快猫短视频

THE core of the Earth is as remote and mysterious as anywhere in the Solar
System, with pressures around 300 million times higher than at the surface and
temperatures reaching 6000 掳C鈥攖he same as the surface of the Sun.
Although the core is made of a familiar material鈥�iron鈥攗苍诲别谤
these extraordinary conditions it behaves in ways that are both unfamiliar and
difficult to detect. In recent years, though, researchers have come to
understand a lot more about this mysterious planet within a planet. The
challenge now is in the detail: how is the magnetic field generated? How can we
explain the fact that from time to time the polarity of the magnetic field
becomes reversed? And if, as geologists now believe, the Earth鈥檚 core has an
inner and an outer layer, how do the two interact with the rocky layers above
them?

The core originated when the Earth formed with the other planets and the Sun
from a whirling disc of gas and dust. Gravity drew particles of dust together,
collecting them into clumps that collided with each other and grew bigger. As
the intense heat from the collisions melted the clumps, dense metals such as
iron collected together and sank to form the core. Earth grew slowly and its
iron core accumulated very early in the planet鈥檚 history (see Inside Science
Nos. 96 and 133). The temperature, together with pressure from the weight of the
outer layers of rock, kept the iron core fluid, swirling around in constant
motion, driven by the rotation of the Earth and by convection as the body began
to cool. The flow of this conducting fluid somehow started the generation of a
magnetic field in the core of our planet.

But how do we know what the core is like, how big it is, for example, or what
exactly it鈥檚 made of? There is little direct evidence to help: most deductions
depend on reasoning and indirect evidence. To begin with, we know that the Earth
is made of much the same stuff as the other planets, the Sun and, importantly,
meteorites. Measurements of the proportions of elements in Earth and in these
other bodies tells us how abundant these elements were in the clouds of gas that
condensed to make the Solar System. Meteorites are fragments of the planetary
bodies that collected together when the Solar System formed. Some meteorites are
made of rock; some of iron and some are a mixture. This tells two important
things. First, the importance of iron in the average composition of the Solar
System. And because some meteorites are made solely of iron and iron alloys with
other metals such as nickel it shows that these planetary bodies formed metallic
cores early in their lives.

The mass of the Earth is yet another important clue. Isaac Newton described
the way in which the gravitational attraction between two bodies depends on
their masses and separation. Sufficiently accurate measurements of the
attraction between different masses and the Earth indicate the mass of the Earth
directly. And Newton established that planet鈥檚 masses determine their distances
from the Sun; these relationships and the orbits of satellites also give
estimates of the Earth鈥檚 mass.

Once you have the Earth鈥檚 mass, you can start to work out what it is made of.
The continents are made of silicates, relatively light minerals
containing silicon, oxygen, aluminium, calcium and so on, essentially in the
form of granite. Such rocks are in fact less dense than the average
density of planet Earth itself, as also is the denser basaltic material
of the mantle(the layer beneath the crust). The core has to be made up of
heavier elements, and the commonest heavy element is iron. Once scientists had
established the density of the core, it became clear from calculations that the
core is made out of iron, perhaps with a few per cent sulphur and oxygen.

So there must be plenty of iron in the centre of the Earth, but how do we
know that it forms a distinct core, and what would that core be like? For most
of the past century, geophysicists have known that the Earth is layered (see
Inside Science No. 6). Seismologists study the pathways of energy spreading
through the planet from earthquakes. Two main forms of elastic waves carry the
energy from a quake through the world. Pressure waves, called P
waves, are compression waves like sound waves in air, in which increases and
decreases in pressure move in the direction of the wave. For shear waves,
known as S waves, the movements are transverse and involve shear
stress.

When an earthquake occurs, seismic waves are set in train and spread through
the planet from the focus of the quake (see Inside Science No. 64). Inside the
Earth the P and S waves behave like other waves, bending and bouncing back when
the properties of the rocks through which they pass change. Reflection and
refraction reveal significant changes in the physical properties of the rocks,
changes most simply related to contrasts in density.

The changes that take place in seismic waves reflected from different
boundaries can tell us about the properties of the materials deep in the Earth.
This enables seismologists to build sophisticated models of the layers in the
Earth, and to work out where to look for waves that have passed through
different layers of the Earth. To find out about the boundary between the core
and mantle, for example, you need big earthquakes and arrays of seismometers set
in an arc between 105掳 and 142掳 away from the quake鈥檚 epicentre. This is
the region in which seismologists found a shadow around the Earth where S waves
were not transmitted, a shadow not seen in P waves. The difference between the
two is significant: S waves do not travel through fluids because fluids cannot
support shear stress. The shadow zone corresponds to the outer core, 2900
kilometres in radius, and that outer core must therefore be liquid.

