Did a nearby supernova cause one of Earth’s mass extinctions?
About 2.6 million years ago, an oddly bright light arrived in the prehistoric sky and lingered there for weeks or months. It was a supernova some 150 light-years away from Earth. A few hundred years later, after the new star had long since faded from the sky, cosmic rays from the event finally reached Earth, slamming into our planet. Now, a group of researchers led by Adrian Melott at the University of Kansas believes this cosmic onslaught is linked to a mass extinction of ocean animals roaming Earth’s waters at the time.
The killer radiation came in the form of cosmic rays made up of muons, which are a few hundred times the mass of an electron.
Muons are elementary particles with a half-life of a small fraction of a second. Because they move at near-light velocity and are produced mostly by collisions in Earth’s atmosphere, they could, in principle, be devastating to life on our planet. Melott used modeling to explain that muons would penetrate deep into the oceans, and would increase the lifetime risk of developing cancer in large animals by 16 to 40 percent. A supernova, they suggested, could thus be responsible for a mass extinction 2.6 million years ago that wiped out 36 percent of the genera of large marine animals.
The other piece of the puzzle was pinpointing the event that could have caused that wave of radiation. Iron-60 is a radioactive isotope of iron with a half-life of about 2.6 million years which means that any iron-60 that formed with Earth is now long gone. Thus, the only way we could still find iron-60 today is if it arrived via cosmic means, such as in the wave from a supernova. And there’s a huge spike of iron-60 that was deposited about 2.6 million years ago, indicating the material from a supernova event reached us then.
In spite of the new study by Melott and colleagues, the extreme climatic changes associated with the advance and retreat of glaciers explanation still seems to make the most sense.
In a 2016 paper by Dieter Breitschwerdt from the Technical University Berlin and colleagues, two nearby supernova explosions were identified based on iron isotope signatures in the geologic record.The first one 2.2 million years ago and another 1.5 million years ago. There were no major extinction events at these times.
Even if the times did match up the supernova radiation signal would have had less effect on life than the climatic shifts going on in that tumultuous period of Earth’s history. Furthermore, an increase in cancer rate alone would not trigger a mass extinction. Most of the cancers would arise late in life, after animals would have reproduced already. Higher radiation dosages could have altered evolutionary trajectories due to higher mutation rates, but it’s difficult to see a link to extinction.
https://www.liebertpub.com/doi/10.1089/ast.2018.1902
https://www.nature.com/articles/nature17424
https://en.wikipedia.org/wiki/Ice_age
Phedias Hadjicharalambous.
Cyprus Astronomy Organisation.
