Can nearby supernova explosions threaten life on Earth?


Earth’s protective atmosphere has supported life for billions of years, creating a refuge where evolution has produced complex life forms like ours.

The ozone layer plays a vital role in protecting the biosphere from deadly UV rays. It blocks 99% of the Sun’s powerful UV rays. The Earth’s magnetosphere also shelters us.

But the Sun is relatively docile. How effective are ozone and the magnetosphere in 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 within 100 parsecs (326 light years) of Earth. We know this because our solar system is inside a massive bubble in space called the local bubble.

This is a cavernous region of space where the density of hydrogen is much lower than that outside the bubble. A series of supernova explosions over the previous 10 to 20 million years widened the bubble.

Supernovae are dangerous, and the closer a planet is to one, the more deadly its effects. Scientists have speculated about the effects of supernova explosions on Earth, wondering whether they triggered mass or at least partial extinctions.

The gamma-ray burst and cosmic rays from a supernova can deplete Earth’s ozone layer and allow ionizing UV rays to reach the planet’s surface. These effects can also create more aerosol particles in the atmosphere, increasing cloud cover and causing global cooling.

A new research article in Nature Communications Earth and Environment examines supernova explosions and their effects on Earth. It’s called “Earth’s atmosphere protects the biosphere from nearby supernovae.”

The lead author is Theodoros Christoudias of the Climate and Atmospheric Research Center at the Cyprus Institute in Nicosia, Cyprus.

The local bubble is not the only evidence of nearby core-collapse supernovae (SNe) over the past few million years. Ocean sediments also contain 60Fe, a radioactive isotope of iron with a half-life of 2.6 million years.

SNe expel 60Fe in space when they explode, indicating that a nearby supernova exploded about 2 million years ago. There is also 60Fe in sediments that indicate another SN explosion about 8 million years ago.

This graphic from the research article shows the potential atmospheric and climate impacts of a nearby supernova.  Gamma rays can deplete the ozone layer, allowing more UV rays to reach the Earth's surface.  Some UV rays are ionizing, meaning they can damage DNA.  Cosmic rays can also create more condensation nuclei, meaning more clouds and potential global cooling.  Image credit: Christoudias et al.  2024
This graphic from the research article shows the potential atmospheric and climate impacts of a nearby supernova. Gamma rays can deplete the ozone layer, allowing more UV rays to reach the Earth’s surface. Some UV rays are ionizing, meaning they can damage DNA. Cosmic rays can also create more condensation nuclei, which means more clouds and potential cooling of the planet. (Christoudias et al., 2024)

The researchers correlated an explosion of SN with the Late Devonian extinction around 370 million years ago. In one paper, researchers discovered plant spores burned by UV light, indicating that something powerful has depleted Earth’s ozone layer.

In fact, terrestrial biodiversity declined for about 300,000 years before the Late Devonian extinction, suggesting that multiple SNe may have played a role.

The ozone layer on Earth is constantly changing. When UV energy hits it, it breaks down ozone (O3) molecules. This dissipates the UV energy and the oxygen atoms combine again into O3. The cycle repeats itself.

This is a simplified version of the atmospheric chemistry involved, but it serves to illustrate the cycle. A nearby supernova could disrupt the cycle, reducing the density of the ozone column and allowing more deadly UV 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 and provides sufficient protection against SNe within a 100 parsec radius.

While previous researchers have modeled the Earth’s atmosphere and its response to a nearby SN, the authors claim to have improved on this work.

They modeled the Earth’s atmosphere with an Earth Systems Model with Atmospheric Chemistry (EMAC) to study the impact of nearby SNe explosions on the Earth’s atmosphere.

Using EMAC, the authors claim to have modeled “the complex atmospheric circulation dynamics, chemistry, and process feedbacks” 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 into CCNs” (cloud condensation nuclei).

