- If you can't journey to the centre of the Earth, why
not bring the churning heart of the planet to your lab? It's scary stuff,
but Adrian Cho thinks it might tell us why we're still alive
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- Take 14 tonnes of highly explosive metal, melt in a large
vessel and stir vigorously. Stand well back. Intrepid researchers at the
University of Maryland plan to try out this recipe, and, needless to say,
the fire marshal is already having sleepless nights. But it will be well
worth the trouble if they solve the long-standing puzzle of how the Earth
produces its magnetic field.
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- It might even be a matter of life or death. The Earth's
field is one of nature's great gifts, shielding us from lethal cosmic radiation
and possibly stopping our atmosphere being stripped away by the ravages
of the solar wind. If our magnetic field were to switch off entirely, the
Earth could become as sterile as Mars.
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- Our protective shield is unlikely to fail permanently,
but a temporary shutdown may be imminent. It could happen within as little
as 2000 years. Measurements of the Earth's field show that it is getting
weaker, and suggest that we are heading for a field reversal, in which
the north and south magnetic poles will swap. When the reversal is in full
swing, there will be a time when the field sinks almost to zero before
cranking up again. This unprotected period might only last for a few years,
or it could go on for thousands. To know for sure, we'll need a very precise
model of the Earth's core.
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- The core is a ball of iron 6960 kilometres across, at
a temperature of more than 5000 °C. The outer 2260 kilometres are liquid,
the inner part is squeezed solid. Convection roils the outer portion of
the core, as cooler, denser fluid sinks under the pull of gravity, while
hotter, less dense liquid rises to take its place.
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- So how could this swirling molten metal create a magnetic
field? Magnetism, electricity and motion are like a three-for-two special
offer: if you have two of them, the third one comes free. In a bicycle
light dynamo, for example, a magnet and the spinning rear wheel of your
bike generate electricity. In the Earth's core, researchers believe that
the magnetism of a "seed field" from, say, a nearby star, works
with the motion of the churning metal to generate electric currents. The
electricity in turn feeds the magnetic field. Given the right conditions
for this "magnetic dynamo", the seed field will stretch, twist
and grow as the molten metal moves. Eventually, the field will become strong
enough to influence the motion of the fluid, effectively controlling its
own growth. Once at this point, the magnetic dynamo can produce a stable,
self-sustaining field.
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- Whorls and eddies
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- However, this is still a matter of faith among physicists-they
can write the equations that describe the motion of a conductor and the
evolution of a magnetic field, but they can't explain exactly how it reaches
a steady state. That's mainly because the fluid flow inside the Earth is
turbulent, teeming with whorls and eddies. "We don't have enough computer
memory and power to resolve the really small eddies," says Gary Glatzmaier,
a computational physicist at the University of California in Santa Cruz.
And so models must rely on simplifications and approximations.
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- What they need is something real they can use to refine
their computer models-a turbulent core they can play with. Several research
groups are now building them. To capture the effects of turbulence, they
have to make devices that allow liquid metal to flow freely. Researchers
in Cadarache, France, have built a small device that will be filled with
330 litres of molten metal, and another team at the University of Wisconsin,
Madison, will soon rev up a spherical mock-up of the Earth's core 1 metre
in diameter.
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- But Daniel Lathrop, Daniel Sisan and Woodrow Shew at
the University of Maryland have by far the most ambitious plan. For the
moment they are working with a pair of small devices, but they are drawing
up plans for a ball 3 metres across that will contain 14 tonnes of sodium.
It will be heated to more than 110 ¡C to melt the metal, and propellers
will churn the liquid to simulate the effect of convection in the core.
The entire ball will spin seven times a second to mimic the Earth's rotation.
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- If you know your chemistry, alarm bells should be ringing
by now. Sodium may be a wonderful conductor of electricity, but it is also
rather reactive. Chemists keep the metal in oil to avoid contact with air
or water-otherwise it can burn or even explode. When just 100 kilograms
of sodium exploded at the French nuclear research centre in Cadarache in
1994, a worker was killed. To ensure safety in Maryland, the entire device
will sit inside a big metal box. "That makes the fire marshal and
the safety officer feel a whole lot better," says laboratory technician
Donald Martin.
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- Despite the risk, the sphere really does need to be as
big as possible. Size matters because the magnetic fields need space to
stretch, twist and grow. Field lines confined to a small space tend to
resist this sort of deformation.
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- Researchers in Riga, Latvia, and in Karlsruhe, Germany,
have generated magnetic fields in somewhat smaller vessels, but only by
forcing sodium to flow along helical paths. This doesn't mimic the more
complicated workings of the Earth's core, says Agris Gailitis at the University
of Latvia. "It is really low turbulence", he says. In the Earth,
as in any free-flowing dynamo, the fluid will be highly turbulent.
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- So the only way to get anywhere close to mimicking the
Earth's core is to have a huge volume of madly churning molten metal. The
faster it goes, and the bigger the volume of the fluid, the more the field
will twist, stretch and grow towards a steady state. So far, no one has
yet managed to persuade such a freely churning fluid to generate a magnetic
field. But a sphere 3 metres across might do the trick.
