Scientists have discovered radioactive debris from relatively nearby stars that exploded a few million years ago, raising questions about whether cosmic rays released by the supernovae impacted Earth's past climate.
The evidence comes from samples taken from the floors of the Pacific, Atlantic and Indian oceans. All contained a radioactive isotope of iron, known as iron-60, which is produced in the cores large stars and in supernovae, which are exploding stars.
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The particles were transported via interstellar dust grains to Earth between 1.7 million and 3.2 million years ago, according to a paper published in this week's Nature.
A second analysis, also published in Nature, traces the iron-60 to two explosions, the first occurring about 2.3 million years ago and the second 1.5 million years ago.
The exploded stars, which were roughly 9.2 and 8.8 times bigger than the sun, respectively, were about 300 light-years away at the time -- close enough to be visible during the day.
"We have shown that the iron-60 must come from outer space, it cannot originate from the solar system," lead researcher Anton Wallner, with the Australian National University's Department of Nuclear Physics, wrote in an email to Discovery News.
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Wallner and colleagues also were able to ferret out when the iron-60 was deposited, indicating that there was more than one nearby supernova explosion.
"It's not a single event in the last 10 million years or so, it's rather a series of supernovae," Wallner said.
The timing of the supernova explosions coincides with a period of time when Earth cooled, shifting from what is known as the Pliocene into the Pleistocene periods.
"We have now a consistent and coherent picture of what happened around the solar system in the last 20 million years and we know how close these supernovae were. We can now proceed to find out if there might have been any biological effects," astronomer Deiter Breitschwerdt, with the Berlin Institute of Technology, wrote in an email to Discovery News.
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The supernova explosions would have generated cosmic rays, which although unlikely were powerful enough to kill off life on Earth, might have triggered increased cloud cover, lightning and other climate changes.
"We do not know if there is a link between supernova activity and colder temperature" – a variation that may have been one of the conditions that led to human evolution, University of Kansas astronomer and physicist Adrain Melott wrote in an related essay in Nature.
"The new studies will open up ... deeper insight into what might have happened on Earth over the past 10 million years as a result of nearby stellar fireworks," Melott said.