Extinction by supernova
Earth’s fossil record suggests its ozone layer took a protracted beating
Earth’s fossil record suggests our planet’s ozone layer took a protracted beating
One of the worst extinction events in Earth’s history may have been triggered by a supernova, the violent death of a distant star.
About 75 per cent of all species on Earth died out at the end of the Devonian Period, nearly 360 million years ago. Rocks from this era preserve many thousands of spores that appear to be scorched by ultraviolet (UV) radiation, indicating that something went seriously wrong with our protective ozone layer.
The destructive force may have come from very far afield. “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, a professor of physics and astronomy at the University of Illinois at Urbana-Champaign. “Instead we propose that one or more supernovae, 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 – well outside the kill distance of 25 light years,” said Adrienne Ertel, a graduate student in Fields’ research group.
Supernovae, which end the lives of giant stars, can hit Earth life with a powerful one-two punch. Highly energetic UV, X-ray and gamma radiation delivers the first wallop, and the second comes from swarms of charged particles called cosmic rays that are accelerated to tremendous speeds by the explosion. This combo can damage Earth’s ozone layer for 100,000 years or so.
Fossil evidence suggests that biodiversity decreased substantially for about 300,000 years at the end of the Devonian, often called the
‘Age of Fishes’ because of its tremendous fish diversity. The Late Devonian extinction may have involved several different dramatic events – perhaps two or more nearby supernovae. “This is entirely possible,” said Jesse Miller, another graduate student in Fields’ lab. “Massive stars usually occur in clusters with other massive stars, and other supernovae are likely to occur soon after the first explosion.”
The team suggested a way to test the hypothesis: look for the radioactive isotopes plutonium-244 and samarium-146 in rocks and fossils from the Late Devonian. Isotopes are versions of chemical elements with different numbers of neutrons in their nuclei. “Neither of these isotopes occur naturally on Earth today, and the only way they can get here is via cosmic explosions,” said Zhenghai Liu, also at Illinois.
Such a search has not yet happened. Though Fields and his team aren’t the first researchers to find possible links between supernovae and extinction events. For example, a different team recently proposed that a supernova contributed to the minor mass extinction at the end of the Pliocene Epoch about 2.6 million years ago.
The Late Devonian and Late Cretaceous events are two of the five mass extinctions that scientists have traditionally recognised. However, there’s a growing consensus that we’re now living through a sixth mass extinction – one caused primarily by humanity, with global warming and habitat destruction two of the biggest drivers.
“Supernovae could have been responsible for the protracted loss of ozone”
Brian Fields