Did Radioactive Debris Shower the Earth?

A super nove.

An artist’s impression of a supernova. (Image: Greg Stewart via SLAC, National Accelerator Lab)

Was the Earth showered with radioactive debris from a series of massive supernova explosions between 3.2 and 1.7 million years ago? Scientists say the evidence suggests the answer is yes!

The team of scientists believes the evidence they have discovered proves that a series of massive supernova explosions near our solar system would have rained radioactive debris down on Earth.

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Deep within the Pacific, Atlantic, and Indian Oceans, scientists have found radioactive iron-60 in sediment and crust samples. What is surprising is that the iron-60 was concentrated between 3.2 and 1.7 million years ago, which in astronomical terms is relatively recent.

Research leader Dr. Anton Wallner, a nuclear physicist at the ANU Research School of Physics and Engineering, said in a statement:

“We were stunned that there was debris spread across 1.5 million years. It suggests there were a series of supernovae, one after another. It’s an interesting coincidence that it corresponded with when the Earth cooled and moved from the Pliocene into the Pleistocene period.”

The international team also found evidence from an older supernova (a massive star explosion as it runs out of fuel and collapses) around eight million years ago that overlaps with global faunal changes in the late Miocene. Some theories suggest the cosmic rays from the supernovae may have increased cloud cover.

Supernova explosions create radioactive debris.
A false-color image of Cassiopeia, a supernova remnant. (Image: JPL-Caltech via NASA)

The scientists believe the supernovae, in this case, would have been less than 300 light-years away, close enough to be visible during the day and comparable to the brightness of the moon. According to Australian National University, 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.

Supernova explosions create radioactive debris

Supernova explosions create various heavy elements and radioactive isotopes that are then thrown into the cosmic neighborhood. One of these isotopes is iron-60, which decays with a half-life of 2.6 million years.

So any iron-60 from Earth’s formation that was more than four billion years ago would have long since decayed. Furthermore, the iron-60 atoms that did reach Earth from these more recent events were in minuscule quantities; therefore, extremely sensitive techniques were needed to identify the interstellar iron atoms.

Dr. Wallner explains: “Iron-60 from space is a million-billion times less abundant than “the iron that exists naturally on Earth.”

When Dr. Wallner heard that a team from TU Munich had found hints of iron-60 in samples from the Pacific Ocean floor, he became intrigued. He then pulled together an international coalition to search for the interstellar dust from 120 ocean-floor samples spanning over the past 11 million years.

Anton Wallner at ANU’s Heavy Ion Accelerator. (Image: Stewart Hey via ANU)

The first step consisted of extracting all of the iron from the ocean cores. Then using the Heavy-Ion Accelerator at ANU, the team separated all the tiny traces of interstellar iron-60 from the other terrestrial isotopes. The scientists discovered that iron-60 occurred all over the globe.

To determine the core’s age, the team measured the decay of other radioactive isotopes in the radioactive debris (beryllium-10 and aluminum-26) by using accelerator mass spectrometry (AMS) facilities. The dating indicated that the fallout had only occurred during two time periods — 3.2 to 1.7 million years ago and eight million years ago.

The team has two possible theories of the source of the supernovae; an aging star cluster that has since moved away from Earth, or the cluster has no large stars left, suggesting they have already exploded as supernovae, throwing out waves of radioactive debris.

The research has been published in Nature.

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