Exploding stars may have assaulted ancient Earth

By Daniel CleryJul. 15, 2021 , 11:19 AM

For our Australopithecus ancestors who roamed Africa 2.5 million years ago, the bright new star in the sky surely would have aroused curiosity. As luminous as the full Moon, it would have cast shadows at night and been visible during the day. As the supernova faded over the following months, it probably also faded from memory. But it left other traces, now coming to light.

Over the past 2 decades, researchers have found hundreds of radioactive atoms, trapped in seafloor minerals, that came from an ancient explosion marking the death of a nearby star. Its fusion fuel exhausted, the star had collapsed, generating a shock wave that blasted away its outer layers in an expanding ball of gas and dust so hot that it briefly glowed as bright as a galaxy—and ultimately showered Earth with those telltale atoms.

Erupting from hundreds of light-years away, the flash of x-rays and gamma rays probably did no harm on Earth. But the expanding fireball also accelerated cosmic rays—mostly nuclei of hydrogen and helium—to close to the speed of light. These projectiles arrived stealthily, decades later, ramping up into an invisible fusillade that could have lasted for thousands of years and might have affected the atmosphere—and life.

In a flurry of studies and speculation, astronomers have sketched out their potential effects. A cosmic ray barrage might have boosted mutation rates by eroding Earth’s protective ozone layer and generating showers of secondary, tissue-penetrating particles. Tearing through the atmosphere, the particles would have also created pathways for lightning, perhaps kindling a spate of wildfires. At the same time, atmospheric reactions triggered by the radiation could have led to a rain of nitrogen compounds, which would have fertilized plants, drawing down carbon dioxide. In that way, the celestial event could have cooled the climate and helped initiate the ice ages 2.5 million years ago, at the start of the Pleistocene epoch. Even taken together, the effects are “not like the dinosaur extinction event—it’s more subtle and local,” says Brian Thomas, an astronomer at Washburn University who has studied the earthly effects of cosmic catastrophes for nearly 2 decades.

Few astronomers are suggesting that the supernovae caused any great extinction at the time, and even fewer paleontologists are ready to believe them. “Death from space is always really cool,” says Pincelli Hull, a paleontologist at Yale University. “The evidence is interesting but has not quite really reached the threshold to incorporate into my mental register.”

Yet the supernova hunters believe other blasts, more distant in time, went off closer to Earth. And they think these supernovae could explain some extinction events that lack customary triggers such as volcanic outbursts or asteroid impacts. Adrian Melott, an astronomer at the University of Kansas, Lawrence, who explores how nearby cosmic cataclysms might affect Earth, says it’s time to more carefully probe Earth’s history for ancient supernova strikes. Not only will that help astrophysicists understand how the blasts shaped the neighborhood of the Solar System and seeded it with heavy elements, but it could also give paleontologists a new way to think about bouts of global change. “This is new and unfamiliar,” Melott says. “It will take time to be accepted.”

Astronomers believe a few supernovae go off in the Milky Way every century. By the law of averages, a handful must have exploded very close to Earth—within 30 light-years—during its 4.5-billion-year lifetime, with potentially catastrophic effects. Even blasts as far as 300 light-years away should leave traces in the form of specks of dust blown out in the shell of debris known as a supernova remnant. When physicist Luis Alvarez set out in the 1970s with his geologist son Walter Alvarez to study the sediment layers associated with the dinosaurs’ extinction 65 million years ago, they were expecting to find supernova dust. Instead, they found iridium, an element that is rare on Earth’s surface but abundant in asteroids.

The Alvarezes didn’t have the tools to look for supernova dust, in any case. Because Earth is already largely made of elements forged in supernovae billions of years ago, before the Sun’s birth, most traces of more recent explosions are undetectable. Not all of them, however. In the 1990s, astrophysicists realized supernova dust might also deposit radioactive isotopes with half-lives of millions of years, far too short to have been around since Earth’s birth. Any that are found must come from geologically recent sprinklings. One key tracer is iron-60, forged in the cores of large stars, which has a half-life of 2.6 million years and is not made naturally on Earth.

