Cancers in animals could have increased slightly after our planet was blanketed with radiation from two prehistoric supernovae, according to new research.
The star explosions took place between 1.7 million and 8.7 million years ago, irradiating the landscape with high-energy cosmic rays. Cosmic rays hit Earth every day, but the stellar explosions would have increased the radiation striking our planet.
When the cosmic rays hit molecules in our atmosphere, a cascade of secondary particles such as X-rays, protons and muons are produced. Some muon radiation is normal, but the supernovae would have temporarily tripled the dose for land animals and animals living in shallow waters, said co-author Adrian Melott, a University of Kansas physics researcher, in a statement.
"I was expecting there to be very little effect at all," he said. "The supernovae were pretty far way — more than 300 light years — that's really not very close."
The radiation would have given animals the equivalent dose of one additional CT scan in radiation per year. CT scans take X-ray images from several angles to let doctors look inside tumors and other objects. The night sky also would have bathed in blue light from the explosions, making it difficult for animals to sleep.
Some researchers have speculated that there could be a link between increased cosmic rays and a cooler climate on Earth, although that link isn't proven for sure. (Simply put, the theory associates cosmic rays with aerosols, which could produce clouds, which in turn reduces the amount of solar radiation on the surface.) If a connection could be made, Melott said it is possible the supernovae were associated with a known, smaller mass extinction 2.59 million years ago when Earth went through repeated ice ages.
"There was climate change around this time," Melott said. "Africa dried out, and a lot of the forest turned into savannah. Around this time and afterwards, we started having glaciations — ice ages — over and over again, and it's not clear why that started to happen. It's controversial, but maybe cosmic rays had something to do with it."
The research is available on preprint website arXiv and has been accepted in Astrophysical Journal Letters. It was led by Brian Thomas, a physicist and astronomer at Washburn University in Kansas.
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