Two international research teams have found evidence of a series of ‘near-Earth’ supernova explosions, which showered our planet with radioactive debris. Both teams have published their findings in separate papers in the journal Nature.

One of the teams, led by Dr. Anton Wallner of the Australian National University, proves the existence of ancient seabed deposits of iron-60 isotopes, tracing their source to supernovae occurring about 325 light-years from Earth.

The scientists also estimate explosion times of these supernovae, isolating two events: one 1.7 to 3.2 million years ago, and the other around 8 million years ago.

They found radioactive iron-60 in sediment and crust samples taken from the Pacific, Atlantic and Indian Oceans.

“The iron-60 was concentrated in a period between 3.2 and 1.7 million years ago,” Dr. Wallner said.

“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.”

“It’s an interesting coincidence that they correspond with when the Earth cooled and moved from the Pliocene into the Pleistocene period.”

Dr. Wallner and co-authors also found evidence of iron-60 from an older supernova around 8 million years ago, coinciding with global faunal changes in the late Miocene.

The second team, headed by Dr. Deiter Breitschwerdt of the Berlin Institute of Technology, identified a likely source of the ancient supernovae.

“A possible source is an ageing star cluster, which has since moved away from Earth,” the scientists said.

“The cluster has no large stars left, suggesting they have already exploded as supernovae, throwing out waves of debris.”

“This research essentially proves that certain events happened in the not-too-distant past,” said Prof. Adrian Melott of the University of Kansas, who was not involved in the research.

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A. Wallner et al. 2016. Recent near-Earth supernovae probed by global deposition of interstellar radioactive 60Fe. Nature 532, 69-72; doi: 10.1038/nature17196

D. Breitschwerdt et al. 2016. The locations of recent supernovae near the Sun from modelling 60Fe transport. Nature 532, 73-76; doi: 10.1038/nature17424