In the late 1990s, Gunther Korschinek, an astroparticle physicist at the Technical University of Munich (TUM), decided to look for it, partly because the university had a powerful accelerator mass spectrometer (ASM) suited to the task. After ionizing a sample, an ASM boosts the charged particles to high energies and shoots them through a magnetic field. The field bends their path onto a string of detectors; the heaviest atoms are deflected least because of their greater momentum.

Separating atoms of iron-60 from the similarly hefty but differently charged nickel-60 is especially challenging, but TUM’s ASM, built in 1970, is one of the few in the world powerful enough to tease them apart.

Korschinek also needed the right sample: a geologic deposit laid down over millions of years in which an iron signal might stand out. Antarctic ice cores wouldn’t work: they only go back a couple of million years or so. Most ocean sediments accumulate so fast that any iron-60 is diluted to undetectable levels. Korschinek ended up using a ferromanganese crust dredged from a North Pacific seamount by the German research ship Valdivia in 1976. These crusts grow on patches of seabed where sediments can’t settle because of a slope or currents. When the pH of the water is just right, metal atoms selectively precipitate out of the water, slowly building up a mineral crust at the rate of a few millimeters every million years.

Korschinek and his team sliced their sample up into layers of different ages, chemically separated out the iron, and fired the atoms through their mass spectrometer. They found 23 atoms of iron-60 among the thousands of trillions of atoms of normal iron, with the highest abundance from a time less than 3 million years ago, the team reported in Physical Review Letters in 1999. The era of supernova geochemistry had begun. “We were the first ones to start experimental studies,” Korschinek says.

Others followed. Iron-60 was found in ocean crusts from other parts of the world and even in ocean sediment microfossils, remains of living things that, helpfully for the supernovae hunters, had taken up and concentrated iron in their bodies. Most results pointed to a local supernova between 2 million and 3 million years ago—with hints of a second one a few million years earlier.

Although the remnants from these blasts have long since swept past Earth, a drizzle of the atoms they blew out continues. In 2019, Korschinek’s team ran iron from a half-ton of fresh Antarctic snow through its ASM and found a handful of iron-60 atoms, which he estimates fell to Earth in the past 20 years. Another team found a smattering of the atoms in cosmic rays detected by NASA’s Advanced Composition Explorer at a position partway between the Sun and Earth. Researchers have even found iron-60 in lunar soil brought back by the Apollo missions. “The Moon confirmed that it was not just some Earth-based phenomenon,” says astronomer Adrienne Ertel of the University of Illinois, Urbana-Champaign (UIUC).

Dieter Breitschwerdt is trying to trace the iron to its source in the sky. When the astronomer at the Technical University of Berlin learned of Korschinek’s results, he was studying the local bubble, a region of space around the Solar System swept clear of most of its gas and dust. Supernovae were the likely brooms, and so he began to track gangs of stars in the Solar System’s neighborhood to see whether any passed close enough to the Sun to deposit iron-60 on Earth when some of their members exploded.

Using data from Hipparcos, a European star-mapping satellite, Breitschwerdt looked for clumps of stars on common trajectories and rewound the clock to see where they would have been millions of years ago. Two clumps, now a part of the Scorpius-Centaurus OB Association (Sco OB2), seemed to be in the perfect spot—300 light-years from Earth—about 2.5 million years ago. “It looked like a miracle,” he says. The odds of a detonation at the right time were good. Core-collapse supernovae take place in massive stars. Based on the ages and masses of the 79 stars remaining in the clumps, Breitschwerdt estimates that a dozen former members exploded as supernovae in the past 13 million years.

