Asteroids, vast volcanic provinces, supernovae, and over-ambitious humans top the list of potential causes of catastrophe. We have strong evidence for three of these triggering mass extinctions, but we're less sure about exploding stars. Now, however, some scientists are arguing that a series of supernovas caused a major decline in life's abundance at the end of the Devonian era..
The Devonian era 400 million years ago, often called the Age of Fishes, ended with two major extinctions around 10 million years apart, known as the Kellwasser and Hangenberg events. Their causes remain unknown, but recently a paper has provided evidence for ozone depletion at the time of the Hangenberg event.
Although some have seen this as a pointer to volcanic activity, Professor Brian Fields of the University of Illinois, Urbana-Champaign, noted the radiation from a nearby supernova would also damage their ozone layer. In Proceedings of the National Academy of Sciences, Fields sets out the case for why a nearby stellar explosion better fits what we know.
A nearby star exploding would release gamma rays, X-rays, and high-frequency ultraviolet light that would break the bonds holding ozone molecules in the upper atmosphere together. Until the ozone layer recovers, damaging photons from the Sun usually blocked in the stratosphere would bathe the Earth, something that could last a lot longer than the initial burst of radiation.
Moreover, interactions between the supernova blast and surrounding gas would produce a lasting upswing in cosmic rays that would prevent the ozone layer from recovering for thousands of years. Fields thinks this more extended round of depletion fits better with the Hangenberg evidence than other events whose impact would be more short-lived.
The situation is complicated by the fact that supernovas don't give much warning, yet the Hangenberg event was preceded by a slow decline in species abundance lasting some 300,000 years. The life forms of the day couldn't have known an explosion was coming. Instead, the paper proposes a series of supernovas, with the earlier ones causing modest harm to the planet and the last one being the most damaging.
“This is entirely possible," said graduate student Jesse Miller, who co-authored the study, in a statement. "Massive stars usually occur in clusters with other massive stars, and other supernovae are likely to occur soon after the first explosion.”
At the moment, the team's claims are still quite speculative. To back them up, it would be necessary to find evidence of radioactive plutonium-244 or other radioactive supernova products in the era's rocks. By now, most would have decayed. Still, it's possible that enough is left that its presence could be detected with sufficient effort.
Discussions such as this may dent some people's enthusiasm to see Betelgeuse explode in their lifetimes, but Fields says we shouldn't worry. He estimates the Hangenberg supernovas would have been around 65 light-years from Earth.
“To put this into perspective,” he said. “One of the closest supernova threats today is from the star Betelgeuse, which is over 600 light-years away.”