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Space + FlightSpace & Flight

A String of Supernovae May Have Caused Earth’s Recent Ice Ages

ByConor GearinNOVA NextNOVA Next

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You may have thought Earth was safe from supernovae, the giant explosions from dying stars. Astronomers are now saying we should think again.

Two teams of scientists reached the same surprising conclusion: a cluster of supernova blasts rained radioactive iron on the Earth. One of them blew up as recently as 0.8 million years ago.

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The road to this discovery began in 1999, when scientists took samples from the ocean’s crust, in a layer of rock that formed 2.2 million years ago. A radioactive isotope called iron-60 turned up in the samples. It’s an unstable atom with a short half-life. That means it’s a much younger atom than earthly elements. Had it been around when the planet took shape, it would have degraded long ago. Instead, it must have landed in the crust from outer space.

Supernova illustrationUSE
An illustration of the radioactive iron cloud from one of the supernovae.

Until recently, astronomers thought that this iron came from just one nearby supernova from an unknown direction. But the two new studies suggest that, in fact, a string of supernovae went off over the past 10 million years—just a blink of an eye in galactic terms. And two of them were near enough to hit Earth with radioactive particles.

Here’s Daniel Clery, reporting for Science Magazine:

Their analysis suggests that two of those supernovae were close enough and recent enough to have contributed to iron-60 on Earth: the first 2.3 million years ago, the second 0.8 million years later; both about 300 light-years from Earth.

“We may never be able to identify [the individual stars],” comments Neil Gehrels, an astrophysicist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, “but we can see the regions of intense star formation” where they lived and died.

These supernovae weren’t close enough to incinerate life on Earth. But all that iron in the air may have changed the planet’s climate. The timing of the explosions happens to correspond to the transition into the Pleistocene epoch , the icy glacial period dominated by mastodons, mammoths, and saber-toothed cats. It’s possible that the radioactive particles in the atmosphere could have seeded clouds—thereby increasing cloud cover—and contributed to the ice ages of the Pleistocene.

To sort out exactly where in the galaxy the explosions came from, it might take new missions to the moon. Since Earth’s moon has no atmosphere, particles from supernovae could still be positioned exactly as they were when they first fell. Astrophysicists could use iron-60 samples from the moon to point the way to their source.