Distant Star's Explosion Blamed for Mass Extinction on Earth

Most all of us have heard the theory that the extinction of the dinosaurs 65 million years ago was caused by a comet or asteroid striking Earth. It's plausible and there's evidence to support the theory. But this extinction was not the only mass extinction in Earth's history. About 440 million years ago the second-largest extinction in the Earth's history, the killing of two-thirds of all species, may have been caused by a supernova perhaps 10,000 light years away.

A supernova happens when a massive star explodes, often becoming a billion times brighter than our Sun. If such an explosion took place within 10,000 light years of Earth, gamma ray bursts from the explosion could have destroyed the chemistry of the Earth's atmosphere, allowing our Sun's ultraviolet rays to cook fragile unprotected life forms.

This particular extinction is called the Ordovician extinction and is named after the geologic period it occurred in. It is the second most severe of Earth's five great periods of extinction. Fossil records from this time show an abrupt disappearance of two-thirds of all species on the planet. The records also show that an ice age began at that time and lasted for more than half a million years.

Previously, scientists had blamed the ice age itself for the mass extinction. More recently astronomers think there may be a far more distant cause. Gamma rays striking the Earth could explain both the extinction and the ice age.

If a gamma ray hit the Earth it would break down molecules in the Earth's upper atmosphere. Nitrous oxide and other chemicals would form which would in turn destroy the ozone layer and shroud Earth in a brown smog.

Earth's sky would be a murky brown and there would be intense ultraviolet radiation striking the surface with no ozone layer to block it. Such radiation might be fifty times above normal: a more than lethal dose for any exposed life forms. At the same time, the brown smog, blocking sunlight would cause Earth to cool, triggering the ice age. Prior to the Ordovician extinction, the Earth was very warm. Climate experts have found no other explanation for sudden onset of an ice age.

During this time, there was only primitive plant life on land, all other life lived in the oceans. Most of the life that became extinct was primitive sea creatures that lived at on near the surface of the water. They would have little protection from the radiation. The animals which lived in deep water survived. The plants too would have been affected both from radiation and the sudden cooling.

Astronomers can pinpoint supernova even millions of years after they occurred. Supernova leave behind clouds of gas, shock waves and black holes that can be detected by telescopes. Supernova aren't all that uncommon events, and the odds are that in the past 500 million years Earth has been hit by gamma rays from such an event. In fact, it's likely that a gamma ray burst could strike Earth every few hundred million years. And that means it will almost certainly happen again. Let's hope if it does happen, it will be far enough in the future that humans have another home to go to!

Constellations

The Solar System