- (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.
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- 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.
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- 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.
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- 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.
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- 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.
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- 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.
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- 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.
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- Geological records show there have been
five major mass extinctions in the past 500 million years.
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- 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.
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- "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.
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- 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.
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- 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.
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- 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.
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- 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.
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- "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.
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- 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.
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- Examination of the fossil record also
shows a clear correlation between the extinction pattern of a species and
its vulnerability to ionizing radiation.
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- 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.
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- 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."
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- 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.
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- 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.
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- 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.
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