According to geological evidence on Earth, a supernova occurs once every million years within a radius of 300 light years. Such massive starbursts lead to sharp increases in gamma and cosmic radiation. This could have led to mass extinctions on Earth. However, a research group with Austrian participation considers this unlikely. Their simulations show that Earth’s atmosphere protects the biosphere from the effects of a supernova.

Massive stars end in a big bang. In such supernovae, heavy chemical elements are formed and thrown into space. The detection of the iron isotope Fe-60 in oceanic sediments and the presence of a local gas- and dust-poor bubble in the Milky Way suggest that several supernovae have occurred within the past ten million years, about 300 light-years away. from earth.

Are supernovae responsible for mass extinctions?

Because such stellar explosions are associated with intense bursts of gamma rays and massive increases in cosmic rays, there is a theory that supernovae are capable of causing mass extinctions. “There are two consequences in how a supernova can affect the atmosphere – and thus the Earth’s climate and life,” Dominik Stolsenburg from the Department of Physical Chemistry of the Atmosphere at the Technical University of Vienna (TU) explained to APA.

On the one hand, increased radiation protection can lead to the depletion of stratospheric ozone, which leads to higher levels of UV light exposure. On the other hand, radiation can lead to an increase in aerosols in the atmosphere. This leads to more nuclei, in which water vapor condenses, resulting in denser cloud cover and a cooling climate.

The Earth system model aims to simulate supernova impacts

Stolsenburg, along with Theodoros Christotias of the Cyprus Institute and colleagues from the CLOUD experiment at the CERN European Laboratory for Particle Physics near Geneva, used a complex Earth system model to simulate the effects of a nearby supernova on the atmosphere and climate. . In their work, published in the journal “Nature Communications Earth and Environment,” they hypothesized that stellar explosions increase cosmic radiation by a factor of 100.

“We are not biologists, and our study does not take into account direct health risks to humans and animals as a result of increased ionizing radiation,” emphasized the atmospheric chemist. Overall, the simulations show that it is unlikely that a nearby supernova would cause mass extinction on Earth.

Currently, the average annual equivalent dose from all natural sources of ionizing radiation is 2.4 millisieverts (mSv), of which 0.35 mSv comes from cosmic radiation. If this were increased 100-fold due to a near-Earth supernova, that would mean radiation exposure of 35 mSv per year, which would correspond to a lifetime dose of 2 Sv in the tropics and mid-latitudes, the researchers write in their paper. Paper.

Missing data

Data on long-term exposure at this level are limited, with 1–10 Sv for mammals and birds, 10–100 Sv for reptiles and fish, and 1,000–10,000 Sv when 50 percent of the population dies; For bacteria and insects. “This means that a nearby supernova has consequences for life on Earth and individual individuals fall victim to high levels of radiation, but not all life on Earth is destroyed,” Stolsenburg said.

In particular, the just-published study shows that the effects on atmospheric chemistry are sometimes smaller than assumed. According to the new calculations, depletion of stratospheric ozone due to more intense cosmic radiation is moderate and similar in magnitude to current ozone loss in the polar regions due to anthropogenic emissions. The increase in aerosols is unlikely to be strong due to the formation of highly charged particles in the atmosphere. More clouds form, which leads to cooling. “At 2.4 watts per square meter, that’s about the same magnitude as current climate change — only with the opposite sign,” Stolsenburg said. It’s not enough to trigger a mass extinction.

Overall, the researchers found that “the planet’s atmosphere and geomagnetic field effectively shield the biosphere from the effects of a stellar explosion,” Stolsenburg said. Thus, life on Earth was able to evolve for hundreds of millions of years, despite nearby supernovae.

Service: Website: https://doi.org/10.1038/s43247-024-01490-9