As a star exhausts its fuel, it succumbs to gravitational forces and collapses. When a star over eight times the mass of our sun collapses, it can result in a supernova, a tremendous explosion that releases more energy in just a few seconds than what the sun produces over 10 billion years.

During a supernova explosion, high-energy particles known as Cosmic Rays of Galaxy and a violent outpouring of electromagnetic waves, referred to as Gamma rays, are generated. These emissions are termed Ionizing radiation because they dislodge electrons from the molecules they encounter, resulting in ionization. This process can devastate everything from biomolecules like DNA to atmospheric particles like aerosol. Consequently, researchers believe that supernovae pose significant threats to nearby life forms.

While humans have not witnessed a supernova explosion close to Earth, our ancestors may have been less fortunate. A nearby supernova could eject radioactive elements encapsulated in interstellar dust grains, which can travel through the solar system and eventually reach Earth. Geologists have traced these grains in marine mud over the last 10 million years and estimate that a supernova has likely exploded within 100 parsecs of our planet in the last million years. The Earth is positioned about 8,000 parsecs from the center of the Milky Way, making these stellar explosions relatively close in cosmic terms.

Historically, scientists have speculated that nearby supernovae may have influenced animal diversity by contributing to mass extinction events over the past 500 million years. Some researchers propose that cosmic rays emitted from supernovae could potentially deplete the Earth’s ozone layer every hundred million years, exposing surface dwellers to harmful UV radiation. Others suggest that ionizing radiation can interact with aerosols to form clouds that block sunlight. However, scientists remain divided on the extent of ozone depletion, how severe a supernova’s impact could be, its effects on climate, and how catastrophic it might be for the biosphere.

Recently, researchers have revisited the potentially destructive impact of nearby supernovae using models that simulate interactions among planetary atmospheres, oceans, land, and biospheres. Earth system models employ atmospheric chemistry frameworks, such as EMAC, to capture complex processes previously overlooked, including air circulation and chemical reactions. Specifically, EMAC utilizes data from outdoor experiments conducted by CERN to calculate how ions interact with aerosol particles.

The research team modeled the Earth as it exists today, with 21% atmospheric oxygen, normal radiation levels, and an intact ozone layer. They simulated an explosion of ionizing radiation equivalent to a supernova 50 parsecs away, increasing the gamma rays in their model tenfold for a few seconds and boosting cosmic rays in the galaxy by a factor of ten per annum.

The team investigated the effects of ionizing radiation bursts on the ozone layer. Their findings confirmed that ionizing radiation strips electrons from atmospheric nitrogen and oxygen atoms, leading to the formation of highly reactive molecules known as radicals, which can destroy ozone. However, they discovered that certain reactions occurred at slower rates than anticipated, resulting in less ozone depletion than expected. They also found that ionizing radiation interacts with water vapor to produce hydroxyl radicals, which, when combined with nitrogen radicals, actually contribute to ozone formation.

Based on their findings, the team estimated that supernovae could potentially deplete up to 10% of Earth’s ozone layer. This level of ozone loss is comparable to the 6% depletion caused by human-made fluorocarbons and is far from lethal. They repeated the model to account for an Earth with just 2% atmospheric oxygen, simulating conditions around 500 million years ago when life transitioned to land. This modeling revealed repeated UV protection in the ocean, and they found that at this reduced oxygen concentration, only 10% to 25% of the ozone layer was lost.

The team then analyzed how radiation from the supernova influences cloud formation and climate. They calculated that ionizing radiation could increase the number of cloud-forming particles by about 10% to 20% globally. This alteration is quite similar in magnitude to recent anthropogenic warming and could cool the Earth by approximately 2.5 watts per square meter. While they acknowledged that these changes might disturb the environment, they believe it wouldn’t lead to sudden extinction.

The researchers concluded that radiation from nearby supernovae is unlikely to trigger mass extinction events on Earth. Since our early ancestors first emerged, the atmosphere has functioned as a protective barrier, safeguarding us from immediate harmful effects. Nevertheless, they cautioned that their model does not account for the risks associated with long-term exposure to elevated levels of ionizing radiation, which remains largely unexplored. They suggested that future research should seek safe methods to investigate the direct impacts of cosmic radiation on humans and animals.

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