- The asteroid that wiped out the dinosaurs was thrown
to Earth in a moment of 'planetary madness'. And scientists can now predict
when the heavens will go haywire again.
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- There's something badly wrong with the pendulum clock
in the corner of the room. Normally, it ticks rhythmically, its bob swinging
back and forth with hypnotic regularity. Over time, however, the size of
the swing gradually gets larger, the ticks louder and louder. And, very
occasionally - in fact, so occasionally that nobody has yet ever observed
it - the clock goes stark-staring mad, ticking completely erratically as
the pendulum bob swings first to one side, then twice or three times as
far to the other side.
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- Surely, there is no clock that behaves like this? According
to a team of geophysicists and mathematicians, there is: the clock in the
sky. "For tens of millions of years, the planets circle the Sun with
the predictability of clockwork," says Michael Ghil of the Ecole Normale
SupÈrieure in Paris and the University of California at Los Angeles
(UCLA). "Then, without the slightest warning, everything goes utterly
haywire."
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- The heavens are generally considered to be a paragon
of predictability so this is a radical stuff. But it is only the beginning.
Ghil and his colleagues, Ferenc Varadi and Bruce Runnegar at UCLA, believe
the last time the solar system went insane was roughly 65 million years
ago. "It seems too much of a coincidence," says Ghil. "We
think it may have been connected with the extinction of the dinosaurs."
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- The kind of planetary madness Ghil and his colleagues
are talking about goes by the name of "chaos". Chaos is defined
as erratic motion with no sign of any regularity. Loosely speaking, chaotic
systems are infinitely sensitive to initial conditions, like a hurricane
in the Caribbean that was triggered by the flutter of a butterfly's wings
in distant Hawaii.
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- In the solar system, the most important drivers of chaos
are Jupiter and Saturn because they are the most massive of the planets.
In their investigation of planetary chaos, it is therefore these two planets
that Ghil and his colleagues have focused their attention on. The Jupiter-Saturn
system is actually not inherently chaotic. However, it is known to skate
close to the edge of chaos. The possibility therefore exists that, occasionally,
something might cause it to teeter over the edge into planetary insanity.
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- Ghil and his colleagues considered the possibility that
the "something" might be fluctuations in the pressure exerted
on Saturn by sunlight and the wind of subatomic particles blowing from
the Sun. Over tens of millions of years, their combined buffeting could
have a significant effect on Saturn's orbit. The researchers guessed that
solar variability might change the planet's "semi-major axis"
- a measure of the length of its elliptical path round the Sun - by as
much as 0.1 per cent. "We think this is perfectly plausible,"
says Ghil.
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- To see what changing Saturn's semi-major axis did to
the Jupiter-Saturn system, Ghil and his colleagues used a "digital
orrery". This is a purpose-built computer rigged to simulate the motion
of the planets under their mutual gravity. The researchers also incorporated
a novel feature of the behaviour of Jupiter and Saturn.
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- Jupiter orbits the Sun "about" five times for
every two times Saturn goes round. If the ratio of the orbital periods
was precisely 5:2, the combined effect of the gravity of two massive planets
on other bodies in the solar system would be greatest every 10 years -
that is, when the two planets are on the same side of the Sun and pulling
together. But, because this 5:2 "resonance" is not exact, the
planets are in perfect alignment on the same side of the Sun only every
1,000 to 2,000 years. "What this means is that the effect of Jupiter
and Saturn on the other bodies in the solar system rises to a crescendo
every 1,000-odd years," says Ghil.
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- Until now, researchers who have used computers to simulate
the long-term future of the solar system have assumed that this effect
is of no consequence, guessing that over long periods of time its effect
"averages out". "We had a hunch, however, that this wasn't
true," says Ghil. Using their digital orrery and taking this effect
into account, Ghil and his colleagues discovered that as the semi-major
axis of Saturn's orbit changes, the Jupiter-Saturn system drifts back and
forth between motion which is regular and motion which is totally chaotic.
"The system trips over into chaos every few tens of millions of years,"
says Ghil.
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- The team's most remarkable discovery, however, is that
in a wide range of simulations in which the semi-major axis of Saturn is
allowed to vary, a burst of chaos arises around 65 million years before
the present. "The timing coincides strikingly with the Cretaceous-Tertiary
[geological] boundary which marks the extinction event that wiped out the
dinosaurs," says Ghil.
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- As yet, says Ghil, it is impossible to tell how long
the burst of chaos persisted. Nevertheless, it is possible to investigate
the effect it would have had on other bodies in the solar system - specifically,
asteroids. The asteroids are thought to be the left-over rubble of a planet
which was prevented from congealing out of the "proto-planetary nebula"
by the disruptive effect of Jupiter. Vast numbers of asteroids - ranging
in size from pebbles to rocky bodies 1,000 kilometres across - circle the
Sun between the orbits of Jupiter and Mars.
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- Ghil and his colleagues simulated the effect on the asteroids
of a burst of chaos in the Jupiter-Saturn system. They found a wealth of
effects. "The most important are abrupt changes in the semi-major
axis of asteroid orbits," he says. "These would lead eventually
to complete ejection of bodies from the asteroid belt." Some of these
could easily end on a collision course with Earth.
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- The sequence of events revealed by the simulations is
complex. Some asteroids suffer small jumps in the size of their semi-major
axis, others large jumps. Some move to smaller orbits, some to longer orbits.
"A population of asteroids can drift back and forth through a succession
of different orbits," says Ghil.
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- Crucially, bodies whose elliptical orbits become ever
more elongated eventually come under the influence of the gravity of other
planets and are tugged free of the asteroid belt. "They get catapulted
out of the asteroid belt, some into orbits which cross the Earth's orbit,"
says Ghil. This is precisely what Ghil and his colleagues think might have
happened 65 million years ago. "A burst of chaos in the Jupiter-Saturn
system caused a flurry of Earth-crossing asteroids," says Ghil. "Among
them was one which struck the Earth off the coast of Central America, providing
the killer blow which finished off the dinosaurs."
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- If Ghil and his colleagues are right, the demise of the
dinosaurs cannot be attributed to an entirely random event. As the dinosaurs
grazed unawares, the great clock of the solar system went temporarily out
of kilter. The dinosaurs may have been victims of an event hardwired into
the dynamics of the solar system. "And they may not have been the
only victims," says Ghil.
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- The team's simulations reveal that another burst of planetary
chaos occurred about 250 million years ago. This seems to correspond precisely
with another major mass extinction at the Permian-Triassic boundary. "As
yet, however, we aren't totally confident about this," says Ghil.
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- The new paradigm which seems to be emerging is of a solar
system which evolves quietly for tens of millions of years but which goes
through occasional periods of madness. And what has happened in the past
will happen again. The simulations show another burst of chaos is due in
the future. "I wouldn't lose sleep over it," says Ghil. "The
due date is AD30 million, so there's plenty of time to evacuate the Earth!"
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- - Marcus Chown is author of 'The Universe Next Door'
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- © 2004 Independent Digital (UK) Ltd http://news.independent.co.uk/world/science_medical/story.jsp?story=506699
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