Astronomers may have found an explanation for the origins of a strange, radioactive material found deep inside Earth’s oceanic crusts. Much of the material, called iron-60, seems to come from two supernovae explosions that occurred millions of years ago, according to a new study published today in the journal Nature. The explosions appear to be the closest to Earth within the last few million years.

"All the iron-60 we find here must come from outer space."

Prior to this study, the presence of iron-60 on Earth has been puzzling for scientists. The radioactive material has a half life of 2.6 million years, meaning that’s how long it takes for half of the material in a sample to decay. Because Earth and other nearby planets are thought to have formed 4.5 billion years ago, any iron-60 that formed during the first years of the Solar System would be long gone by now. "All the iron-60 we find here must come from outer space," said study author Dieter Breitschwerdt, an astrophysicist at the Berlin Institute of Technology.

Supernovae, huge stellar explosions that occur when massive stars run out of fuel, have long been thought to be the source of iron-60. When a star explodes, its core collapses, causing different types of elements to form inside the star. "Supernovae are element factories," said Brian Fields, an astronomer at the University of Illinois, who did not work on the study. "New elements are created during the life of the star and during the supernova explosion, and one thing you get is radioactivity — unstable nuclei that decay over sometime."

This theory about the origins of Earth's iron-60 inspired Breitschwerdt to determine which supernovae explosions might have been responsible for the element's presence. To figure this out, he and his team didn’t observe supernovae directly, but they looked at the motions of other stars moving hundreds of light years away. The stars were located inside the Local Bubble — a region of hot gas spanning hundreds of light years across around our Solar System. The Local Bubble is believed to be leftover from supernovae explosions, and the researchers thought that maybe some of these star deaths that formed the bubble may have also spread the iron-60.

A simulation of how the far the iron-60 was distributed. (Michael Schulreich)

Using data from the European Space Agency's Hipparcos satellite, the researchers focused on a moving cluster of stars within the Local Bubble that are about 300 to 400 light years away from Earth. Based on the stars’ properties, the team was able to determine the age of the cluster and that it once passed by our solar neighborhood. By calculating mass distribution across the cluster, researchers also realized that some of the bigger stars in the group must have already exploded. "We can calculate from the still existing stars how many high-mass stars there must have been in the past," said Breitschwerdt.

Two supernovae are thought to have contributed about half of all the iron-60

Ultimately, the research team found that 16 of the cluster's supernovae were responsible for forming the Local Bubble, but only two of the closest explosions brought iron-60 to our planet. "The further away the explosion is, the less iron comes to Earth," said Breitschwerdt. The closest explosion is thought to have happened 2.3 million years ago and about 296 light years away from our Solar System; the other explosion occurred much more recently, about 1.5 million years ago and about 313 light years away. These two supernovae are thought to have contributed about half of all the iron-60 found in Earth's deep-sea crusts.

Breitschwerdt's work is all theoretical, so the study's findings aren't definitive proof that these supernovae took place, according to Fields. That means there are other possible scenarios that could explain how iron-60 got to Earth. But Fields says that this study paints a very consistent picture of how the element may have come here, in the context of how our galactic neighborhood formed. "They even have specific suggestions for where in the sky the supernovae came from," said Fields. "It's a nice sort of piece of detective work."