Effects of radioactive supernovae debris 1.7 million years ago researched
Earth was showered by radioactive debris from supernovae - exploding stars - as recently as 1.7 million years ago, research has shown.
A series of stellar blasts are thought to have occurred around 300 light years away, close enough to have been visible in daylight.
Fortunately they were well beyond the "kill zone" of 30 light years, within which a supernova could wipe out life on Earth.
But scientists believe they may still have had an impact on the planet and its climate, and may even have influenced human evolution.
Evidence of massive supernova explosions was found in the form of radioactive iron in sediment and crust samples taken from the Pacific, Atlantic and Indian oceans.
The iron-60, an isotope or atomic "strain" of iron not found naturally on Earth, was deposited in material flung through space by the supernovae, the scientists said.
Careful dating revealed two "fall-out" events, one spanning a period between 3.2 to 1.7 million years ago, and the other around eight million years ago.
The likely source was an ageing star cluster which has since moved away from our corner of the galaxy.
A supernova occurs when a massive star runs out of fuel at the end of its life and explodes.
Tellingly, the cluster has no large stars left, suggesting they have already exploded.
Dr Anton Wallner, from the Australian National University (ANU), whose team investigated the earlier event, said: "We were very surprised that there was debris clearly spread across 1.5 million years.
"It suggests there were a series of supernovae, one after another."
Seen from Earth, the supernovae would have been about as bright as the moon, said the scientists whose findings are reported in the journal Nature.
Supernovae pour out energetic particles, x-rays and gamma rays that could potentially strip the Earth of its protective ozone layer and trigger mass extinctions.
In this case, they were too far away to cause such destruction.
Astrophysicist Dr Adrian Melott, who wrote an accompanying paper in Nature entitled Supernovae in the Neighbourhood, said: "Our local research group is working on figuring out what the effects were likely to have been.
"We really don't know.
"The events weren't close enough to cause a big mass extinction or severe effects, but not so far away that we can ignore them either."
It is possible cosmic rays from the supernovae could have increased the Earth's cloud cover and altered its climate, the scientists believe.