From Inside Outwards

Earth鈥檚 ever-growing core

In 1936 Danish seismologist Inge Lehmann suggested that there must also be an
inner core, to explain some seismic energy that she had detected in the
shadow zone. Observations suggested that the inner core was solid, and had a
radius of around 1400 kilometres, close to today鈥檚 best estimate of 1215
kilometres. The density of the inner core is hard to establish, but it is
probably somewhere in the range 0.25-1 gram/centimetre. It seems strange to have
a solid, inner, hotter part of the core, surrounded by cooler molten outer core,
but this is how iron behaves under such great pressures. The core is solidifying
in this unusual way from inside out because of the effect of the very high
pressure. Best estimates suggest that the inner core is growing by a few cubic
metres per second, and that it started to solidify around 2000 million years
ago. Before that, the Earth would have had an entirely liquid core.

As seismologists began to look at the core in greater detail, they realised
that they would need lots of earthquake data for more reliable observation at
such great depths. To get the maximum information about how the seismic waves
change when they interact with the distant boundaries between different rock
layers you need to record as much of the information in the waves as possible.
For this seismologists use instruments called broadband seismometers which can
record the vibrations from an earthquake across a very broad band of
frequencies. They also need, of course, a source of fairly regular, powerful
earthquakes. At one time, seismologists were using the effects of explosions at
a Soviet nuclear testing site, but generally, seismologists seek places where
earthquakes are fairly common naturally. Frequent moderate quakes happen in
subduction zones where the Earth鈥檚 plates converge and one overrides the
other (see Inside Science No. 64).

Seismologists have set up arrays of such sensitive instruments in the places
where they expect to pick up reflections from the core. There is a big change in
the physical properties at the edge of the core鈥攕omething like the change
at the surface of the Earth, from rock to air. But in this case, the change is
from the solid rocks of the deep mantle to the fluid iron of the outer core
(Figure 1).

The outer core is where the Earth鈥檚 magnetic field is generated, by a
mechanism known as the geodynamo. The flow of this conducting material is
enough to reinforce and maintain the existing magnetic field鈥攁n
arrangement known as a self-sustaining dynamo. Such systems are the
subject of much theoretical modelling, using equations that tie together fluid
dynamics, magnetism and the other properties of the fluid in the core.

This subject, magnetohydrodynamics, has led to several workable models
for the way in which the Earth鈥檚 magnetic field is generated. To match the
Earth鈥檚 field, a model field has to match certain characteristics. All the
models should produce a field that is symmetrical. It has to be generated by
flow in the iron of the outer core. Models also have to allow for reversed
polarity, because the Earth鈥檚 field has often switched in the past. During a
period of reversed polarity, a compass needle that once would have pointed to
the Magnetic North Pole would point south. This state is just as stable and
鈥渘ormal鈥� as a period of normal polarity, so any model must also be able to
reverse its polarity and still look much the same. It also has to be able to
swap polarity fairly fast geologically speaking: estimates from the rock record
suggest that it takes a few thousand years for the field to swap over.

Models that meet these demands tend to consist of arrays of cylindrical
eddies, aligned parallel to the Earth鈥檚 axis, like a bundle of drainpipes lashed
around a beach ball (Figure 4).
The patterns of flow are complex, but can exist
in ways that produce a field like that of the Earth.

Finding The Field

Sediment, ships and satellites

Modelling the geodynamo has been hampered by the fact that there is very
little useful data on what the Earth鈥檚 field looks like. 快猫短视频s鈥� pictures of
the magnetic field today are generally put together from measurements taken at
scattered places over the Earth鈥檚 surface, or from measurements made in planes
or satellites. Instruments called magnetometers measure the strength of
the field at different locations. Once you take away the effects of different
magnetic minerals in rocks, for example, the results can be combined to give a
snapshot of the field at a particular time. Some satellites have been set up to
measure the field from their orbits, giving a worldwide picture. Magsat did this
in 1980, measuring the magnetic field vector around the globe and producing a
map of the field. Oersted is doing the same this year, so there will be two maps
taken 20 years apart.

