Supernova explosions once battered the Earth, changing how life developed

Multiple supernovae explosions that occurred in the vicinity of our solar system showered the Earth with radioactive debris, an international team led by scientists at the Australian National University (ANU) reported Wednesday in the journal Nature.

In the study, ANU nuclear physicist Dr. Anton Wallner and his colleagues explained that they had discovered iron-60 isotopes in sediment and crust samples taken from the Atlantic, Pacific, and Indian Oceans. Furthermore, they said that the radioactive material had been concentrated between 3.2 and 1.7 million years ago – relatively recently, astronomically speaking.

“We were very surprised that there was debris clearly spread across 1.5 million years. It suggests there were a series of supernovae, one after another,” Dr. Wallner explained in a statement. “It’s an interesting coincidence that they correspond with when the Earth cooled and moved from the Pliocene into the Pleistocene period.”

He and his co-authors also found evidence of iron-60 from a supernova that had taken place as much as eight million years ago and coincided with global changes to fauna that occurred during the late Miocene. The findings imply that some of these isotopes were gathered by dust grains and then deposited on Earth, and indicate that multiple supernovae and other massive star events took place during the last 10 million years at distances of less than 300 light years.

Source was likely a star cluster that has since drifted away

If their research is correct, those supernovae would have been close enough to have been seen during the day and would have appeared about as bright as the Moon. The planet’s surface would have been bombarded by cosmic rays, they noted, but the radiation would not have been strong enough to have caused mass extinctions, or any direct biological damage, for that matter.

However, the explosions would have generated heavy elements and radioactive isotopes such as iron-60, which has a half-life of 2.6 million years. These elements and isotopes are spread across the surrounding cosmos, and if any iron-60 did make it to the Earth during the planet’s formation some four billion years ago, it would have disappeared a long time ago, the study authors said.

To find it, Dr. Wallner’s team needed to use extremely sensitive techniques designed to locate and identify atoms of the interstellar material, which he explained is “a million-billion times less abundant than the iron that exists naturally on Earth.” Nonetheless, the researchers collected 120 samples from the three ocean floors, and began by extracting all of the iron from those samples. Next, they separated the interstellar iron-60 from all other isotopes of the element.

They found that the interstellar isotopes were found in all of the samples, and determined the cores to have come from two different periods: one between 3.2 and 1.7 million years ago, and another about eight million years ago. They believe that the source of the iron-60 was an aging star cluster that has since drifted away from our solar system – a cluster which no longer contains any massive stars, as they all likely already exploded as supernovae.

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Image credit: Greg Stewart, SLAC National Accelerator Lab

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