Can a gamma-ray burst cause a mass extinction? 

440 million years ago in the late Ordovician period, a mass extinction wiped out 85% of marine species. Most research indicates that glaciation was to blame, since it looks like the extinction was associated with a cooling climate and sea level decline, but a handful of scientists have a different idea: What if it was caused by a gamma-ray burst? 

A gamma-ray burst (GRB) is an intense flash of high-energy radiation thought to result from the collapse of a massive star. If one occurred close enough to Earth (within a radius of about 3,000 light years), the radiation could severely damage our biosphere. One study estimates that a GRB could occur within this radius twice every billion years. 

Gamma-ray bursts would be harmful because they would expose the Earth to highly energetic photons. When these photons collide with our atmosphere, they break apart molecules there, including the molecular oxygen that makes up the ozone layer - and destroying ozone allows damaging UV radiation in. The energetic photons would also produce nitrogen dioxide in the atmosphere, blocking out sunlight and causing global cooling, which would be lethal to photosynthesising organisms. 

Image Credit: Wikimedia

The damage a GRB can do is fairly consistent with what we know about the Ordovician extinction, more notably the fact that species who lived in environments more exposed to UV radiation seemed to suffer more severely. The surviving fauna of the extinction seem to have come from deeper water, where UV radiation can’t penetrate effectively or from higher latitudes. 

But currently, we don’t have any evidence that a nearby gamma-ray burst actually happened - so maybe a GRB alone might not be sufficient to completely explain the extinction. 

(By the way, recent studies have shown that gamma-ray bursts just aren’t that common in our galaxy anymore, because most of the massive stars that would collapse and produce GRBs have already done so - so don’t get too worried about GRBs causing a mass extinction in our lifetime!)  

Want to hear more about gamma-ray bursts and their threat to Earth? Have a listen to this UniverseToday podcast.