Supernovae bombarded Earth with cosmic rays

The remnants of a supernova that exploded 300 lightyears from Earth have been discovered on the floors of our oceans, according to a new study.

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Led researcher Anton Waller at ANU’s Heavy Ion Accelerator, which was able to isolate the heavy isotopes that show two supernovae bombarded Earth with radioactive particles.
Credit: Stuart Hay, ANU

Two supernova explosions showered Earth with radioactive debris that may have been deposited on our ocean floors, according to two new research papers.

Scientists exploring the phenomenon found radioactive iron-60 isotopes in samples taken from the Pacific, Atlantic and Indian Oceans, linking their presence to two supernovae that exploded about 325 lightyears from Earth.

These supernovae are thought to have occurred as two separate events: one 1.7 to 3.2 million years ago and the other 6.5 to 8.7 million years ago.

Supernovae are explosions that occur when massive stars reach the end of their lives. In this case, researchers say the explosions would have been close enough to Earth to be visible during the day and would have been comparable to the brightness of the Moon.

Coincidentally, the supernovae occurred just as Earth was cooling and moving from the Pliocene into the Pleistocene period. Some theories suggest cosmic rays from the explosions may have increased cloud cover on Earth.

Despite this, the supernovae were still too far from our planet to have caused biological damage or species extinction through radiation.

Supernova explosions generate heavy elements and radioactive isotopes, one of which is iron-60. This isotope decays with a half-life of 2.6 million years, so scientists can infer that any traces left over from the formation of Earth would have disappeared a long time ago.

The researchers were able to detect miniscule quantities of iron-60 on Earth's seabeds and link it to past supernova explosions.

"Iron-60 from space is a million-billion times less abundant than the iron that exists naturally on Earth," says research leader Dr Anton Wallner from the Australian National University (ANU).

Dr Wallner began researching the phenomenon following the discovery of traces of iron-60 in samples from the Pacific Ocean floor a decade ago by scientists at TU Munich. He put together a group of scientists to look for interstellar dust from 120 samples taken from ocean floors. The Heavy-Ion Accelerator at ANU was then used to separate traces of iron-60 from isotopes found naturally on Earth, revealing it is found right across the planet.

Dating techniques concluded that the fallout occurred in two time periods: 3.2 to 1.7 million years ago and eight million years ago.

The effects of these supernovae on the development of life on Earth is as yet unclear. Astrophysicist Adrian Melott of the University of Kansas says: “Our local research group is working on figuring out what the effects were likely to have been. We really don’t know. The events weren’t close enough to cause a big mass extinction or severe effects, but not so far away that we can ignore them either. We’re trying to decide if we should expect to have seen any effects on the ground on the Earth.”


Supernova remnant Cassiopeia A. Did supernova explosions like this one bombard Earth with radioactive debris?
Credit: NASA/JPL-Caltech/STScl/CXC/SAO
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