Over the last 13 million years, 16 massive stars collapsed and then spectacularly exploded in our galactic neighborhood, leaving a gigantic cavity of hot gas in their wake.

Using radioactive dust buried globally at the bottom of oceans, two teams have identified the location and date of the closest of these supernovas. The pair of investigations used different tactics, but the conclusion is the same: These massive fireworks from supermassive stars littered our planet with radioactive cosmic dust, which in the end, may have altered human evolution.

“This is a reminder that life on Earth does not proceed in isolation. In addition to being Earthlings, we’re citizens of a larger cosmos, and sometimes the cosmos intrudes on our lives,” said astronomer Brian Fields of the University of Illinois at Urbana-Champaign.

Fields wasn’t involved with these studies, but in 1996, his theoretical predictions sparked a new branch of astronomy. He calls it supernova archaeology.

“In addition to being Earthlings, we’re citizens of a larger cosmos, and sometimes the cosmos intrudes on our lives.”When a massive star dies, it creates a tremendous explosion known as a supernova. Its stellar core becomes a blizzard of nuclear reactions that boil 10 times hotter than the sun’s, Fields said. If a supernova occurred near us, it would obliterate the Earth. The radiation and cosmic rays would peel away our ozone layer and roast life on Earth. Twenty years ago, Fields’s graduate school advisers — David Schramm and John Ellis — estimated a supernova’s ‘kill radius’ to be 32 light years away and then asked an interesting question.

“Think about how we would know. What are the signatures that we could use to prove that a supernova happened nearby?” Soon after, Fields’s attention turned to a radioactive chemical isotope, iron-60.

Fields predicted that iron-60 embedded in the Earth’s crust would be a great time capsule for spotting recent supernova events. On a cosmic scale, this element is relatively short-lived, only painting a picture of the last 10 million or so years of supernova history. Plus, supernovae are the only known source of interstellar debris like iron-60. Comets and meteorites in our solar system can’t be the source of iron-60 deposits on Earth.

But damn if iron-60 isn’t hard to find.

A cosmic needle in an ocean haystack

Interstellar iron-60 levels in the ground are 15 to 16 orders of magnitude lower than iron from Earth,” said Australian National University nuclear physicist Anton Wallner, who led one of the new studies published today in the journal Nature.

For reference, if you had a pizza made of raw earth, you would need to evenly chop it one trillion times, and then another 1,000 times to perceive the relative abundance of iron-60. Its scarcity explains why, until today, only a single excavation of Pacific Ocean crust in 1999 had yielded enough evidence to support the occurrence of a recent supernova from 2.8 million years ago.

Wallner’s team wanted to find more interstellar particles inside Earth’s terrestrial archives, so they and a set of international collaborators collected deep sea sediments — in cores — from the Indian Ocean, the Atlantic Ocean and the Pacific Ocean.

“With many cores, you have for the first time a good idea of how much iron-60 came here,” said University of Kansas physicist Adrian Melott, who wasn’t involved in the research. “The deposition of this stuff on the Earth could be extremely patchy. Big in some places, smaller, even absent in other places. So by just looking at one core, you really don’t have a good idea of the amount of iron-60 that actually hit the Earth.”

Wallner’s team focused on sediment cores removed from ferromanganese crusts — pieces of earth rich in iron and manganese that grow like tree rings. Each ring corresponds to a date or era in our Earth’s history.

To detect the spurious iron-60, Wallner’s and his colleagues used accelerator mass spectrometry. First, particles in the crust samples are filtered according to mass and energy, before being tossed into a particle accelerator.

“Iron is accelerated to high energies. Then this particle beam is again purified from background and finally sent into a particle detector,” Wallner said. The payoff happens at the detector, where the scientists can count iron-60 atoms one by one.

That’s how they’re able to identify samples with iron-60, even though this interstellar litter makes up one-quadrillionth of the total material. “The low content of Fe-60 (iron-60) resulted in about one Fe-60 (iron-60) detection every hour or two,” Wallner wrote via email. “Overall, it took many weeks of (heavily booked) accelerator beam time to generate this set of data.”

In the end, his team reconfirmed the supernova discovered in 1999, but found a second recent supernova that happened 6.5 to 8.7 million years ago.

“Which is a big deal. The sediments are useful because they give a time history,” Fields said.

Meanwhile, the second study on these two supernovae describe where they occurred. To do so, astrophysicist Dieter Breitschwerdt at the Berlin Institute of Technology and colleagues relied on a star map of our interstellar neighborhood, also known as the Local Bubble. Peanut-shaped and 300 light years in length, the Local Bubble is a cavern of hot gas in the interstellar medium that encompasses our solar system and a few stellar cloud formations within the Milky Way.

Ten million years ago, 14 to 20 supernovae ruptured in quick succession like popcorn, creating this massive cavern of hot gas. The bursts from these explosions pushed around star clusters in our neighborhood, providing Breitschwerdt with an opportunity.

“What controls how much iron-60 gets on Earth is how much the supernova makes and how far away it is. The more spread out, the less you get. By measuring it, you can learn something about the distance,” Fields said.

By estimating the trajectories for how nearby stars moved following this supernova party, his team surmised that 16 supernovae exploded within in the Local Bubble during the past 13 million years. They built a computer model that pinpointed the galactic coordinates of the last two explosions, and these distance calculations were backed by iron-60 residues found in a deep-sea sediment core.

“The two different studies come to very consistent conclusions, even though they started from completely different places about what likely happened,” Melott said.

Cooled by star dust?

Both supernovae occurred 293 to 326 light years away, well outside the supernova kill radius. However, the events may have shifted the course of life on Earth.

“There’s speculation that there have been a rise of extinctions since 2 to 3 millions years ago,” Fields said. “Extinction rates tend to go up and down, in my impression, but now that they’ve found the second supernova around 8 million years ago, it’d be interesting to see if you also see biological effects back then.”

The timing of the most recent supernova does fall in the range of an Earth-shattering climatic change. In an op-ed article, Melott explains how a temperature decline ushered in a prolonged series of glaciations in the Pleistocene epoch — about 2.6 million to 12,000 years ago. “This climatic variation may be one of the conditions that led to human evolution,” he writes.

It’s unclear at this point if supernovae activity leads to cold temperatures, but one emerging theory is that cosmic rays from these star explosions enhance cloud formation.

“And if you have enhanced cloud formation, especially at low altitudes, you can cut down the amount of sunlight that reaches the Earth and cause cooling,” Melott said. “One study suggests correlations between the cosmic ray flux and clouds on Earth, but this idea remains controversial.”

Earth has witnessed five mass extinctions over the last 500 million years — we may be in the midst of a sixth — but none have been tied to supernovae…yet. Fields and Melott argue that these two studies lay the groundwork for unpacking the history of supernovae and their connections to mass extinctions. Accelerated mass spectrometry on other interstellar isotopes buried in the Earth’s crust may reveal earlier chapters in the timeline, for instance.

“If you go back in time hundreds of millions of years, it’s almost certain that you encounter a time when there was something really, really close that had disastrous consequences for the Earth,” Melott said. “One of our mass extinctions is probably from a really close supernova, but we just don’t know enough yet to be able to identify what it would be.”