MARK GARLICK / SCIENTIFIC PHOTO LIBRARY
In February 2020, Betelgeuse, a prominent star 642 light years from the constellation Orion, began to darken, suggesting she was dying. Astronomers’ telescope observations and computer simulations have revealed the real culprit: a roving dust cloud that temporarily crossed in front of the star. When Betelgeuse eventually runs out of fuel and enters the supernova phase of a star’s life, it will generate a brilliant spectacle of stellar fireworks in the night sky.
Astronomers estimate that a handful of stars in our galaxy become supernovas every century. Throughout Earth’s history, it’s likely that some of these stellar explosions were close enough to cause catastrophic damage to our planet and, as some researchers believe, potentially alter the evolutionary trajectory of life. The accusation sparked skepticism, but rekindled debate about life’s sensitivity to cosmic influence.
The explosion of a near-Earth supernova has the potential to trigger a cascade of events that would have a dramatic impact on our planet. Visible light from the supernova would reach Earth first, and the exploded star would appear to shine as bright as the moon for months. While ultimately not harmful to humans, it could be bright enough to alter the biological systems of nocturnal animals, says Adrian Melott, Ph.D., an astronomer at the University of Kansas, Lawrence.
Shortly after the appearance of the first light from a supernova, a shock wave filled with cosmic rays, an amalgamation of high-energy particles, would begin its course towards Earth. “The star’s bowels are launched into space at speeds that are a few percent of the speed of light,” says Brian Fields, Ph.D., astronomer at the University of Illinois Urbana-Champaign . This wave radiates through space, sweeping gas and other interstellar material like a cosmic snowplow. It could take thousands of years for these rays to reach Earth because their trajectory is influenced by the magnetic fields they encounter. If their path is free of magnetic field lines, they will travel in a straight line, says Melott.
I spy: Iron-60
A telltale sign of a near-Earth supernova is the presence of the radioactive isotope Iron-60. The isotope, which is carried to Earth by the gaseous remnants of these exploded stars, has a half-life of millions of years, which means it must have arrived on Earth long after our planet was formed. Traces of iron-60 have been found in rock crusts torn from the seabed, in Antarctic snow, and even in lunar soil collected during the Apollo missions.
The Earth’s atmosphere supports the weight of these charged particles, according to a 2016 study in Letters from astrophysical journals, led by Melott and colleagues. They suggest that cosmic rays generated by a supernova 300 light years away pass through nitrogen molecules in the air, generating nitrogen oxide compounds that can rain and fertilize vegetation. This would cause the plant life on Earth to engulf carbon dioxide from the atmosphere and cool the climate.
Excess nitrogen oxide in the atmosphere could also remove up to 7% of the Earth’s ozone layer, according to the study. Erasing this protective shield would subject animals and plants to sun damage, potentially altering the food web for thousands of years. “You and I would put on a hat and sunscreen, but if you’re a phytoplankton you don’t have that option and just cook,” says Fields.
Additionally, when cosmic rays pass through Earth’s atmosphere, they generate secondary particles called muons, which are similar to electrons but heavier. “[Muons] can descend to the ground and even below the ground, ”says Fields. “You can’t hide from them. These muons would subject animals on the Earth’s surface to three times the normal amount of radiation.
It is more difficult to scour the planet for geological evidence of a near-Earth supernova than, for example, to search for an asteroid crater five miles wide. Yet researchers like Melott and Fields scour geological records for cases where supernovae may have played a role in shaping Earth’s environment and the evolution of life on Earth.
Last year, for example, a team of researchers studying the fossilized leaves of a notable extinction event at the end of the Devonian, some 359 million years ago, found evidence of deformed plant spores, suggesting that these plants might have absorbed excessive amounts of ultraviolet radiation. Fields and his colleagues argued in a subsequent article published in the Proceedings of the National Academy of Sciences last September that the increased radiation could be the result of an ozone-free Earth.
And in a study published in The Journal of Geology Last year, Melott and colleagues suggest that widespread wildfires – possibly caused by cosmic-ray-induced lightning – helped our early human ancestors move from forests to the savannah and embrace bipedalism in the early days. from the Pleistocene, 2.5 million years ago. Deposits of the radioactive isotope Iron-60 (see box) found on Earth and on the Moon appear to correspond to this time.
Fields admits that more evidence is needed to understand exactly what role these stellar explosions may have played in mapping our evolutionary path. Identifying the exact causes of changes on a global scale in the geological record is a difficult task.
One thing is certain, however: Earth is safe from future supernovae. Of the stars in our galaxy that are nearing the end of their lifecycle and may become a supernova in the near future, none of them are likely to cause catastrophic damage. At the most, says Fields, they’ll simply provide a gripping spectacle.
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