Visible evidence for these supernovae in Sco OB2 is long gone: Supernova remnants dissipate after about 30,000 years, and the black holes or neutron stars they leave behind are challenging to spot. But the arrival direction of the iron dust could, in theory, point back to its source. Samples from the sea floor provide no directional information because wind and ocean currents move the dust as it settles. On the Moon, however, “there is no atmosphere, so where it hits is where it stops,” says UIUC astronomer Brian Fields. Because it spins, the Moon cannot provide longitudinal direction, but if more iron-60 was detected at one of the poles than at the equator, for example, that could support Breitschwerdt’s Sco OB2 as the source. Fields and several colleagues want to test that idea and have applied to NASA for samples of lunar soil, to be collected and returned by any future robotic or human missions.

Korschinek’s team now has a rival in the hunt for supernova iron: a group led by Anton Wallner, a former postdoc of Korschinek’s, who has used an upgraded ASM at Australian National University (ANU) to analyze several ferromanganese crusts dredged off the Pacific Ocean floor by a Japanese mining company. “Now we pushed Munich,” Wallner says.

This year, in Science Advances, Wallner’s team probed the timing of the recent supernovae more precisely than ever by slicing a crust sample into 24 1-millimeter-thick layers, each representing 400,000 years. “It’s never been done before with this time resolution,” says Wallner, now at the Helmholtz Center Dresden-Rossendorf. The 435 iron-60 atoms they extracted pinned the most recent supernova at 2.5 million years ago and confirmed the hints of an earlier one, which they pegged at 6.3 million years ago. Comparing the abundance of iron-60 in the crust with models of how much a supernova produces, the team estimated the distance of these supernovae as between 160 and 320 light-years from Earth.

Wallner’s team also found 181 atoms of plutonium-244, another radioactive isotope, but one that may have been forged in the supernova blast itself rather than in the precursor star, like iron-60. But its source is hotly debated: Some researchers think plutonium-244 is tough for supernovae to make in any great amounts. Instead, they see it as the product of collisions between neutron stars—cinders left behind by supernovae.

These collisions, called kilonovae, are 100 times rarer than supernovae, but are much more efficient at making the heaviest elements. “Neutron star mergers have an easy time making plutonium,” says Rebecca Surman, an astrophysicist at the University of Notre Dame. “For supernovae it’s much harder.”

Surman still sees a role for supernovae. She takes the reported seafloor plutonium-244 as a sign that a kilonova, deep in the past, dusted our interstellar neighborhood with heavy elements. When the two recent supernovae went off, their expanding remnants may have swept up and delivered some of that interstellar plutonium-244 along with their own iron-60, she speculates. Korschinek, however, says it will take more data on the plutonium signal and its timing to convince him that multiple rare events happened so near and so recently.

Beyond dusting Earth with rare nuclei, what impact might nearby supernovae have had? In 2016, a team led by Melott and Thomas estimated the flux of various forms of light and cosmic rays likely to reach Earth from an explosion 300 light-years away. Writing in Astrophysical Journal Letters, they concluded that the most energetic, potentially damaging photons—x-rays or gamma rays—would have minimal impact. “There is not a lot of high energy radiation,” Thomas says. They suggested a few weeks of the bright light would have little more impact than disrupting sleep patterns.

Cosmic rays—the particles accelerated to near light speed by shock waves in the supernova’s expanding fireball—are another story. Because they are charged, they can be deflected away from Earth by galactic magnetic fields. But the local bubble is thought to be mostly devoid of fields, so cosmic rays from just 300 light-years away would have a relatively clean shot.

The atmosphere would have been subjected to a drawn-out barrage, Melott and Thomas found. “The ramp up is a slow process, decades at least,” Thomas says, reaching a peak about 500 years after the supernova flash and causing a 10-fold increase in ionization of atmospheric gas that would persist for 5000 years. Using an atmospheric chemistry model developed by NASA, they estimated that chemical changes caused by the ionization would deplete ozone by about 7% or more in places and would boost the creation of fertilizing nitrogen oxide compounds by 30%. The resulting surge in plants might be enough to cool the climate and usher in the Pleistocene.

