Science & Technology

An Ancient Supernova May Have Triggered a Mass Extinction on Earth

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When people talk about what killed the dinosaurs, they usually talk about asteroids. Scientists generally agree that was the trigger for one huge extinction, but there have been many more mass die-offs in Earth's history with many more causes. One extinction event that's more recent — both in its timescale and in its discovery — happened about 2.6 million years ago, killing off large numbers of ocean animals and ending what's known as the Pliocene epoch. How did it happen? New research is suggesting that an ancient supernova is to blame.

Related Video: What Really Killed the Dinosaurs?

Pumping Iron-60

The evidence lies in the fact that Earth is full of an isotope called iron-60. An isotope is just an atom with an unusual number of neutrons in its nucleus; iron-60 has 34 neutrons and 26 protons while your garden-variety iron (aka iron-56) has 30 neutrons and 26 protons.

Back in the 1990s, an astrophysicist named Brian Fields suggested that finding this particular isotope in the fossil record could tell researchers when supernovae occurred over Earth's history. That's because iron-60 is radioactive; it decays over time to eventually become cobalt and then nickel. That means that any iron-60 that formed with our planet would have long ago morphed into another element, so if it's here, it came from outer space — specifically from an exploding star, also known as a supernova. Find out how long ago it landed here, and you can find out when that supernova occurred.

In 2016, two papers published in Nature used this very isotope as evidence of ancient supernovae. One paper from researchers in Australia found evidence of iron-60 isotopes in an ancient seabed and successfully traced their source to supernovae that happened about 325 light-years away. The other, from researchers in Germany, timed them out, figuring that one supernova occurred around 2 million years ago and the other occurred about 7 million years ago.

However, the researchers noted that 325 light-years is really far away — not so far that iron-60 couldn't reach us, but much farther than the generally agreed-upon 30 light-year "kill zone" an explosion would need to be in to do real damage to life on Earth. Direct damage, that is.

According to a new paper out of the University of Kansas, these distant supernovae could have showered the planet with something less instantaneous, but just as deadly.

Death From the Skies

"I've been doing research like this for about 15 years, and always in the past it's been based on what we know generally about the universe — that these supernovae should have affected Earth at some time or another," said lead author Adrian Melott in a statement. "This time, it's different. We have evidence of nearby events at a specific time. We know about how far away they were, so we can actually compute how that would have affected the Earth and compare it to what we know about what happened at that time — it's much more specific."

The first thing the team did was provide even more evidence for these ancient supernovae in the form of the Local Bubble. That's a hollowed-out space in the collection of gas, dust, and cosmic rays between stars known as the interstellar medium. When a star explodes, it clears out almost all of the gas and dust in its vicinity, which creates a bubble. If multiple stars explode next to each other, those bubbles will combine. That's what our Local Bubble looks like, and that lends even more credence to the idea of multiple supernovae exploding in the past.

But that still doesn't explain what killed all the marine megafauna at the end of the Pleistocene — megafauna like megalodon, a species of shark that was the size of a school bus. For that, the researchers point to the muon, a subatomic particle a few hundred times more massive than an electron. These heavy hitters sprinkle the Earth all the time, arriving via cosmic rays from outer space that collide with the atmosphere.

Megalodon pursuing two Eobalaenoptera whales.

You have muons passing through you all the time — they're just sparse enough not to cause real damage. "Nearly all of them pass through harmlessly, yet about one-fifth of our radiation dose comes by muons," says Melott. "But when this wave of cosmic rays hits, multiply those muons by a few hundred. Only a small fraction of them will interact in any way, but when the number is so large and their energy so high, you get increased mutations and cancer — these would be the main biological effects."

The researchers estimate that for animals the size of a human, this muon shower would boost the cancer rate by about 50 percent. And the bigger the animal, the worse that rate gets. The Pleistocene extinction event decimated about 36 percent of genera (singular "genus," the organism classification above "species"), most of which were in coastal areas where large, surface-dwelling marine animals lived. The muons wouldn't do as much damage in deeper waters, but they could still reach hundreds of yards down.

Death wouldn't have been as quick as from an asteroid impact or volcano eruption, but it definitely could have happened. "There really hasn't been any good explanation for the marine megafaunal extinction," Melott said. "This could be one."

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Find out about all of Earth's extinctions in "The Ends of the World: Volcanic Apocalypses, Lethal Oceans, and Our Quest to Understand Earth's Past Mass Extinctions" by Peter Brannen. We handpick reading recommendations we think you may like. If you choose to make a purchase, Curiosity will get a share of the sale.

Written by Ashley Hamer January 11, 2019

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