“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,” says astronomer Brian Fields of the University of Illinois at Urbana-Champaign. In 1996, his theoretical predictions sparked a new branch of astronomy –supernova archaeology. “In addition to being Earthlings, we’re citizens of a larger cosmos, and sometimes the cosmos intrudes on our lives.”
Every century, about two massive stars in our Milky Way galaxy explode, producing magnificent supernovae. In the universe at large, a supernova event occurs every second. Stephen Hawking believed that these massive explosions may be responsible for killing off advanced extraterrestrial civilizations –perhaps a major factor in the “Great Silence” of the Fermi Paradox.
In February 1987, Neil Gehrels, then a young researcher at NASA’s Goddard Space Flight Center boarded a military plane bound for the Australian Outback, carrying some peculiar cargo: a polyethylene space balloon and a set of radiation detectors he had just finished building back in the lab.
“Gehrels,” writes Julian Rosen in Natutil.us, “was in a hurry to get to Alice Springs, a remote outpost in the Northern Territory, where he would launch these instruments high above Earth’s atmosphere to get a peek at the most exciting event in our neck of the cosmos: a supernova exploding in one of the Milky Way’s nearby satellite galaxies.”
Gehrels –later a renowned global figure as chief of the Goddard Astroparticle Physics Laboratory, passed away on Feb. 6, 2017–was hoping to gather evidence that answered the question how close does a supernova need to be to devastate life on Earth by annihilating the ozone layer, exposing plants and animals to harmful ultraviolet light, and possibly cause a new mass extinction.
Armed with new data from SN 1987A, the supernova Johannes Kepler spotted one in our own Milky Way galaxy in 1604, Gehrels could calculate a theoretical radius of doom, inside which a supernova would have grievous effects, and how often dying stars might stray inside it.
“The bottom line was that there would be a supernova close enough to the Earth to drastically affect the ozone layer about once every billion years,” says Gehrels.
In a post in The Galaxy on May 28, we reported that a supernovae eight-million years ago created atmospheric ionization that triggered an enormous upsurge in cloud-to-ground lightning strikes, igniting forest fires around the globe. These infernos could be one reason ancestors of homo sapiens developed bipedalism—to adapt in savannas that replaced torched forests in northeast Africa leading proto-humans to walk on two legs, “eventually resulting in homo sapiens with hands free to build cathedrals, design rockets and snap iPhone selfies.”
“Over the last 13 million years,” observed PBS in a 2016 post, “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, scientists have identified the location and date of the closest of these supernovas. These massive fireworks from supermassive stars littered our planet with radioactive cosmic dust, which in the end, may have altered human evolution.”
The paper in the Journal of Geology by lead author Adrian Melott, professor emeritus of physics and astronomy at the University of Kansas, makes the case: Supernovae bombarded Earth with cosmic energy starting as many as 8 million years ago, with a peak some 2.6 million years ago, initiating an avalanche of electrons in the lower atmosphere and setting off a chain of events that feasibly ended with bipedal hominins such as homo habilis, dubbed “handy man.”
“It is thought there was already some tendency for hominins to walk on two legs, even before this event,” said Melott. “But they were mainly adapted for climbing around in trees. After this conversion to savanna, they would much more often have to walk from one tree to another across the grassland, and so they become better at walking upright. They could see over the tops of grass and watch for predators. It’s thought this conversion to savanna contributed to bipedalism as it became more and more dominant in human ancestors.”
Based on a “telltale” layer of iron-60 deposits lining the world’s sea beds, astronomers have high confidence supernovae exploded in Earth’s immediate cosmic neighborhood—between 100 and only 50 parsecs (163 light years) away—during the transition from the Pliocene Epoch to the Ice Age.
In an earlier 2016 post, PBS reported about supernova blast was so close, it littered the ocean floor with radioactive dust. The image below shows predicted distribution of Iron-60 (by mass density) through the Local Bubble and a neighboring superbubble Loop, 2.2 million years ago. The whitish red region, near where Earth is located (not drawn to scale), represents material that has been expelled by recent supernova explosions. Photo by Michael Schulreich/Berlin Institute of Technology
“We calculated the ionization of the atmosphere from cosmic rays which would come from a supernova about as far away as the iron-60 deposits indicate,” Melott said about . “It appears that this was the closest one in a much longer series. We contend it would increase the ionization of the lower atmosphere by 50-fold. Usually, you don’t get lower-atmosphere ionization because cosmic rays don’t penetrate that far, but the more energetic ones from supernovae come right down to the surface—so there would be a lot of electrons being knocked out of the atmosphere.”
According to Melott and co-author Brian Thomas of Washburn University, ionization in the lower atmosphere meant an abundance of electrons would form more pathways for lightning strikes.
“The bottom mile or so of atmosphere gets affected in ways it normally never does,” Melott said in the current KU study. “When high-energy cosmic rays hit atoms and molecules in the atmosphere, they knock electrons out of them—so these electrons are running around loose instead of bound to atoms. Ordinarily, in the lightning process, there’s a buildup of voltage between clouds or the clouds and the ground—but current can’t flow because not enough electrons are around to carry it. So, it has to build up high voltage before electrons start moving. Once they’re moving, electrons knock more electrons out of more atoms, and it builds to a lightning bolt. But with this ionization, that process can get started a lot more easily, so there would be a lot more lightning bolts.”
The KU researcher said the probability that this lightning spike touched off a worldwide upsurge in wildfires is supported by the discovery of carbon deposits found in soils that correspond with the timing of the cosmic-ray bombardment.
“The observation is that there’s a lot more charcoal and soot in the world starting a few million years ago,” Melott said. “It’s all over the place, and nobody has any explanation for why it would have happened all over the world in different climate zones. This could be an explanation. That increase in fires is thought to have stimulated the transition from woodland to savanna in a lot of places—where you had forests, now you had mostly open grassland with shrubby things here and there. That’s thought to be related to human evolution in northeast Africa. Specifically, in the Great Rift Valley where you get all these hominin fossils.”
Melott said no such event is likely to occur again anytime soon. The nearest star capable of exploding into a supernova in the next million years is Betelgeuse, some 200 parsecs (652 light years) from Earth.
“Betelgeuse is too far away to have effects anywhere near this strong,” Melott said. “So, don’t worry about this. Worry about solar proton events. That’s the danger for us with our technology—a solar flare that knocks out electrical power. Just imagine months without electricity.”
“People estimated the ‘kill zone’ for a supernova in a paper in 2003, and they came up with about 25 light years from Earth,” Melott said. “Now we think maybe it’s a bit greater than that. They left some effects out or didn’t have good numbers, so now we think it may be a bit larger distance. We don’t know precisely, and of course it wouldn’t be a hard-cutoff distance. It would be a gradual change. But we think something more like 40 or 50 light years. So, an event at 150 light years should have some effects here but not set off a mass extinction.”
While no mass-extinction events happened 2.8 million years ago, writes Rosen, “some drastic climate changes did take place—and they may have given a boost to human evolution. Around that time, the African climate dried up, causing the forests to shrink and give way to grassy savanna. Scientists think this change may have encouraged our hominid ancestors as they descended from trees and eventually began walking on two legs.”
Image Credit top of page: Double Supernova X-ray: NASA/CXC/U.Illinois/R.Williams & Y.-H.Chu; Optical: NOAO/CTIO/U.Illinois/R.Williams & MCELS coll.; Radio: ATCA/U.Illinois/R.Williams et al.) This composite image of DEM L316 combines data from Chandra (X-ray, blue), the Curtis-Schmidt telescope at CTIO (optical, red) & ATCA, the Australia Telescope Compact Array (radio, green).