that occurred 359 million years ago, at the boundary between the Devonian and Carboniferous periods.
“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,” Fields said. “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.
Brian and his team explored the mechanism of ozone depletion involving increased water vapor in the lower stratosphere caused by enhanced convection due to higher surface temperatures. These causes include 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.
Instead, the researchers focus on supernovae (SNe) that can be divided into two types:
1. massive stars that explode as core-collapse SNe (CCSNe);
2. white dwarfs.
The explosion of SNe immediately bathes Earth with damaging UV, X-rays, and gamma rays. According to the study a CCSN close enough to cause a massive extinction would also deliver supernova debris to the Earth as dust grains — micron- or submicron-sized particles created early after the explosion. The portion that reaches the Earth would deposit in the atmosphere live (undecayed) radioactive isotopes.
But fossil evidence indicates a 300,000-year decline in biodiversity leading up to the Devonian-Carboniferous mass extinction, suggesting the possibility of multiple catastrophes.
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 deposited at the time of extinction. “Neither of these isotopes occurs naturally on Earth today, and the only way they can get here is via cosmic explosions,” said undergraduate 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 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.”
Researchers have yet to search for Pu-244 or Sm-146 in rocks from the Devonian-Carboniferous boundary. Fields’ team said its study aims to define the patterns of evidence in the geological record that would point to supernova explosions.
“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.”
Story sources:
Materials provided by University of Illinois at Urbana-Champaign, News Bureau. Originally written by Lois Yoksoulian.
“Supernova triggers for end-Devonian extinctions”. Originally written by Brian D. Fields, Adrian L. Melott, John Ellis, Adrienne F. Ertel, Brian J. Fry, Bruce S. Lieberman, Zhenghai Liu, Jesse A. Miller, Brian C. Thomas. PNAS first published August 18, 2020, https://doi.org/10.1073/pnas.2013774117
