Supernova explosions in a local cluster of stars could have caused a wave of extinction on Earth two million years ago, according to Narciso Benítez of Johns Hopkins University in the US and colleagues. The astronomers have calculated that supernova explosions in the ‘Scorpius-Centaurus association’, which is relatively close to Earth, took place at around the time of the extinction. An excess of iron-60 deep in the Earth’s crust – which is old enough to have been deposited by the supernovas – appears to support their theory (N Benítez et al 2002 Phys. Rev. Lett. 88 081101).
Palaeontologists believe that the so-called Pliocene-Pleistocene marine extinction took place after a surge of ultraviolet light entered the Earth’s atmosphere, killing the small organisms that are crucial to the ecosystem of the ocean. Benítez and colleagues now propose that intense cosmic rays from nearby supernovas damaged the ozone layer, allowing ultraviolet radiation from the Sun to stream into the atmosphere.
The team suspected that a well-known astronomical feature known as the ‘Local Bubble’ could explain this event. This local region of hot, low-density gas is about 490 light years across, and astronomers believe that it was created by a series of supernovas. Benítez and colleagues calculated that these explosions took place about two million years ago, when the Scorpius-Centaurus cluster of stars was just 130 light years from Earth. Rotation of our galaxy means that the cluster is now 424 light years away.
Palaeontologists have previously calculated that deposits of iron-60 in the Earth’s crust are two million years old. The iron-60 itself would not have affected life on Earth, but Benítez and colleagues believe that the age of the deposits links them to the supernova bursts in Scorpius-Centaurus, and in turn to the Pliocene-Pleistocene extinction. Previous studies had simply attributed the iron-60 deposits to ‘a background of radioactive iron’ in the Sun’s neighbourhood.
“Many palaeontologists are already considering our theory as a strong alternative to more conventional explanations of the extinction”, Benítez told PhysicsWeb.
Benítez and co-workers are now trying to establish more accurately when the supernovas occurred. To add weight to their theory, they need to show that the explosions took place when the star cluster was in a position to cause maximum damage to the Earth’s atmosphere.