Gamma-Ray Wipe-Out

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How a gamma-ray burst could exterminate life on Earth

The Ordovician extinction occurred around 440 million years ago, when Earth was a water-world teeming with life. Lieberman describes the planet in those times: “If an alien from outer space was visiting they would probably characterise life on Earth as dominated by trilobites.” Trilobites were a highly diverse family of crab-like creatures. In the earliest mass extinction event, they died out - along with two-thirds of all other species.

For years, scientists thought that this wipe-out was caused by a global ice age. However, there were problems with this hypothesis: there was seemingly no trigger to this sudden big freeze. Lieberman was unconvinced that an ice age alone could have killed so many species: “We have had other glaciations in the geological past that do not produce mass extinctions on scale that we see at the end of the Ordovician. So that’s why we can’t treat it as a smoking gun.”

Working together, Melott and Lieberman pieced together an alternative theory, trying to work out what might have happened. They believe that, around 440 million years ago, a supernova within 10,000 light years of Earth exploded in a gamma-ray burst. The radiation destroyed the composition of the Earth’s atmosphere, ripping apart the nitrogen and oxygen molecules. The planet became enveloped in brown smog - made up of lethal nitrogen dioxide - which blocked out the Sun’s heat. Though the gamma ray burst lasted just a few seconds, the resulting smog triggered an ice age.

The nitrogen dioxide cloud blocked the Sun’s infrared rays, but it didn’t obstruct the shorter wavelengths of the ultraviolet. This was doubly bad news for the Earth, because the gamma-rays had also produced nitrogen oxide which destroyed the ozone layer. As a result, our planet’s surface had no protection from a 300% increase in UV radiation. The Sun’s deadly ultraviolet radiation ripped through the DNA of every living creature. Only those living deep underwater stood a chance of surviving.

Melott believes that the Earth has probably been fried by gamma ray bursts several times in its 4.5 billion year history. Could it happen again? Melott thinks so: “It could happen tomorrow or it could be in millions of years.” Is it worth worrying about? No. Gamma-ray bursts hit without warning. We can’t predict when or where they come from. And we can’t do a thing to stop them.

For more information

Hooper Virtual Paleontological Museum - mass extinctions
http://park.org/Canada/Museum/extinction/tablecont.html

NASA - the low-down on gamma-ray bursts
http://imagine.gsfc.nasa.gov/docs/science/know_l1/bursts.html