April 7, 2016 – An international team of scientists showed evidence of a series of massive supernova explosions near our solar system, which showered the Earth with radioactive debris. The research paper was published in the journal Nature.

“Through data collected around the globe and analyzed in various nuclear physics labs  we found traces of an isotope of iron that is the fingerprint to series of supernovae exploding about 2-3 million years ago,” said Professor Michael Paul from the Racah Institute of Physics at The Hebrew University of Jerusalem, a member of the research group.

The team also includes scientists from Australian National University (ANU), Austria’s University of Vienna, Japan’s Shimizu Corporation and Nihon University and Germany’s Helmholtz-Zentrum Dresden-Rossendorf (HZDR).

The scientists used extremely sensitive techniques to identify interstellar iron atoms in deep-sea sediment and crust samples taken from the Pacific, Atlantic and Indian Oceans. They found radioactive iron-60 that decays with a half-life of 2.6 million years, and was concentrated in a period between 3.2 and 1.7 million years ago.

They also found evidence of iron-60 from an older supernova, around 8 million years ago, which is relatively recent in astronomical terms.

iStock_000018487816_MediumA supernova is a massive explosion of a star after it runs out of nuclear fuel and collapses, in the process creating many heavy elements and radioactive isotopes, which are strewn out into the cosmic neighborhood. Data indicates that the debris spread across 1.5 million years, suggesting that there was a series of supernovae, one after another.

Dr. Anton Wallner from ANU, Professor Michael Paul from Hebrew University and collaborators followed up first hints of iron-60 in samples from the Pacific Ocean floor, found a decade ago by a research group at Technical University Munich.

Using the Heavy-Ion Accelerator at ANU, they found tiny traces of iron-60 all over the globe. Using lab data in Austria, Germany and Japan, they found that the fallout had occurred in two time periods, 3.2 to 1.7 million years ago and 8 million years ago.

The scientists believe the supernovae were less than 300 light years away, close enough to be visible during the day and comparable in brightness to the moon.

A possible source of the supernovae is an aging star cluster, which has since moved away from Earth. The cluster has no large stars left, suggesting they have already exploded as supernovae, throwing out waves of debris. This scenario is consistent with the flow direction of interstellar medium material towards Earth.

Although Earth would have been exposed to an increased cosmic ray bombardment, the radiation would have been too weak to cause direct biological damage or trigger mass extinctions.

Some theories suggest that cosmic rays emanating from supernovae could have increased cloud cover, which had an impact on the Earth’s climate.