Neutron Star Could Kill Us All, But Perhaps Not Today

(June 26, 1998 2:09 p.m. EDT http://www.nandotimes.com) - As if you didn't have enough to worry about already, it turns out that we're 100 million years overdue for mass extinction.

Keep watching the skies if you want some warning. When you see an eerie blue glow, slightly bigger than a full moon, it means that you've got just a few days before the apocalypse.

The glow is caused by a burst of gamma rays hitting the upper atmosphere. Following close behind is a high-energy jet of deadly cosmic rays. Once they hit the atmosphere, your chances of survival are not good: researchers believe that the last few times cosmic ray jets hit the Earth, they wiped out up to 95 percent of animal life on the planet.

To get more than a few days warning, you'd have to monitor the orbits of all the nearby neutron star pairs. The collapse and merger of these super-dense balls of matter is the source of the lethal cosmic rays. As the pairs of neutron stars circle around each other, their gravitational pull moves them closer together and spins them faster and faster.

Eventually they collapse into each other and form a black hole, releasing energy as a tightly focused beam of cosmic rays. The beam can travel for up to a million light years before losing its power, so any planet in the line of fire had better watch out.

Any starlight that gets in the way of the cosmic ray jet is kicked out in front of it like a ball; this acceleration pumps the photons up to gamma ray energies and makes them travel faster than the cosmic rays -- hence their early arrival at the Earth. Low-intensity gamma ray bursts, thought to be from neutron star mergers in distant galaxies, are detected by astronomers about once a day.

This whole scenario may sound like science fiction, but it's getting the attention of serious researchers. Calculations of the timings of nearby neutron star collapses show that, just like mass extinctions on Earth, they seem to occur about once every 100 million years. Disturbingly, for the inhabitants of Earth, the evidence suggests the last one probably happened 200 million years ago.

Geological records show there have been five major mass extinctions in the past 500 million years.

Scientists believe the most recent one, which wiped out the dinosaurs 64 million years ago, was caused by the impact of a meteorite. Some 300,000 tons of the element iridium was laid down in the Earth's crust at this time, and high levels of iridium have also been found in asteroids.

"What caused all the other extinctions is still an open question," says Arnon Dar, a space physicist at the Israel Institute of Technology. Suggested explanations have included high volcanic activity blocking sunlight and poisoning the atmosphere, and supernova explosions. But there's no geological evidence for coincident volcanic activity, and supernova explosions don't occur close enough at a sufficiently high rate.

Dar was the first to suggest that collapsing neutron stars might be to blame. He and his colleagues have studied the likely effects of the cosmic ray jets flung out by neutron star collapses, and their conclusions, to be published next week in the journal Physical Review Letters, make chilling reading.

Cosmic rays are a very serious threat. Entering the Earth's atmosphere, the jets create showers of lethal high-energy subatomic particles known as muons. As they rain down on the Earth, the muons have enough energy to irradiate and kill almost every living thing in their way.

Dar calculates an "average" muon shower occurrence will give about 100 times the ionizing radiation dose needed for a 50 percent chance of mortality in humans -- in other words, enough to kill everyone. Such an intense dose would destroy the central nervous system, causing death within a couple of days.

The cosmic ray burst can last up to a month, during which time muons would also destroy the ozone layer, irradiate the environment and damage vegetation, severing the food chain. Thanks to the Earth's rotation, and radiation borne on atmospheric winds, the effect would be quickly spread around the globe. The muons also boast massive penetrating power; the radiation can be fatal even hundreds of yards underwater or underground.

"Unlike the other suggested extraterrestrial mechanisms, a lethal burst of atmospheric muons can explain the massive extinctions deep underwater," says Dar. Suddenly, the fossil record's reported extinction of marine life, as well as continental life, begins to make sense.

Dar's doomsday scenario also explains other features of previous mass extinctions that current theories leave to one side. The powerful radiation causes biological mutations that would account for the fast appearance of new species after massive extinctions.

Examination of the fossil record also shows a clear correlation between the extinction pattern of a species and its vulnerability to ionizing radiation.

Insects, for example, have been the great survivors of mass extinctions. According to Dar, this is not surprising as insects can, in general, tolerate up to 20 times the radiation dose that kills most vertebrates. The only time they were severely affected was in the largest mass extinction, 251 million years ago. Even then, only 30 percent of insect species were destroyed, compared with up to 95 percent of other orders of species.

Dar is keen to point out that although things might look bad, being 100 million years overdue for an apocalypse doesn't make it any more likely to happen today. "The chance of extinction doesn't increase with passing time," he says. "The fact that you have not been killed in a car accident so far doesn't increase the chances of it happening in the next 10 years."

Looking to assess the actual time we have left, astronomers have examined the orbits of the five pairs of neutron stars observed in our galaxy -- it seems that we could have a breathing space of about 50 million years before the first ones collapse.

There's just one problem, though. The data seem to indicate that our galaxy also contains other neutron star pairs that no one has yet seen. Until we see them, we can't know when they will merge. So nobody can actually be sure that the apocalypse is not just around the corner.

Of course, although there's evidence to support Dar's idea, it's still just a theory. To take it further will require elaborate investigations of the effects of cosmic ray jets and their biological, geological and radiological fingerprints.

By MICHAEL BROOKS, The Guardian. Distributed by Scripps Howard News Service.