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Supernovae Showered Earth with Radioactive Debris

A series of massive supernova near our solar system showered the Earth with radioactive debris, according to evidence derived from radioactive iron-60 found in sediment and crust samples taken from the Pacific, Atlantic and Indian oceans.

The iron-60 was concentrated in a period 1.7–3.2 million years ago, which is relatively recent in astronomical terms, said research leader Dr Anton Wallner of The Australian National University. “We were very surprised that there was debris clearly spread across 1.5 million years,” Wallner said. “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.”

The international research team 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 research has been published in Nature (http://tinyurl.com/hwukuxs).

Supernovae eject heavy elements and radioactive isotopes into the cosmos. One of these isotopes is iron-60, which decays with a half-life of 2.6 million years. Therefore any iron-60 dating from the Earth’s formation more than 4.6 billion years ago has long since disappeared.

Wallner was intrigued by the first hints of iron-60 in samples from the Pacific Ocean floor a decade ago, so he assembled an international team to search for interstellar dust from 120 ocean-floor samples spanning the past 11 million years.

The first step was to extract all the iron from the ocean cores and then separate the tiny traces of interstellar iron-60 from the other terrestrial isotopes using the Heavy-Ion Accelerator at the ANU. The team found that it occurred all over the globe.

The age of the cores was determined from the decay of beryllium-10 and aluminium-26 radioactive isotopes. The dating showed that the fallout had only occurred in two time periods, 1.7–3.2 million years ago and 8 million years ago.

A possible source of the supernovae is an ageing star cluster that has since moved away from Earth. The cluster has no large stars left, suggesting they have already exploded as supernovae and thrown out waves of debris.