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- Theorist David Sweet, working with Lathrop and his colleagues
at the University of Maryland and the Los Alamos National Laboratory, has
shown how this giant ball of sodium should produce a self-sustaining magnetic
field (Physics of Plasmas, vol 8, p 1944).
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- They studied how churning liquid metal responds to a
magnetic "seed" pulse that kick-starts a self-sustaining field.
At a low flow speed, the field inside the liquid decays as soon as the
pulse is turned off. But the rate of this decay decreases as the flow increases.
Eventually, it won't decay at all.
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- When the experimenters subject their giant ball of churning
sodium to brief blasts of a magnetic seed field, the dynamo should spring
to life. But it won't be steady straight away-the dynamo starts up like
a sputtering old lawnmower, says Sweet. His calculations show that the
field comes on full blast, drops to zero, and then returns to full blast
later. These bursts are common to all turbulent magnetic dynamos, Sweet
says, and are the signs that Lathrop and his colleagues will look for to
see if they've created one. As the flow speed increases further, the field
will eventually stop bursting.
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- The researchers will also try to observe "saturation",
when the flowing fluid does not just produce a magnetic field, but the
field in turn controls the flow of the fluid-this is what allows the field
to sustain itself. Getting this right will require careful stirring, warns
Cary Forest, a physicist at the University of Wisconsin in Madison. The
flow has to have a particular character in order to generate a self-sustaining
field. "If the flow is not right you're not going to get a dynamo,"
he says. Get the flow wrong and you could end up simulating the core of
the wrong planet. Earth and Venus are similar in size and basic composition,
yet Earth has a field while Venus doesn't. No one knows why, but flow might
be the key.
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- They may not know the precise recipe for successful flow,
but theorists believe there are two essential ingredients. The first appears
to be differential rotation, which will stretch any stray magnetic field
lines around and around the axis-like a kid stretching a wad of chewing
gum round and round his finger.
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- The second ingredient is flow parallel to the spin axis,
creating loops of magnetic field bulging out of the tightly spiralling
lines-imagine the kid pulling a single strand of the wound-up gum towards
the end of his finger. As the fluid continues to rotate, these loops of
magnetic field can twist off, the two ends joining to form independent
field lines.
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- Lathrop believes the required flow probably arises out
of the interplay between turbulence and steady rotation. "The rotation
tends to organise the turbulence," he says. Unlike the Earth, Venus's
crust hasn't split into tectonic plates. This reduces the effectiveness
of the planet's convection cooling system and suppresses any turbulence.
Venus may also rotate too slowly to calm and organise any turbulence that
does arise. Whichever is lacking, something in the flow seems to stop Venus's
core generating a field. Only by building mock-ups of the Earth's core
will we find out what's really going on.
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- Meanwhile, there's another, more urgent question that
needs addressing. If Lathrop's experiment does produce bursts of magnetic
field, rather than a steady field, does that mean we are lucky enough to
be living in the middle of a burst of the Earth's dynamo? Could it be about
to cut out?
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- That's a worry, because the Earth's field deflects high-energy
particles crashing in from space. These cosmic rays can cause cancer and
other diseases. The field also deflects the solar wind, the torrent of
ionised gas streaming from the Sun. This ill wind may have blown away most
of Mars's atmosphere when the Red Planet lost its magnetic field roughly
4 billion years ago (New Scientist, 10 February, p 4).
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- The Earth's dynamo appears to be operating beyond the
bursty turn-on transition, Glaztmaier says. If he's right, the field won't
cut out entirely-at least, not until the planet has cooled for a few billion
years, slowing the convection. But without a more thorough understanding
of the role of turbulence in generating the field, it's hard to be entirely
sure.
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- Sinister portent
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- What's more, Earth's field has a well-known penchant
for reversing its poles every now and then. These reversals are recorded
in the magnetism of ancient rocks. And measurements of the field show that
its strength is decreasing at the moment.
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- Interpreting that decline is difficult, says Sten Odenwald,
a researcher on NASA's IMAGE project to investigate the Earth's magnetosphere,
the region of space dominated by the planet's magnetic field. "We
don't really know if the decline is just a natural ripple, or a portent
of something far more sinister."
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- If we're heading for a field reversal, then for a while
the Earth will be hit by much more radiation than it currently receives.
"There's going to be a long period of time-possibly many generations-when
we're going to have to find a way to deal with all this extra energy,"
says James Green, another IMAGE researcher. "I don't know that anyone's
done a proper scientific investigation of what will happen. It's certainly
one of the things we should be looking into."
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- If all goes well, Lathrop and his colleagues intend to
have their giant sodium ball up and spinning within two years. Meanwhile,
Forest intends to roll out his 1-metre ball at Wisconsin this summer, and
believes he will be first to generate the dynamo effect in a freely flowing
fluid.
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- Whoever wins the race, says Glatzmaier, these experiments
should give physicists benchmarks against which to test their dizzyingly
complicated programs. "We'll be able to apply our computer models
to the experiments instead of a planet or a star, and see if we can match
them," he says. This, it seems, could be the start of something big.
After decades of quiet research, dynamo physics might be about to explode-metaphorically
speaking, of course.
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- http://www.newscientist.com
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