Exploding star strafed Earth

A supernova may have caused mass extinction two million years ago.

The explosion of a dying star could have ended much of marine life on Earth two million years ago. The supernova could have strafed the Earth’s atmosphere with cosmic rays, severely damaging the ozone layer and exposing living organisms to high levels of the Sun’s hazardous ultraviolet rays, US researchers propose¹.

This idea dates back to the 1950s, but now Narciso Benítez of Johns Hopkins University in Baltimore, Maryland, and colleagues have come up with the first plausible evidence. Their proposal remains tentative, but is consistent with what is known about the likelihood of nearby stellar explosions and the telltale signatures of these events on our planet.

Supernovae are the death throes of large stars. When such stars run out of fuel for nuclear fission, they collapse under their own gravity, heat up rapidly and explode, releasing huge amounts of matter and energy. Fortunately for us, such outbursts are rare: the most recent one in our own galaxy was spotted in 1604, and was too far away to pose any risk. Several supernovae have been observed more recently in other galaxies.

One-fifth of all supernovae occur in large groups of relatively young stars that are thought to have coalesced from the same gas cloud. One such cluster in our own galaxy is the Scorpius-Centaurus association, which comprises three subgroups of stars.

Each subgroup would have generated supernovae at different times: about 10, 7 and 2 million years ago, say Benítez’ group. The most recent episode could have included a supernova as little as 130 light years from Earth. This is not close enough to fry our planet. But it would have left a mark that the researchers think has already been found.

Three years ago, scientists in Germany reported high concentrations of iron-60 in two layers of ocean rock, dated at about 0-3 and 4-6 million years old2. Iron-60 is a rare form produced on Earth by nuclear reactions involving cosmic rays, such as those that supernovae generate.

Putting these arguments together, Benítez’ team proposes that the most recently formed iron-60 layer could be the result of a nearby supernova in the Scorpius-Centaurus association around two million years ago. The supernova could have left another signature – in the fossil record.

Two million years ago, many marine creatures, such as bivalve molluscs, died out suddenly all over the planet. As mass extinctions go, this was a mild one. But no one knows what caused it.

Benítez and his colleagues think that a nearby supernova at this time could have showered the Earth with cosmic rays. These charged subatomic particles collide with atoms in the air, initiating chemical reactions. Copious cosmic rays are thought to produce nitrogen monoxide, which can destroy ozone molecules.

The researchers calculate that a supernova 130 light years away could have thinned the ozone layer by up to 60 per cent, exposing marine organisms to ultraviolet rays from the Sun. This could have killed off plankton, and thence the molluscs that live off them.

To support this hypothesis, astronomers now need to find the smoking gun: remnants of ancient supernovae in nearby star clusters.

References

1. Benítez, N., Maíz-Apellániz, J. & Canelles, M. Evidence for nearby supernova explosions. Physical Review Letters, 83, 081101, (2002).
2. Knie, K. et al., Indication for supernova produced 60Fe activity on Earth. Physical Review Letters, 83, 18 – 21, (1999).