Radioactive Isotopes in The Oceans May Be Remnants of Ancient Supernovae

Cosmic dust may have altered life on Earth as we know it

supernova remnants
The remnants of a supernova. NASA/JPL-Caltech

For years, scientists have puzzled over where a radioactive iron isotope discovered deep beneath the ocean floor may have come from. Now, two studies published this week in the journal Nature suggest that the source of radioactive material may be two nearby supernovae that detonated millions of years ago. Not only does this shed new light on the history of our cosmic neighborhood, but scientists suggest that these events may have affected the development of life on Earth.

In 1999, the discovery of large amounts of the radioactive isotope iron-60 embedded in the ocean floor left scientists scratching their heads. This was surprising, as the only known sources of iron-60 in the universe are supernovae, Nsikan Akpan reports for the PBS Newshour. It can’t come from comets or asteroid impacts.

"All the iron-60 we find here must come from outer space," astrophysicist and study author Dieter Breitschwerdt tells Loren Grush for The Verge.

Not only that, but iron-60 has a half-life of about 2.6 million years—that’s the amount of time that it takes for half of the radioactive material in a sample to become inert. Since Earth is about 4.5 billion years old, any iron-60 that might have been around in the early days of the solar system should have disappeared by now. But its continued presence in the ocean means that these materials must have arrived on Earth much more recently, Avaneesh Pandey reports for International Business Times.

So Breitschwerdt started looking for signs of ancient supernovae that might have seeded Earth with iron-60. They used data from the European Space Agency’s Hipparcos satellite to examine the movement of stars in a hot, gaseous region surrounding our solar system known as the Local Bubble, Grush reports.

Scientists believe that the Local Bubble was created by 14 to 20 nearby supernovae that erupted about 10 million years ago and pushed many stars in new directions. By tracking the stars back to their starting points at the time the Local Bubble formed, Breitschwerdt and his team identified two nearby supernovae that occurred about 1.5 to 2.3 million years ago that were distant enough from Earth to not annihilate it, but close enough that they could have showered the planet with radioactive dust, Akpan reports.

“This research essentially proves that certain events happened in the not-too-distant past,” University of Kansas astrophysicist Adrian Melott, who was not involved in the research, said in a statement. “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. We're trying to decide if we should expect to have seen any effects on the ground on the Earth.”

Breitschwerdt’s work was supported by another study also published this week, which came to similar conclusions based on different data. What’s interesting about the timing of the supernovae is that the older of the two coincides roughly with the end of the Pliocene epoch, a time when the planet was starting to cool. It’s possible that a shower of radioactive particles from the supernova may have triggered this climactic change by creating more cloud cover, Akpan reports. In turn, scientists believe the cooling planet helped direct human evolution. While this work is theoretical, it suggests that our ancestors were affected by distant cosmic events.

“This is a reminder that life on Earth does not proceed in isolation,” University of Illinois astronomer Brian Fields, who was not involved with the studies, tells Akpan. “In addition to being Earthlings, we’re citizens of a larger cosmos, and sometimes the cosmos intrudes on our lives.”

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