Imagine sitting out on a park at night reading an immersive work of fiction under the light of a supernova burst that's slightly brighter than a full moon on a cloudless night. Except you would neither be reading nor appreciating the scenery, because you would be dead due to the radiation.
And that's how scientists think at least one mass extinction event came to be. Also, detecting certain radioactive isotopes on Earth could be a way to confirm this hypothesis.
A study recently published in the journal Proceedings of the National Academy of Sciences investigates the possibility that an astronomical event of great extent was to blame for an extinction event 359 million years ago.
The team has its focus primarily on the boundary between Devonian and Carboniferous periods because there were findings of multiple generations of plant spores from that era that showed signs of UV light burns. Which could be a marker for a long-lasting ozone depletion event.
Hey, where did the ozone layer go?
Astronomy and physics professor at the University of Illinois who also is the leader of the study told Phys.org "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," and added, "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 Adrienne Ertel, a graduate student co-author points out that Betelgeuse, which is one of the closest supernova threats, is over 600 light-years away and we're well outside of its 25 light-years kill range.
The team also investigated other probable causes for an ozone depletion of this extent; meteorite impacts, solar eruptions, and gamma-ray bursts. Another graduate co-author Jesse Miller added that "... these events end quickly and are unlikely to cause the long-lasting ozone depletion that happened at the end of the Devonian period"
On the other hand, a supernova explosion could be a good candidate for such a long-term ozone depletion event. An exploding star immediately radiates UV, X-rays, and gamma-rays all around it in massive amounts. Such an event could damage the Earth and its ozone layer for up to 100,000 years.
But there's another interesting thing going on here. The fossil evidence indicates that biodiversity decline lasted for about 300,000 years. This may mean that we have gone through multiple catastrophic events. It could even mean multiple supernovae going off in a cluster. Miller finds this idea feasible: "Massive stars usually occur in clusters with other massive stars, and other supernovae are likely to occur soon after the first explosion."
How do we prove it?
The team says, to prove this whole scenario, we'd need to find the radioactive isotopes plutonium-244 and samarium-146 in rock and fossil samples deposited at the time of extinction. As co-author Zhenghai Liu points out "Neither of these isotopes occurs naturally on Earth today, and the only way they can get here is via cosmic explosions. Because they decay over time, "it's like a smoking gun" says Fields.
Fields finished his talk with thought-provoking words: "The overarching message of our study is that life on Earth does not exist in isolation. We are citizens of a larger cosmos, and the cosmos intervenes in our lives—often imperceptibly, but sometimes ferociously."