“We assume a nearby representative SN with GCR (galactic cosmic ray) ionization rates in the atmosphere that are 100 times higher than current levels,” they write. This corresponds to a supernova explosion about 100 parsecs or 326 light years away.

These panels from the research letter show the decrease in column ozone percentage versus a 100-fold increase in GCR intensity from nominal.  The left vertical axis represents the Earth's latitude and the X axis represents the time of year.  Ozone loss is more pronounced at the poles due to the effect of Earth's magnetosphere, where it is weaker.  a is present-day Earth, while b represents an ancient Earth with only 2% oxygen in the Precambrian.  Image credit: Christoudias et al.  2024
These panels from the research letter show the decrease in column ozone percentage versus a 100-fold increase in GCR intensity from nominal. The left vertical axis represents the Earth’s latitude and the X axis represents the time of year. Ozone loss is more pronounced at the poles due to the effect of Earth’s magnetosphere, where it is weaker. a is present-day Earth, while b represents an ancient Earth with only 2% oxygen in the Precambrian. (Christoudias et al., 2024)

“The maximum ozone depletion over the poles is less than the current anthropogenic ozone hole over Antarctica, which is equivalent to a column ozone loss of 60-70%” , explain the authors.

“On the other hand, there is an increase in ozone in the troposphere, but it remains well within the limits resulting from recent anthropogenic pollution.”

But let’s get straight to the point. We want to know whether the Earth’s biosphere is safe or not.

The maximum average stratospheric ozone depletion due to ionizing radiation 100 times higher than normal, representative of near SN, is approximately 10% globally. This is approximately the same reduction as that caused by our anthropogenic pollution. This would not affect the biosphere much.

“Although significant, such changes in ozone are unlikely to have a major impact on the biosphere, particularly because most ozone loss occurs at high latitudes,” the authors explain.

But that’s for modern Earth. In the Precambrian, before life exploded into a proliferation of forms, the atmosphere contained only about 2% oxygen. How would an SN affect this?

“We simulated a 2% oxygen atmosphere because this would likely represent conditions under which the emerging biosphere on Earth would still be particularly sensitive to ozone depletion,” the authors write.

“Ozone loss is about 10 to 25 percent at midlatitudes and an order of magnitude lower in the tropics,” the authors write. At minimum ozone levels at the poles, ionizing radiation from an SN could actually end up increasing column ozone.

“We conclude that it is unlikely that these changes in atmospheric ozone had a major impact on the emerging terrestrial biosphere during the Cambrian,” they conclude.

What about global cooling?

Global cooling would increase, but not to a dangerous extent. Over 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 polluted atmosphere of today.”

They say it would cool the atmosphere about as much as we warm it now.

These two panels from the research help illustrate the global cooling effect of a nearby SN exposing the Earth to 100 times more ionizing radiation.  b shows the fractional change in CCN from today.  d shows the fractional change in outgoing solar radiation from today due to increased cloud albedo.  Image credit: Christoudias et al.  2024
These two panels from the research help illustrate the global cooling effect of a nearby SN exposing the Earth to 100 times more ionizing radiation. b shows the fractional change in CCN from today. d shows the fractional change in outgoing solar radiation from today due to increased cloud albedo. (Christoudias et al., 2024)

The researchers emphasize that their study concerns the entire biosphere and not individuals. “Our study does not take into account direct health risks to humans and animals resulting from exposure to high ionizing radiation,” they write.

Depending on individual circumstances, individuals could be exposed to dangerous levels of radiation over time. But overall, the biosphere would continue to hum despite a 100-fold increase in UV radiation. Our atmosphere and magnetosphere can handle it.

“Overall, we find that nearby SNe are 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 neighboring SNe, which allowed life to evolve on earth over the past hundreds of millions of years.”

This study shows that Earth’s biosphere won’t suffer much as long as supernova explosions stay at bay.

This article was originally published by Universe Today. Read the original article.



Source link

Leave a Comment

Your email address will not be published. Required fields are marked *

Scroll to Top