Records of how the field has changed come from magnetism preserved in rocks
when they formed. Lavas containing iron minerals lock in the direction of the
magnetic field when they solidified, as do sedimentary rocks held together by
minerals containing iron. Given rock samples from enough places around the
world, it is possible to build up a pattern of changes in the field over
hundreds of thousands of years (Figure 3).
Such readings are also used to map
the positions of the continents in the distant past.

But although seismologists have snapshots of the field now (Figure 2), and a
record of how it changed over many thousands of years in the past, there is very
little information between times. Generally there鈥檚 has been a lack of
information on how the magnetic field as a whole behaves over centuries鈥�
information that would help the modellers to refine their models. Looking at
historical records is one way to remedy this.

Ever since European nations began to explore the wider world in their ships,
they have used compasses and recorded their readings in the ships鈥� logs.
In the 18th century, navigators carefully measured the angle between the
direction of magnetic north (or south) and the corresponding geographical pole,
determined from celestial navigation. Knowing the local magnetic
declination helped them both to deduce the position of the ship and to
correct the steering compasses for variations in the local magnetic field.

The lack of an accurate position for ships makes these sequences of magnetic
readings hard to interpret for today鈥檚 geophysicists: quite often the latitude
recorded in the logs changes by a few degrees once the ship sighted land and
recognised its position by other means. This was in the early 18th century,
before accurate navigation, when the British carpenter John Harrison and a
number of scientists were striving to develop a marine chronometer that would
keep time accurately for long periods in rough seas and thus enable them to
measure longitude. But with all their inaccuracies, the exploratory and trading
voyages, for example, of the East India Company vessels, are a treasure trove
for geomagnetists. The voyage data will make it possible to build a database of
observations spanning from 1600 to 1900.

Such information will help researchers with their models of the magnetic
field. The field today, for example, is clearly not exactly like that of a bar
magnet: there is no simple change in polarity at the Equator. Instead, the field
has lobes of northern and southern fields, and areas of stronger and less strong
fields. The challenge for modellers is to reproduce the overall north-south
patterns and to fill in the details.

Backwards And Forwards

How does the field change?

How the geodynamo behaves during reversals of the Earth鈥檚 magnetic
field has given clues to its origin. Every few hundred thousand years, the
magnetic field reverses, without the strength of the field dropping to zero. The
pattern is irregular and includes, between these reversals, episodes in which
the magnetic poles move away from their usual positions, then return. These are
called excursions. More precise methods of dating the rocks have revealed
an underlying pattern to the changes, with reversals happening two or three
times every million years, and excursions lasting between 5000 and 10,000 years
taking place more frequently in between.

Geomagnetists have suggested that this results from the different properties
of the inner and outer core. The outer liquid core can reverse its field
polarity relatively quickly, because it flows as a fluid. It takes just 500
years, for example, for convection to overturn the field completely. The inner
core is solid and any changes in the polarity of the field there happen much
more slowly, and are limited by the rate of diffusion through solid iron. So,
the theory goes, field excursions happen when the outer core reverses, however a
full reversal does not take place unless the field stays reversed there for long
enough for the inner core to reverse too. But not all geophysicists agree with
this idea.

What geophysicists have not had until this year has been a working physical
model of any self-sustaining dynamo. Now several groups of researchers have
produced lab models that generate and keep a magnetic field going as a result of
flowing conducting fluids. The models do not try to recreate conditions in the
Earth鈥檚 core: the pressure and temperature are too extreme. What they have done
instead is to find a material that behaves like the iron in the core does, in
circumstances that are easier to model, and considerably faster. The key to
mimicking the important aspects of the geodynamo is to find a fluid in
which the magnetic field moves, and generates an electric current, faster than
the electric current dies away because of the resistance of the fluid. This
balance is measured by the magnetic Reynolds number, which should be
above a critical value to set up a self-sustaining dynamo. A good material, from
this point of view, is liquid sodium: it is an excellent conductor and it has a
high enough magnetic Reynolds number. But sodium is not a promising material
from the point of view of an experimenter: solid sodium burns easily in air and
explodes in water. Even small pieces are dangerous. The liquid form is much more
reactive and the experiments demand tanks of the stuff a metre or more across.
Nonetheless, experimenters have made models that work, in different ways, with
pumps and propellers that move the fluid around much as convection currents
would in the core. They produce magnetic fields that grow from small starting
fields and keep themselves going. Although this is an achievement in itself, it
is also a boost for the complicated calculations that theorists are confident can
explain the origin of the Earth鈥檚 magnetic field.

