Science & Environment

Exploding stars left recent, radioactive mark on Earth

supernova illustration Image copyright Greg Stewart, SLAC National Accelerator Lab
Image caption A supernova is the giant explosion that ensues when a star runs out of fuel

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.

Image copyright Michael Schulreich
Image caption Modelling the distribution of iron-60 in the local bubble; "X" marks the Solar System

"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.

Image caption The famous Crab Nebula is formed by the expanding debris from a supernova

Together, these new samples cover 11 million years of Earth's geological history - and they reveal an increased smattering of iron-60 between 1.5 and 3.2 million years ago.

"We were very surprised that there was debris clearly spread across 1.5 million years," said that study's lead author Anton Wallner, a nuclear physicist at the Australian National University in Canberra.

"It suggests there were a series of supernovae, one after another."

Coincidental cooling?

Dr Wallner and his colleagues also detect a spike in iron-60 a little earlier, between 6.5 and 8.7 million years ago - but it is the more recent deposit that aligns remarkably well with Prof Breitschwerdt's simulation.

Even the teams' predicted distances match: the simulations and the ocean-floor data both place the recent explosions 300 light-years away, or less.

The timing of the supernovae also has some fascinating implications: "It's an interesting coincidence that they correspond with when the Earth cooled and moved from the Pliocene into the Pleistocene period," Dr Wallner adds, referring to the epoch of regular ice ages that took hold some 2.5 million years ago.

The idea that nearby stellar explosions could have triggered key transitions in our planet's natural history is not a new one. But it has found little scientific support over the years.

Image copyright Science Photo Library
Image caption The supernovae would have been close enough to light up the sky as much as the Moon

Adrian Melott from the University of Kansas, US, specialises in studying those possible effects. He was not involved in either of the new studies but wrote a commentary for Nature on their implications.

"The events [described in the new findings] weren't close enough to cause a big mass extinction or severe effects, but not so far away that we can ignore them either," he said.

Prof Melott's own research team will be following up on the results, he added - particularly the new, precise estimates for when the two nearest supernovae flung their debris in our direction.

"We're trying to decide if we should expect to have seen any effects on the ground on the Earth."

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