Earth's protective atmosphere has protected life for billions of years, creating a refuge where evolution produced complex life forms like us. The ozone layer plays a critical role in protecting the biosphere from deadly ultraviolet radiation. Blocks 99% of the sun's powerful UV ray emission. Earth's magnetosphere also protects us.
But the Sun is relatively tame. How effective are ozone and the magnetosphere at protecting us from powerful supernova explosions?
Every million years (a small fraction of Earth's 4.5 billion years of life) a massive star explodes 100 parsecs (326 light years) from Earth. We know this because our Solar System is located inside a huge bubble in space called Local bubble. It is a cavernous region of space where the density of hydrogen is much lower than outside the bubble. A series of supernova explosions in the previous 10 to 20 million years created the bubble.
Supernovae are dangerous and the closer a planet is to one, the more lethal their effects are. Scientists have speculated about the effects that supernova explosions have had on Earth, wondering whether they caused mass extinctions or at least partial extinctions. The gamma ray burst and cosmic rays from a supernova can deplete Earth's ozone and allow ionizing ultraviolet radiation to reach the planet's surface. The effects can also create more aerosol particles in the atmosphere, increasing cloud cover and causing global cooling.
A new research paper in Nature Communications Earth and Environment examines supernova explosions and their effect on Earth. Its titled “Earth's atmosphere protects the biosphere from nearby supernovae.The lead author is Theodoros Christoudias of the Climate and Atmosphere Research Center of the Cyprus Institute, Nicosia, Cyprus.
The local bubble is not the only evidence of nearby core-collapse supernovae (SNe) in the last few million years. Ocean sediments also contain 60Fe, a radioactive isotope of iron with a half-life of 2.6 million years. SNe expel 60Faith to space when they explode, indicating that a nearby supernova exploded about 2 million years ago. There's also 60Faith in sediments indicating another SN explosion about 8 million years ago.
Researchers have correlated an SN explosion with the Late Devonian extinction About 370 million years ago. on a paper, researchers found plant spores burned by ultraviolet light, an indication that something powerful depleted Earth's ozone layer. In fact, Earth's biodiversity declined for about 300,000 years before the Late Devonian extinction, suggesting that multiple SNe could have played a role.
The Earth's ozone layer is constantly changing. When ultraviolet energy reaches it, it breaks down ozone (O3) molecules. This dissipates the ultraviolet energy and the oxygen atoms combine again into O3. The cycle repeats. This is a simplified version of the atmospheric chemistry involved, but it serves to illustrate the cycle. A nearby supernova could nullify the cycle, depleting the density of the ozone column and allowing more deadly ultraviolet rays to reach the Earth's surface.
But in the new paper, Christoudias and his fellow authors suggest that Earth's ozone layer is much more resilient than previously thought, providing ample protection against SNe over 100 parsecs. While previous researchers have modeled Earth's atmosphere and its response to a nearby SN, the authors say they have improved on that work.
They modeled Earth's atmosphere with an Earth Systems Model with Atmospheric Chemistry (EMAC) to study the impact of nearby SNe explosions on Earth's atmosphere. Using EMAC, the authors say they have modeled “the complex dynamics of the atmospheric circulation, chemistry and feedback process” of Earth's atmosphere. These are needed to “simulate stratospheric ozone loss in response to elevated ionization, leading to ion-induced nucleation and particle growth to CCNs” (cloud condensation nuclei).
“We assume a representative nearby SN with GCR (galactic cosmic ray) ionization rates in the atmosphere that are 100 times current levels,” they write. This correlates with a supernova explosion about 100 parsecs or 326 light years away.
“The maximum ozone depletion at the poles is less than the current anthropogenic ozone hole in Antarctica, which is equivalent to a loss of column ozone of 60-70%,” the authors explain. “On the other hand, there is an increase in ozone in the troposphere, but it is within the levels resulting from recent anthropogenic pollution.”
But let's get to the point. We want to know if Earth's biosphere is safe or not.
The mean maximum depletion of stratospheric ozone due to 100 times more ionizing radiation than normal, representative of a nearby SN, is approximately 10% globally. That's about the same decline caused by our anthropogenic pollution. It wouldn't affect the biosphere much.
“Although significant, such changes in ozone are unlikely to have a major impact on the biosphere, especially since most ozone loss occurs at high latitudes,” the authors explain.
But that's for modern Earth. During the Precambrian, before life exploded into multiplying forms, the atmosphere had only about 2% oxygen. How would that affect an SN? “We simulated an atmosphere with 2% oxygen, as this would likely represent conditions in which the emerging biosphere on Earth would still be particularly sensitive to ozone depletion,” the authors write.
“Ozone loss is about 10% to 25% in mid-latitudes and an order of magnitude less in the tropics,” the authors write. With minimum ozone levels at the poles, ionizing radiation from an SN could end up increasing the ozone column. “We conclude that these changes in atmospheric ozone are unlikely to have had a major impact on the emerging biosphere on Earth during the Cambrian,” they conclude.
What about global cooling?
Global cooling would increase, but not to a dangerous degree. In the Pacific and Southern Oceans, the CCN could increase by up to 100%, which seems like a lot. “These changes, although climatically relevant, are comparable to the contrast between the pristine pre-industrial atmosphere and the current polluted atmosphere.” They say it would cool the atmosphere by about the same amount as we warm it now.
The researchers point out that their study refers to the entire biosphere, not individuals. “Our study does not consider direct health risks to humans and animals resulting from exposure to elevated ionizing radiation,” they write. Depending on individual circumstances, people could be exposed to dangerous levels of radiation over time. But overall, the biosphere would continue to function despite a 100-fold increase in ultraviolet radiation. Our atmosphere and magnetosphere can handle it.
“Overall, we find that the nearby SNe is unlikely to have caused mass extinctions on Earth,” the authors write. “We conclude that our planet's atmosphere and geomagnetic field effectively protect the biosphere from the effects of the nearby SNe, which has allowed life to evolve on Earth over the past hundreds of millions of years.”
This study shows that Earth's biosphere will not suffer much as long as supernova explosions remain at a distance.
