Exploding stars left recent, radioactive mark on Earth

[nitpicker]

http://www.bbc.com/news/science-environment-35976498

Exploding stars left recent, radioactive mark on Earth

By Jonathan Webb
Science reporter, BBC News
6 April 2016

Two new studies confirm that multiple exploding stars, called supernovae, have showered the Earth with radiation within the last few million years. One study reports traces of radioactive iron-60, a strong indicator of supernova debris, found buried in the sea floor right across the globe. A second paper models which specific supernovae are most likely to have splattered this isotope across our historic, galactic neighbourhood. Both appear in the journal Nature.

The periods of bombardment highlighted by the two teams do not coincide with any mass extinction events - and indeed, the predicted locations of the culprit supernovae are not quite close enough to unleash that level of destruction. But the blasts may nonetheless have affected the Earth's climate and thus, the evolution of life.

Importantly, the two sets of results are entirely consistent, according to Dieter Breitschwerdt from the Berlin Institute of Technology, Germany, who led the modelling research. His team has spent years studying the "local bubble": a ballooning region of hot gas, 600 light-years across, that surrounds the Solar System and dominates our stellar neighbourhood.

It was formed, Prof Breitschwerdt and his colleagues have found, by upwards of a dozen supernovae all blowing up within a nearby, moving clump of stars. Their new paper pinpoints those explosions. "We now can make a table of the stars - what mass they had, when they exploded, and where they were," he told the BBC News website.

Specifically, his team calculated how much iron-60 those supernovae would have sprayed into space - and how much the Earth could have swept up, based on the Solar System's path as it orbits around the Milky Way. The tiny quantities of this isotope found in the Earth's crust - first detected in samples from the bottom of the Pacific Ocean in 1999 - show a peak at about two million years ago. So, do the closest explosions in Prof Breitschwerdt's table match that peak? The short answer is yes. The nearest blast in the simulation took place 2.3 million years ago, and the second-nearest 1.5 million years ago. That is quite a spread - but a prolonged, recent scattering of iron-60 is precisely what the other Nature paper reports, based on atom-counting measurements from 120 sea-bed samples spanning the Indian, Pacific and Atlantic Oceans.
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