Earth's atmosphere is our best defense against nearby supernovae, study suggests

Earth's atmosphere has long been recognized as a shielding barrier that safeguards life on our planet, facilitating the growth and evolution of complex organisms. Central to this defense system is the ozone layer, which adeptly blocks and absorbs up to 99% of the Sun's harmful ultraviolet (UV) radiation. Additionally, Earth's magnetic field, known as the magnetosphere, provides further protection against potentially harmful cosmic radiation. This was reported by SSPDaily.

While we have relied on these protective layers, it begs the question: How effective are they when faced with the powerful explosions of supernovae?

Research has revealed that roughly once every million years, a massive star explodes within a proximity of 100 parsecs (equivalent to 326 light-years) to Earth. This fascinating insight is possible due to our solar system's location within a sizable region of space known as the Local Bubble.

This expansive area displays significantly lower hydrogen density compared to the surrounding environment, a result of multiple supernova explosions that took place within the last 10 to 20 million years, leading to the creation of this bubble.

Supernovae are known to pose significant hazards, with proximity to these explosive events amplifying their detrimental effects on planets. Scientists have long speculated about possible mass extinctions or significant disruptions to life caused by supernova explosions.

When a supernova occurs, it releases a gamma-ray burst and cosmic rays that can deplete the ozone layer and allow ionizing UV radiation to penetrate the Earth's surface. Furthermore, these explosions can generate an increase in aerosol particles in the atmosphere, subsequently leading to enhanced cloud coverage and global cooling.

In a recent article published in Communications Earth & Environment titled "Earth's atmosphere protects the biosphere from nearby supernovae," lead author Theodoros Christoudias from the Climate and Atmosphere Research Center at the Cyprus Institute in Nicosia, Cyprus, explores the impact of supernovae explosions on Earth. The study demonstrates that Earth's protective mechanisms offer greater resilience than previously recognized, shielding us against nearby supernovae within a 100 parsec range.

While previous studies have attempted to model Earth's atmosphere and its response to proximity to a supernova explosion, Christoudias and his colleagues have refined this research. Utilizing the Earth System Model with Atmospheric Chemistry (EMAC) model, they simulated and evaluated multiple dynamics, including atmospheric circulation, chemistry, and various process feedbacks.

Through these simulations, the authors investigated stratospheric ozone depletion resulting from enhanced ionization, in turn, leading to ion-induced nucleation and particle growth for cloud condensation nuclei.

For their representative nearby supernova, the researchers assumed galactic cosmic ray (GCR) ionization rates in the atmosphere that were 100 times normal levels. This corresponds to a supernova explosion located approximately 100 parsecs or 326 light-years away.

In the authors' analysis, they note that "the maximum ozone depletion over the poles is less than the present-day anthropogenic ozone hole over Antarctica, which amounts to an ozone column loss of 60–70%." Conversely, there is a limited increase in ozone concentrations within the troposphere; however, these levels remain within the range attributed to recent human-induced pollution.

Ultimately, considering the concern for Earth's biosphere, the maximum average stratospheric ozone depletion resulting from 100 times higher ionizing radiation than usual, as representative of a nearby supernova, is projected to be around 10% globally. This reduction is comparable to the impact of human-induced pollution and thus is not expected to significantly impact the biosphere.

The authors further explain, "Although significant, it is unlikely that such ozone changes would have a major impact on the biosphere, especially because most of the ozone loss is found to occur at high latitudes."

The team then considers Earth's atmosphere during the pre-Cambrian period, characterized by an oxygen content of approximately 2%. They simulated conditions for a 2% oxygen atmosphere since it would likely represent circumstances in which the developing land biosphere would be more sensitive to ozone depletion. The findings indicate ozone loss of about 10%-25% at mid-latitudes and an even lower impact within the tropics. At its lowest levels around the poles, ozone depletion caused by ionizing radiation could potentially lead to a slight increase in the ozone column. Based on their assessments, the researchers conclude that such atmospheric ozone changes would not have had a major impact on the emerging land biosphere during the Cambrian period.

As for global cooling, the study predicts a moderate increase in this phenomenon. Over the Pacific and Southern Oceans, cloud condensation nuclei are projected to multiply by up to 100%. The researchers liken these changes to the discernible disparity between the pristine pre-industrial atmosphere and the more polluted present-day atmosphere, indicating a consequential cooling effect comparable to our current global heating trend.

While the study presented a comprehensive analysis of the biosphere as a whole, it did not delve into the specific health risks posed to humans and animals by elevated ionizing radiation exposure. Individual circumstances may result in dangerous radiation levels over time. However, the study underscores that, overall, Earth's biosphere would largely sustain its functionality despite a 100-fold increase in UV radiation. It is perhaps reassuring to know that our atmosphere, in concert with the protective influence of the magnetosphere, can effectively withstand such circumstances.

The authors succinctly express their conclusion, stating, "Overall, we find that nearby SNe [supernovae] are unlikely to have caused mass extinctions on Earth. We conclude that our planet's atmosphere and geomagnetic field effectively shield the biosphere from the effects of nearby SNe, which has allowed life to evolve on land over the last hundreds of million years."

This comprehensive investigation reaffirms the crucial role played by Earth's atmospheric defenses, assuring us of our biosphere's safety as long as supernova explosions remain sufficiently remote.