Earth’s protective atmosphere has protected life for billions of years, creating a haven where evolution created complex life forms like us.
The ozone layer plays a key role in protecting the biosphere from deadly UV radiation. Blocks 99% of the Sun’s strong UV radiation. Earth’s magnetosphere also protects us.
But the Sun is relatively tame. How effective are the ozone and magnetosphere in protecting against powerful supernova explosions?
Every million years—a tiny fraction of Earth’s 4.5 billion years—a massive star within 100 parsecs (326 light years) of Earth explodes. We know this because our Solar System sits inside a massive bubble in space called the 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 over the previous 10 to 20 million years created the bubble.
Supernovae are dangerous, and the closer it is to a planet, the more deadly its effects. Scientists have speculated about the effects that supernova explosions had on Earth, wondering if they triggered mass extinctions or at least partial extinctions.
A supernova’s gamma-ray burst and cosmic rays can damage Earth’s ozone and allow ionizing UV radiation to penetrate the planet’s surface. The effects can also create more aerosol particles in the atmosphere, increase cloud cover, and cause global cooling.
A new research paper in Nature Communication Earth and environment examines supernova explosions and their effect on Earth. It is titled “Earth’s Atmosphere Protects Biosphere from Nearby Supernovae”.
The lead author is Theodoros Christoudias of the Climate and Atmosphere Research Center, Cyprus Institute, Nicosia, Cyprus.
The Local Bubble is not the only evidence of nearby core-collapse supernovae (SNe) in the past few million years. Oceanic sediments also contain 60Fe, a radioactive isotope of iron with a half-life of 2.6 million years.
SNe banish 60Fe into space when they explode, suggesting that a nearby supernova exploded about 2 million years ago. It’s there too 60Fe in sediments that indicate another SN explosion about 8 million years ago.
The researchers correlated the SN explosion with the Late Devonian extinction about 370 million years ago. In one paper, researchers found plant spores burned by UV light, suggesting that something powerful had damaged Earth’s ozone layer.
In fact, Earth’s biodiversity was declining for about 300,000 years before the Late Devonian extinction, suggesting that more SNe may have played a role.
The Earth’s ozone layer is in constant flux. When UV energy reaches it, it breaks down ozone (O3) molecules. This dissipates the UV energy and the oxygen atoms recombine to form O3. The cycle repeats itself.
This is a simplified version of atmospheric chemistry, but serves to illustrate the cycle. A nearby supernova could override the cycle, reducing the density of the ozone layer and allowing more deadly UV to reach Earth’s surface.
But in a new paper, Christoudias and his co-authors suggest that Earth’s ozone layer is much more resilient than previously thought, providing sufficient protection against SNe up to 100 parsecs.
While previous researchers have modeled the Earth’s atmosphere and its response to a nearby SN, the authors say they have improved on this work.
They modeled Earth’s atmosphere using the 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 modeled “the complex dynamics of atmospheric circulation, chemistry and feedback processes” of Earth’s atmosphere.
These are needed to “simulate the loss of stratospheric ozone in response to increased ionization, leading to ion-induced nucleation and growth of particles on CCN” (cloud condensation nuclei.)
“We hypothesize a representative nearby SN with a GCR (galactic cosmic ray) ionization rate in the atmosphere that is 100 times higher than current levels,” they write. This correlates to a supernova explosion at a distance of about 100 parsecs, or 326 light years.
“The maximum ozone depletion over the poles is smaller than the current anthropogenic ozone hole over Antarctica, representing a loss of the ozone column 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 the Earth’s biosphere is safe or not.
The maximum mean depletion of stratospheric ozone due to 100 times more ionizing radiation than normal, representative of the near SN, is about 10% globally. That’s about the same decline as our anthropogenic pollution is causing. It wouldn’t affect the biosphere much.
“Although significant, such ozone changes are unlikely to have a major impact on the biosphere, especially since most ozone losses occur at high latitudes,” the authors explain.
But that’s for modern Earth. During the Precambrian, before life exploded in the proliferation of forms, the atmosphere had only about 2% oxygen. How would this affect SN?
“We simulated a 2% oxygen atmosphere because this would likely represent conditions where the emerging biosphere on land would still be particularly vulnerable to ozone depletion,” the authors write.
“Ozone loss is about 10-25% in the mid-latitudes and an order of magnitude lower in the tropics,” the authors write. With minimum ozone levels at the poles, ionizing radiation from the SN could actually end up increasing the ozone column.
“We conclude that these changes in atmospheric ozone probably did not have a major impact on the emerging biosphere on land during the Cambrian,” they conclude.
What about global cooling?
Global cooling would increase, but not to a dangerous degree. Over the Pacific and Southern Oceans, CCN could increase by as much as 100%, which sounds like a lot. “These changes, while climatically relevant, are comparable to the contrast between a pristine pre-industrial atmosphere and a polluted contemporary atmosphere.”
They say it would cool the atmosphere by about as much as we’re warming it now.
The scientists point out that their study concerns the entire biosphere, not individuals. “Our study does not take into account the direct health risks to humans and animals resulting from exposure to increased ionizing radiation,” they write.
Depending on individual circumstances, individuals may be exposed to dangerous levels of radiation over time. Overall, however, the biosphere would hum despite a hundredfold increase in UV radiation. Our atmosphere and magnetosphere can handle it.
“Overall, we find that nearby SNe are unlikely to cause mass extinctions on Earth,” the authors write.
“We conclude that our planet’s atmosphere and geomagnetic field effectively shield the biosphere from the effects of nearby SNe, which enabled the development of life on land during the last hundreds of millions of years.”
This study shows that Earth’s biosphere will not suffer significantly if supernova explosions keep their distance.
This article was originally published by Universe Today. Read the original article.