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Supernova To Blame? Nearby Explosion Might Have Caused Mass Extinction On Earth

A supernova explosion hundreds of light-years away from our planet may have played a role in a minor mass extinction that occurred about 2.59 million years ago, a new study suggests.

The exploding star blasted out rapid-moving charged particles known as cosmic rays that may have led to a die-off at the end of the Pliocene era and the start of the Pleistocene, researchers say.

Adrian Melott, co-author of the study and an expert from the University of Kansas, says that at that time, Africa dried out and a lot of its forests turned into savannah. Afterwards, glaciations or ice ages began to happen over and over again, although it is not clear why.

"It's controversial," says Melott. "But maybe cosmic rays had something to do with it."

Focusing On Supernovas

Supernovas can transpire in two ways: when a star that is much more massive than our sun runs out of fuel and dies, or when a white dwarf takes so much matter from a neighboring star that it reaches a limit and explodes.

In the new study, Melott and his colleagues created computer models that determined how supernovas might affect the biosphere and climate of Earth.

Scientists were especially interested in supernovas that occurred only about 300 light-years away from Earth because they think such events flared up twice recently.

One supernova explosion may have occurred about 6.5 million to 8.7 million years ago, while another occurred 1.7 million to 3.2 million years ago. The latter may be linked to the Pliocene-Pleistocene mass extinction, researchers say.

Intense Light And Cosmic Radiation

Melott says he was expecting to find very little effect at all because 300 light-years is not near enough. But the results of the study were quite surprising.

Their calculations suggest that supernovas from 300 light-years away bathed the night sky in blue that was so intense and bright that the sleep patterns of prehistoric animals were disrupted for weeks.

More importantly, a surge of cosmic radiation possibly hit terrestrial organisms and those in the shallow parts of the ocean.

"The big thing turns out to be the cosmic rays," says Melott.

High energy cosmic rays are "pretty rare," he says, but they get increased by quite a lot for a few hundred to thousands of years by a factor of a few hundred.

Melott says high energy cosmic rays can penetrate the Earth's atmosphere by tearing up molecules and ripping electrons off atoms. This goes on right down to the ground level, but it typically only happens at high altitudes.

End Result

When cosmic radiation penetrated the atmosphere, the outcome was probably a three-fold increase of the radiation dose at ground level, scientists say.

The level of radiation may have been sufficient to elevate mutation and cancer rates, but not too much, says Melott. Still, if the mutation rate was increased, it might speed up evolution.

Furthermore, the cosmic radiation from the supernova explosion may have affected the climate on Earth.

The simulations indicate that the high influx of cosmic rays allowed the molecules and atoms in the lowest layer of atmosphere to get a negative or positive charge known as "tropospheric ionization."

This process was estimated to have lasted for 1,000 years. Researchers add that the phenomenon could trigger climate change, particularly if there was instability.

Details of the new study are published [PDF] in the Astrophysical Journal Letters.

Photo: NASA Goddard Space Flight Center | Flickr

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