450 million years ago, the Earth's warm oceans teemed with life we'd recognize as seaweed, starfish, clams, and coral reefs. Suddenly, over half these species died. Now scientists say it was caused by cosmic rays - and could happen again.


Image of a proton shower via University of Chicago.

Usually called the Ordovician Extinction, after the geological era when it ended, this mysterious event is well-documented in the fossil record but only hazily understood. Perhaps the most popular theory is that the supercontinent Gondwana drifted south, across the pole, creating massive glaciers that lowered sea levels and destroyed the habitats of creatures used to basking in a greenhouse-like warmth. But astronomer Adrian Melott is working on an alternate theory: That life in the Orodvician was destroyed by a burst of high-energy protons called cosmic rays that ripped apart our ozone layer and bombarded sea life with ultra-violet radiation.


According to The Daily Galaxy:

The biggest effect of cosmic rays on the Earth's atmosphere, says Melott, is on the ozone layer. When radiation events hit the atmosphere-which is 80 percent nitrogen-the bonds between nitrogen atoms break. The nitrogen atoms will then react with anything they can, he explains, and a substantial number of them will react with oxygen molecules or atoms, making oxides of nitrogen, which sets up a chemical reaction that converts ozone back to oxygen. However, stratospheric ozone is a vital shield against ultraviolet light from the sun. So the ozone depletion causes more ultraviolet B light (UVB) than normal to travel through the atmosphere to the ground. "A strong event like a gamma ray burst from a nearby supernova causes even more depletion, resulting in a doubling of the global average UVB," says Melott, "which all the people who experiment on plants and animals with UVB tell us would be a disaster."

Image of cosmic rays via NASA.

Cosmic rays are produced by high-energy events like supernovas. If a star were to explode nearby, Earth would be bomarded by these particles, which would change the molecular composition of our atmosphere - and allow deadly radiation through. Even if there were no explosion, says Melott, it's possible that the Earth is reguarly exposed to cosmic rays as it bobs upward through the galactic disc. Every 60 million years or so, astronomers believe that our planet cycles northward in the galactic plane (for the same reason planets cycle around the sun, only on a much vaster scale). Without the galactic plane's magnetic field shielding us, we might be in for a high dose of radiation. This might be one reason why we see mass extinctions happening with a fair amount of regularity on Earth over the past 500 million years.


Regardless of whether our movement through the galactic plane is to blame, Melott says that it's looking more and more likely that cosmic rays caused the Ordovician Extinction. He says:

We did a bunch of simulations, gamma ray bursts going off at different latitudes, and we found that if one had gone off over the South Pole it would match their data perfectly. From this pattern we could then say the Northern Hemisphere wouldn't be affected very much. We now can predict that if people look at fossils in certain areas that were north of the equator they'll find the extinction event looks different; it won't have the sudden extinction pulse. That's the prediction for the fossil data. We don't have any answers yet, but there are people looking at it.


In other words, humans are not the only force that can destroy the ozone layer and much of life as we know it. Random cosmic rays from outer space will do the job just fine without us.

via The Daily Galaxy