The surface of the Earth was bathed in life-damaging radiation from nearby supernovae on several different occasions over the past nine million years. That is the claim of an international team of astronomers, which has created a computer model that suggests that high-energy particles from the supernovae created ionizing radiation in Earth’s atmosphere that reached ground level. This influx of radiation, the astronomers say, potentially changed the course of the Earth’s climate and the evolution of life.

Earlier this year, two independent teams of astronomers published evidence that several supernovae had exploded some 330 light-years from Earth. Each event showered the solar system in iron-60, an overabundance of which has been found in core samples from the bottom of the Atlantic, Pacific and Indian oceans – and also in deposits on the Moon.

Iron-60 isn’t all that supernovae produce. The exploding stars produce lots of light, as well as cosmic rays, which are composed of high-energy electrons and atomic nuclei. Previous work, in particular by Neil Gehrels of NASA’s Goddard Space Flight Center, indicated that a supernova would have to explode within 25 light-years of Earth to give our planet a radiation dose strong enough to cause a major mass extinction. In contrast, supernovae several hundred light years away were not thought to be capable of affecting either life or Earth’s atmosphere.

Piercing cosmic rays

Now, a team led by Brian Thomas of Washburn University in the US argues that this conclusion is incorrect. The researchers looked at what would happen if a supernova exploded at a distance of 325 light-years and worked-out how its radiation would affect Earth. They found that, other than disrupting circadian rhythms in terrestrial life forms, the blue and ultraviolet light would have no significant effect. However, the cosmic rays accelerated towards Earth by the supernova are a different story. These have energies in the teraelectronvolt (TeV) region and are able to “pass right through the solar wind and Earth’s magnetic field and propagate much further into the atmosphere than cosmic rays normally do”, says Adrian Melott of the University of Kansas, who is part of the team.

It is not the cosmic rays themselves that do the damage to life, but the radiation they create when they interact with the atmosphere. When a cosmic ray strikes an air molecule, it produces a shower of secondary particles that is filled with the likes of protons, neutrons and a strong flux of muons.

Ordinarily this takes place in the upper atmosphere and can be responsible for ionizing and destroying ozone in the stratosphere. However, the supernova cosmic rays are so energetic that they will pass straight through the stratosphere, instead penetrating the lower atmosphere known as the troposphere. There they create showers, which then ionize atoms on or near the ground and up to a kilometre deep into the ocean.

High-energy focus

“Earlier work on supernova damage hadn’t focused on the high-energy regime of the cosmic-ray spectrum,” says Brian Fields, a professor of astronomy and physics at the University of Illinois, who is not part of Thomas’s team. “Adrian, Brian and colleagues have shown that these cosmic rays are important, even though they are less numerous than the ‘garden variety’ cosmic rays at lower energies.”

Today, muons contribute a sixth of our annual radiation dose, but Thomas’s team calculate that a supernova would result in a 20-fold increase in the muon flux that would triple the annual radiation dose of life forms on the planet over the course of several thousand years. This, the team says, is akin to every life form on Earth having a medical CT scan every year. Although this would not wipe out life on Earth, statistically it may have caused some “minor” mass extinctions, as well as cell mutations that potentially sparked a burst in evolution.

There is evidence for a minor mass extinction around 2.59 million years ago, but it is too early to say how strongly it was linked to a nearby supernova that is believed to have occurred around that time. Nevertheless, the team suggests that the range at which supernovae can do damage perhaps needs to be widened. “The cosmic rays increase the ‘kill radius’ for serious biological damage,” says Fields.

Climate change

The cosmic rays may have also changed Earth’s climate. The most recent batch of supernovae came just before Earth entered a series of ice ages, at the end of which paved the way for humans to emerge. One possible link between the supernovae and climate is that muons in the lower atmosphere affected cloud cover, thereby cooling the planet.

“When ionization takes place down in the troposphere, where all our weather occurs, what will that do to our weather and climate?” asks Melott. “I’m not going to claim it causes ice ages, but it’s a possibility that needs to be investigated.”

The next step, says Melott, is to scour the geological record, searching for any evidence that supernovae really did have an effect, while further refining the models describing the propagation of cosmic rays from supernovae through space.

The research is described in The Astrophysical Journal Letters.