Supernovae left radioactive debris on Earth 3 million years ago

The explosive death of stars have always been of interest to scientists. The chemical reactions and emissions from this phenomena usually holds clues to the very formation of the universe itself. Considering that these stars are usually thousands, if not millions, of light years away, by the time we do "see" a supernova, it means it really happened ages ago. So when scientists discover traces of radioactive debris from supernovae still lingering on earth, they are understandably pleasantly surprised and excited.

Dr. Anton Wallner from the Australian National University was intrigued by the discovery made by a group at the Technical University of Munich (TU Munich) of traces of iron-60 from the Pacific Ocean floor. Iron-60 has a half-life of 2.6 million years, so those that come from space during the Earth's formative years four million years ago have all disappeared by now. Gathering an international team of scientists, Wallner discovered that iron-60 samples from the Pacific, Atlantic, and Indian oceans date somewhere between 3.2 to 1.7 million years ago.

The scientist then theorize that a couple of supernovae were so close to the Earth so as to deposit these radioactive debris when they exploded. How close? About 300 light years, close enough to be as bright as the moon, in broad daylight even. What's more, this all probably happened when the Earth itself was starting to cool down, preserving the debris for scientists to find millions of years later.

Luckily, even at that close a distance, the radiation was too weak to have any adverse effects on the development of organic life, causing no mutations or mass extinctions. That much radioactive leftovers also implied that it was a cluster of supernovae that exploded back then. That cluster has also luckily moved away from the earth since then and have no more large stars left to pose a danger of another supernova radiation bombardment.