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A Series of Supernova Explosions Showered Earth with Radioactive Debris 2 to 3 Million Years Ago

04/08/2016 01:14PM

A Series of Supernova Explosions Showered Earth with Radioactive Debris 2 to 3 Million Years Ago
An international team of scientists has found evidence of a series of massive supernova explosions near our solar system, which showered the Earth with radioactive debris. The scientists 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, which is relatively recent in astronomical terms, said research leader Dr Anton Wallner a nuclear physicist in the Australian National University (ANU) Research School of Physics and Engineering in Canberra, Australia.

"We were very surprised that there was debris clearly spread across 1.5 million years," said Dr Wallner. "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 team from leading universities in Australia, Austria, Israel, Japan, and Germany, also found evidence of iron-60 from an older supernova around 8 million years ago, coinciding with global changes in the late Miocene.

A supernova is a massive explosion of a star as it runs out of fuel and collapses. The scientists believe the supernovae in this case were less than 300 light years away, close enough to be visible during the day and comparable to the brightness of the Moon.

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. The supernova explosions create many heavy elements and radioactive isotopes which are strewn into the cosmic neighborhood.

One of these isotopes is iron-60 which decays with a half-life of 2.6 million years, unlike its stable cousin iron-56. Any iron-60 dating from the Earth's formation more than four billion years ago has long since disappeared.

The iron-60 atoms reached Earth in minuscule quantities and so the team needed extremely sensitive techniques to identify the interstellar iron atoms. "Iron-60 from space is a million-billion times less abundant than the iron that exists naturally on Earth," said Dr Wallner.

Dr Wallner was intrigued by first hints of iron-60 in samples from the Pacific Ocean floor found a decade ago by a group at the Technische Universitat in Munich. 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. This time consuming task was performed by two groups, at Helmholtz-Zentrum Dresden-Rossendorf (HZDR) in Germany and the University of Tokyo.

The team then separated the tiny traces of interstellar iron-60 from the other terrestrial isotopes using the Heavy Ion Accelerator at ANU and found it occurred all over the globe.

The age of the cores was determined from the decay of other radioactive isotopes, beryllium-10 and aluminium-26, using accelerator mass spectrometry facilities at HZDR, the University of Tokyo and the University of Vienna.

The dating showed the fallout had only occurred in two time periods, 3.2 to 1.7 million years ago and 8 million years ago.

A possible source of the supernovae is an ageing star cluster, which has since moved away from Earth, independent work led by the Technische Universitat in Berlin. The cluster has no large stars left, suggesting they have already exploded as supernovae, throwing out waves of debris.


Preview of a Forthcoming Supernova

At the turn of the 19th century, the binary star system Eta Carinae was faint and undistinguished. In the first decades of the century, it became brighter and brighter, until, by April 1843, it was the second brightest star in the sky, outshone only by Sirius (which is almost a thousand times closer to Earth). In the years that followed, it gradually dimmed again and by the 20th century was totally invisible to the naked eye.

The star has continued to vary in brightness ever since, and while it is once again visible to the naked eye on a dark night, it has never again come close to its peak of 1843.

The larger of the two stars in the Eta Carinae system is a huge and unstable star that is nearing the end of its life, and the event that the 19th century astronomers observed was a stellar near-death experience. Scientists call these outbursts supernova impostor events, because they appear similar to supernovae but stop just short of destroying their star.

Although 19th century astronomers did not have telescopes powerful enough to see the 1843 outburst in detail, its effects can be studied today. The huge clouds of matter thrown out a century and a half ago, known as the Homunculus Nebula, have been a regular target for Hubble since its launch in 1990.

Eta Carinae is not only interesting because of its past, but also because of its future. It is one of the closest stars to Earth that is likely to explode in a supernova in the relatively near future (though in astronomical timescales the "near future" could still be a million years away). When it does, expect an impressive view from Earth, far brighter still than its last outburst.


For more information:

http://www.anu.edu.au/news/all-news/supernovae-showered-earth-with-radioactive-debris

http://www.nasa.gov/mission_pages/hubble/science/eta-carinae.html

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