An international team of scientists has found evidence of a series of massive supernova explosions near our solar system, which showered the Earth with radioactive debris. The scientists found radioactive iron-60 in sediment and crust samples taken from the Pacific, Atlantic and Indian Oceans.
Some theories suggest cosmic rays from the supernova could have increased cloud cover. The scientists believe the supernova in this case were less than 300 light years away; close enough to be visible during the day and comparable to the brightness of the Moon.
The supernova explosions create many heavy elements and radioactive isotopes which are strewn into the cosmic neighborhood. 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.
Any iron-60 dating from the Earth’s formation more than four billion years ago has long since disappeared. The iron-60 atoms reached Earth in minuscule quantities and so the team needed extremely sensitive techniques to identify the interstellar iron atoms.
The team from Australia, the University of Vienna in Austria, Hebrew University in Israel, Shimizu Corporation and University of Tokyo, Nihon University and University of Tsukuba in Japan, Senckenberg Collections of Natural History Dresden and Helmholtz-Zentrum Dresden-Rossendorf (HZDR) in Germany, also found evidence of iron-60 from an older supernova around eight million years ago, coinciding with global faunal changes in the late Miocene.
The iron-60 was concentrated in a period between 3.2 and 1.7 million years ago, which is relatively recent in astronomical terms, said research leader Dr Anton Wallner from The Australian National University (ANU).
“We were very surprised that there was debris clearly spread across 1.5 million years,” said Dr Wallner, a nuclear physicist in the ANU Research School of Physics and Engineering. “It suggests there were a series of supernova, one after another. “It’s an interesting coincidence that they correspond with when the Earth cooled and moved from the Pliocene into the Pleistocene period.”
The dating showed the fallout had only occurred in two time periods, 3.2 to 1.7 million years ago and eight million years ago. Current results from TU Munich are in line with these findings.
A possible source of the supernova is an ageing star cluster, which has since moved away from Earth, independent work led by TU Berlin has proposed in a parallel publication. The cluster has no large stars left, suggesting they have already exploded as supernova, throwing out waves of debris.