Experimental evidence has also settled the form of the Earth鈥檚 inner core:
crystalline iron. Researchers at the Carnegie Institution in Washington
DC and elsewhere have built diamond anvil cells. Such a cell can exert
pressures as high as 300 gigapascals by concentrating a relatively modest force
into a tiny area between two diamond faces. They put the material, say some
iron, in the diamond cell and then subject the whole thing to pressures and
temperatures approaching those that exist in the deep Earth, and see what
changes occur in the material. The advantage of the diamond cell is that the
researchers can use X-rays to watch and, in particular, take measurements while
the material is at high pressure and temperature. With this method they have
deduced some of the crystal structures of iron in extreme conditions. The
diamond cell has even shown that hydrogen becomes reorganised and assumes a
metallic structure at the sort of temperature and pressures likely to exist in
the core of giant gas planets such as Jupiter. Some sort of convection in such
unusual states of hydrogen may account for the magnetic field of these giant
gas planets, which do not contain enough iron to have a core like
贰补谤迟丑鈥檚.

One feature of crystals widely familiar to geologists is that they transmit
light at different speeds in different directions. They are known as
anisotropic crystals (isotropic minerals do not have these properties). Under
the microscope, geologists can use these differences to identify the orientation
of the crystal. The method has a use in studying the inner core, too. Reasoning
that if the core is crystalline and anisotropic, geophysicists wondered if it
might be possible to find the orientation of the crystal form.

Seismic waves that pass through the inner core will travel at different
speeds depending on the orientation of the crystals. Seismologists measuring the
speed at which the waves are transmitted through the inner core several years
apart had something of a surprise: the speed had changed by a significant amount
over a decade or so. The readings were taken with the same network of
instruments, using energy from earthquakes in the same subduction zone. But it
looked as if the waves had travelled through a different part of the inner core.
The implication was that the inner core had rotated at a different rate to the
rest of the Earth鈥攐therwise the same bit of the inner core should have
been below the surface at both times.

This was a shock. The researchers calculated that the inner core was behaving
like one huge crystal, with a fast direction inclined at about 10掳 to the
rotation axis of the Earth. In between their two sets of results, the fast
direction had rotated away and the seismic waves moved more slowly through the
inner core. This suggested that the inner core is rotating a few degrees a year
faster than the rest of the planet. Not everyone agrees: there are huge problems
with the inner core moving at a different rate from the rest of the planet. But
this observation has fuelled a great increase in research in this area. The
results are less than conclusive, however; further studies suggest that the
inner core is rotating more slowly than the rest of the planet, or at the same
rate.

What is certain is that this enigmatic part of our own home planet is fast
becoming a little better known, and a little less mysterious.

Earth is not the only planet to have a core. The masses of the terrestrial
(rocky) planets鈥擬ars, Venus and Mercury鈥攁nd their orbits show that
they too must have denser cores, almost certainly made of iron. NASA鈥檚 Mars
Global Surveyor has found stripes of rock magnetised in alternating directions.
This pattern is also a feature of the ocean crust rocks on Earth, when lavas
accumulate over the thousands of years in a magnetic field that periodically
changes direction. This is a sign that perhaps Mars once had a field that
reversed from time to time in the past.

Jupiter, Earth, Mercury and Saturn have magnetic fields that spread away from
the planets to form magnetospheres, because they have a core of conducting
liquid. Mercury is close to the Sun, and so it has not cooled as much as Mars or
Venus. Jupiter and Saturn have magnetic fields that originate from their core,
which are probably made of hydrogen. The extremely high pressure forces the
atoms into close contact, making a conducting metallic structure that can flow.
This can generate the strong field around Jupiter, which has a magnetosphere
that extends some 1,600,000 kilometres from the planet, enclosing a volume
bigger than the Sun.

NASA鈥檚 veteran space probe Galileo has been surveying Jupiter and its moons
for 5 years and has discovered how Jupiter鈥檚 strong magnetic field directs dust
from volcanoes on Io into the inner Solar System. It has also discovered that
Jupiter鈥檚 largest moon, Ganymede, has its own magnetic field, probably coming
from a molten core.

One of the more spectacular results of planetary magnetic fields is the
development of auroras near the poles. The magnetic field acts as a shield in
space, deflecting the solar wind, the flow of charged particles that comes from
the Sun. But the protection is not perfect and some of the solar wind penetrates
the ionosphere around the poles. The shifting coloured lights of the aurora
result from these disturbances.