Earth was pummeled by fallout from multiple supernova explosions as recently as a few million years ago, at a time when pre-humans like Australopithecus were still alive. Two research groups, one led by Dieter Breitschwerdt from the Berlin Institute of Technology and another by Anton Wallner from the Australian National University, based their conclusions on an enhanced iron-60 signature found in deep ocean sediments. Iron-60 is a specific isotope of iron, which is produced abundantly in many different types of supernovas. The results appear in this week’s issue of Nature.
According to Breitschwerdt the explosions occurred within the Local Bubble of hot, diffuse plasma in which our Solar System is embedded. The sources were located about 100 parsecs away (about 325 light years) at the time. Wallner’s team calculated that there were two distinct iron-60 fluxes associated with the supernova explosions, one 1.5 to 3.2 million years ago and the other 6.5 to 8.7 million years ago. The Breitschwerdt team used modeling to further distinguish between two explosions within the younger event, one 1.5 million years ago and one 2.3 million years ago. The exploding stars originally had 9.2 and 8.8 times our sun’s mass, respectively.
It’s tempting to speculate what effects these supernova explosions might have had on our planet. In 1954 Otto Schindewolf linked nearby supernova explosions to past mass extinctions, particularly to the Permian extinction, the most catastrophic event in the last 500 million years, during which up to 90 percent of all species went extinct. It was the only such event that also included a mass extinction of insects. The Late Cretaceous extinction 65 million years ago, which ended the age of the dinosaurs, was caused by the impact of a large asteroid, but there have also been less severe extinction events in Earth’s history for which we do not know the cause.
The supernovas investigated by Breitschwerdt and Wallner were much too far away for their radiation to kill life on Earth; the so-called “kill distance” for that to have occurred is about eight parsecs, or 26 light years. However, they might have contributed to the colder temperatures and series of glaciations that started about 2.6 million years ago and ended only very recently. If so, they may have indirectly affected human evolution.