Exploding stars – or ‘supernovae’ – at least 65 light-years from Earth, could have unleashed devastating mass extinction events about 359 million years ago.
The extinction of the Late Devonian saw 50 percent of all genera – groups of species – and 19 percent of all families disappear, the next largest grouping on the tree of life.
Among the groups least affected were the jaw-less fish, reef-building ‘rugose’ corals and the trilobites – iconic marine marine species resembling woodlice.
This episode of mass death has previously been accused of asteroid impact, climate change, sea level changes and large-scale volcanic activity.
However, researchers from the US investigated the boundary in the rock record between the Devonian and the next period, the Carboniferous.
Here lie hundreds of thousands of generations of plant spores that were sunburned by ultraviolet light – suggesting a long-term depletion of the ozone layer.
This could be caused by the impact of one or more near-supernovae explosions on the solar system – and would have been life catastrophic.
Exploding stars – or ‘supernovae’ – at least 65 light-years from Earth, could have unleashed devastating mass extinction events about 359 million years ago. Pictured, an impression of artists from a local supernova as seen from Earth to Earth
“We suggest that one or more supernova explosions, about 65 light-years from Earth, could have been responsible for the prolonged loss of ozone,” said paper author and astronomer Brian Fields of the University of Illinois at Urbana-Champaign. .
‘Disasters on Earth, 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.’
Professor Fields and his colleagues have also ruled out other space events that could have potentially caused ozone depletion – such as deadly gamma-ray bursts, meteorite impact and solar flares.
‘These events are ending faster and are likely to cause the longest ozone depletion that occurred at the end of the Devonian period,’ said paper author and astronomer Jesse Miller, also of the University of Illinois.
In contrast, the researchers explained, a supernova has the potential to have a more lasting impact – especially seeing how they can deliver a ‘one-two punch’.
If a star had gone near nova, Earth would have bathed for a short time in damage from ultraviolet, X-ray and gamma radiation.
This would be followed by the explosion of pun from the explosion that struck the solar system and our planet underwent radiation from the cosmic rays accelerated by the supernova.
Together, the damage to the Earth’s ozone layer would probably have lasted up to 100,000 years, the researchers said.
Although this is not as long as the 300,000-year decline in biodiversity that led to the end of the Devonian period, the supernova could have occurred in tandem with other catastrophes – or even multiple nova explosions.
“It simply came to our notice then. Massive stars normally occur in clusters with other massive stars, and other supernovae are likely to occur soon after the first explosion, ‘Mr Miller explained.
If a star had gone near nova, Earth would have bathed for a short time in damage from ultraviolet, X-ray and gamma radiation. This would be followed by the fallout from the explosion that struck the solar system. Pictured, a simulation of a nearby supernova explosion that bakes and compresses the solar wind – with the Earth’s orbit in blue and the sun itself as the central red dot
However, evidence for the supernova hypothesis would have to come from finding evidence for the radioactive isotopes plutonium-244 and samarium-146 within the rocks and fossils deposited at the time of mass extinction.
“None of these isotopes occur naturally on Earth today and the only way they can get here is through cosmic explosions,” said paper author and astronomer Zhenghai Liu.
Or, as Professor Fields puts it, they are like green bananas.
‘When you see green bananas [here] in Illinois you know they are fresh, and you know they have not grown here. ‘
‘Like bananas, the 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 smoke guns of a nearby supernova. ‘
However, the search for these metaphorical bananas will be a task for the future, the researchers explained.
The extinction of the Late Devonian saw 50 percent of all genera – groups of species – and 19 percent of all families disappear, the next largest grouping on the tree of life. Among the least affected groups were the jaw-less fish (pictured in this artist’s impression) reef-building ‘rugose’ corals and the trilobites – iconic marine arthropods resembling wooden hatches
‘The overarching message of our study is that life on earth does not exist in isolation,’ concluded Professor Fields.
“We are citizens of a larger cosmos, and the cosmos intervenes in our lives – often imperceptibly, but sometimes cruelly.”
The full findings of the study were published in the journal Proceedings of the National Academy of Sciences.
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