The cosmic rays weren’t done yet. When high-energy particles hit the upper atmosphere, they create cascades of secondary particles. Most fizzle out in further collisions, but muons—heavy short-lived cousins of electrons—keep going. Creatures on Earth’s surface would receive triple the normal radiation dose—equivalent to one or two CT scans per year. “An enhanced risk [of cancer], but not radiation poisoning,” Thomas says. Overall, the team thought the effects were “not catastrophic” but could be detectable in the fossil record if, for example, certain vulnerable species disappeared while others survived.

In Astrobiology in 2019, Melott and two colleagues found that if the supernova exploded just 150 light-years away, rather than 300, the muon radiation would have hit marine animals surprisingly hard. Water blocks most particles that rain down from the sky, but muons can penetrate up to 1 kilometer. Marine creatures, normally shielded from nearly all radiation, would experience the largest relative increase in dose and suffer the most. This chimes with an extinction of marine megafauna at the start of the Pleistocene epoch, only recently identified in the fossil record.

Then, last year, supernova proponents suggested a similar scenario could explain a major extinction event 359 million years ago, at the end of the Devonian period. A team led by John Marshall of the University of Southampton had found that the spores of fernlike plants from the time suddenly became misshapen and dark, blaming the changes on ultraviolet radiation. The team didn’t invoke an astronomical cause. But writing in the Proceedings of the National Academy of Sciences, astronomers saw the possible signature of a nearby supernova. They suggested a blast maybe just 60 light-years away could have drenched Earth in ultraviolet by depleting the ozone layer. “It’s pretty speculative,” admits co-author John Ellis, a theorist at King’s College London, as it is currently impossible to identify the radioactive fingerprints of a supernova that far back.

In a 2020 paper in The Journal of Geology, Melott and Thomas took a bigger speculative leap. They noted that by ripping electrons from air molecules, secondary cosmic rays would have created pathways for lightning, making storms more likely, which would not only generate more nitrogen compounds but also spark wildfires. Intriguingly, a layer of soot has been found in the rock record in some parts of the world at the start of the Pleistocene. Melott and Thomas went on to suggest that those supernova-induced forest fires may have pushed early humans out of the trees and onto the savanna, leading to bipedalism, larger brain size, and everything that followed. “It’s fascinating to say that a supernova 2.5 million years ago means we are talking now via Skype,” Korschinek says.

Such scenarios don’t sit well with paleontologists. “Timing is the trivial answer to everything,” Hull says. “There’s always something happening when things become extinct.” Besides, she says, the transition to the Pleistocene “doesn’t stand out as needing an explanation.” She says other events around that time could have had more impact on the global climate, such as the closing of the isthmus of Panama, which profoundly changed ocean circulation.

To make their case, she says, astronomers need to pin down the timing of the ancient supernovae more precisely. They “need to measure more crusts.” But hunting for supernova traces is not getting any easier. In 2019 TUM closed its AMS, leaving only ANU with an accelerator powerful enough to separate iron-60.

In contrast, rarer isotopes such as plutonium-244 could enable researchers to look further back in time, but they require an AMS that emphasizes sensitivity rather than raw power, and Wallner says only a few in the world are up to the job. He has secured funding to build a new AMS facility in Dresden, Germany, specializing in the heaviest elements, that should be open by 2023. To renew the hunt for iron-60, his team has also made a pitch for national funding to build a new high-energy AMS, which could be up and running in 7 years.

For astronomers, a sudden flash of light in the sky today would be the best chance to see how supernova affects Earth. But the odds are slim that we will see a light show like the one that may have dazzled our distant ancestors. Betelgeuse, a restive red giant likely to blow up sometime in the next 100,000 years, has settled down in recent months, and in any case, it lies more than 500 light-years away. Sco OB2 is now heading away from the Sun. And using data from Hipparcos’s successor, Europe’s Gaia mission, Breitschwerdt has tracked another 10 clumps of stars. “None are coming closer,” he says. “The future”—for Earth, not the supernovae—“is bright.”