Life's lottery: a burst of life or death ray

September 7, 2001

Theories of Earth's demise from the BA Festival of Science in Glasgow, September 3-7

Cosmic blasts of energy could cause mass extinctions and devastate Earth. Lorraine Hanlon reports: Cosmic blasts, heatwaves, floods and symmetrical jelly?

The universe is a dangerous place for life. Earth is still the only planet we know of that has successfully survived bombardment by intergalactic bullets, grenades and nuclear bombs, and still managed to raise a civilised, if somewhat dysfunctional, family in the process.

The threats are legion. The mass extinction that eliminated the dinosaurs 65 million years ago may have been caused by a cometary impact that plunged the planet into a chilling nuclear winter.

The presence of a rare radioactive isotope of iron, found in deep-sea core samples, points to a supernova explosion 100 light years away being the culprit for a mini-extinction 5 million years ago. There have been four other mass extinctions and several mini-extinctions. Were they also caused by extraterrestrial monsters?

Of all the monsters in the universe, the scariest by far are the gamma-ray bursts - created, many astrophysicists say, when massive stars collapse to form black holes. They blast their energy in a matter of seconds in the form of gamma rays, the most energetic form of electromagnetic radiation, and leave behind only the faintest cooling embers. Because their vast energy pours out mostly in gamma and X-rays that can penetrate matter easily, we can see these bursts clearly, even when they occur in galaxies at the far reaches of the universe.

Astrophysicists are optimistic that the bursts will prove to be extremely powerful tools that could be used to study the very early universe, when the first stars were forming. But what are the prospects if a burst were to explode closer to home - say in the Milky Way?

Roughly two bursts occur every day. Given that there are about 100 billion galaxies in the universe and that bursts occur in galaxies, we can estimate that about one gamma-ray burst happens per galaxy every 100 million years or so.

But the bursts that occur in our own galaxy could still be very far away from us - up to thousands of light years. Surely we would be safe from any danger in that case?

One number we need to answer that question is the total amount of energy that the burst produces. A reasonable estimate is about 10^44 joules - about the same energy as in a supernova. To put this number in context, a 100-megaton nuclear bomb requires the conversion of about a pound of matter into pure energy. Generating 10^44 joules is equivalent to converting the entire mass of Jupiter into pure energy.

Supernovae put out their energy over weeks or months in less harmful forms. Bursts, however, discharge energy - almost exclusively as X and gamma rays - in seconds. The damage that a burst can inflict is considerably more substantial than that that would be caused by a supernova occurring at the same distance.

A burst 30,000 light years from Earth - which has a high probability of happening over, say, 100 million years - would have considerable effects on earth with, for example, a 10 per cent reduction in ozone and increases in acid rain.

While a burst a mere 100 light years from Earth - which is a long shot (although the odds of this happening are higher than your chances of winning the lottery) - would destroy the ozone layer, cause global cooling, decrease cloud cover and lead to drought. It would result in mass extinction and total devastation.

Some scientists believe that bursts also accelerate charged particles called cosmic rays towards Earth, which could have additional devastating consequences. Cascades of ionising radiation down to sea level would exceed the radiation dose for 50 per cent mortality of humans. Ionising particles could penetrate hundreds of metres below sea level causing extinction of marine species.

However, despite their destructive potential - it has been suggested that bursts are responsible for sterilising environments and inhibiting the development of life in other solar systems - my colleague Brian McBreen and I have proposed that a burst occurring at an earlier stage in the evolution of our solar system could actually have advanced the formation of rocky planets, including our own.

We suggest that intense X and gamma irradiation by a burst in 300 light years of the pre-solar nebula, that is just as the sun was forming, would have caused dust balls to melt in seconds, cooling slowly to form chondrules - the millimetre-sized particles that are found in many meteorites. The rapid creation of chondrules by this mechanism could then have precipitated the formation of the rocky planets.

It is highly likely that bursts have influenced, for good or ill, the formation of many solar systems in the galaxy at some stage in their evolution.

Many of the numbers assumed here are uncertain. However, I have been conservative in my choices. The risk to Earth from a burst according to these calculations is less than that from a cometary impact. However, without a full census of bursts and much deeper study of their properties, we cannot fully know the true risk.

Lorraine Hanlon is a lecturer in the department of experimental physics, University College Dublin.

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