Download What causes the earth`s magnetic field? How hot is the earth`s

What causes the earth’s magnetic field? How hot is the earth’s interior and
what keeps it that way? Will this always be so or is there an eventual fate
awaiting the planet?
Professor Chris Marone (Pennsylvania State University) provides us some answers to these
questions.
A look at the cross-section of the earth reveals three concentric layers. The outermost layer
is a hard crust ranging from 10 to 100 km thick. Under that lies a donut-shaped mantle,
some 2,900 km thick. At the centre of the Earth lies a two-part core: a solid inner core about
the size of our moon, surrounded by an outer core of liquid metal, around 2,300 km
thick. Pieces of the Earth's crust and uppermost mantle, around 100 km thick, comprise the
tectonic plates on which the continents rest.
Source: Google image: bbc.co.uk
The Earth's rotation makes the liquid metal core flow and swirl. These movements produce
electric currents which, in turn, produce the planet's magnetic field (so-called dynamo effect
of the circulating electric current). The moon, by contrast, does not have a liquid metal core
and hence does not possess a global dipolar magnetic field. The Earth's magnetic field is
similar to that of a bar magnet tilted 11 degrees from the spin axis of the Earth. The
magnetic field is, however, not constant—the strength of the field and the location of its
poles vary. Moreover, the poles periodically reverse their orientation in a process
called geomagnetic reversal. The most recent reversal occurred 780,000 years ago.
The Earth’s magnetic field does play a significant role in holding captive energetically
charged particles that originate from the solar wind and cosmic ray collisions with atoms of
the atmosphere. This it does in two torus-shaped belts – the Van Allen radiation belts, with
energetic electrons forming the outer belt and a combination of protons and electrons
forming the inner belt. The Van Allen radiation belts are centred along the earth's magnetic
equator in a region of the upper atmosphere called the magnetosphere, or exosphere. The
inner and more intense belt extends from roughly 600 miles (1,000 km) to 3,700 miles
(6,000 km) above the earth; the outer belt, from roughly 9,300 miles (15,000 km) to 15,500
miles (25,000 km) above the earth. Though above most satellite and space shuttle orbits,
the charged particles in these belts move fast enough to create dangerous radiation to
astronauts that may pass through. The belts are not well-understood, but they are believed
to play a part in auroral activity by streaming charged particles back and forth between the
Earth's North and South magnetic poles. The ozone layer which traps deadly ultraviolet
radiation from the Sun is located well below the Van Allen belts, some 30 to 60 km above
the Earth’s surface, that is, from the middle of the stratosphere to the lower mesosphere.
Source: Wikipedia
What makes the earth rotate? The cooling and subsequent condensation of the primordial
dust and gas left over from making the sun was what led to the formation of the earth and
other planets. The gravitational orbit of the amorphous clump that was to be the earth set it
spinning and its rate increased as the coalescing continued. Professor Kevin Luhman, a
colleague of Marone, draws a parallel here with a figure skater bringing her arms closer to
her body to speed up her rate of spin. Since gravity pulls inward equally from all directions,
the clump eventually became a rounded planet. The conservation of angular momentum
has since then kept the planet spinning on its axis.
The earth’s interior is extremely hot (around 6,000 oC), hotter than the surface of the sun.
The bulk of this heat, almost 90%, is fuelled by the decaying of radioactive isotopes like
Uranium-238, Uranium-235, Thorium-232 and Potassium-40 contained within the mantle.
These have half-lives of 108 to 1010 years which account for their survival since the formation
of the solar system some 4.5 billion years ago. The heat radiated in the decay of these
isotopes remains trapped in the mantle. Smaller contributions to this reservoir of heat also
come from the still un-dissipated gravitational heat and latent heat arising from the core's
expansion (by about a centimetre every thousand years) as the Earth cools from the inside
out. The earth’s crust and waters of the oceans, however, are directly heated by the sun
which releases a tremendous amount of energy due to the nuclear fusion in its core,
converting hydrogen to helium.
Professor Marone predicts that billions of years in the future the core and mantle could cool
and solidify enough to meet the crust. If that happens, Earth will become a cold, dead planet
like the moon. But a probability that cannot also be discounted is that within a span of five
or six billion years the Sun will run out of hydrogen to burn. Having already been in
existence for 5 billion years, it has burned up half of its hydrogen fuel supply. Throughout
its normal lifetime, a star like the Sun is an uneasy battleground between the force of
gravity which tries to compress it and the energy of nuclear fusion which tries to blow it
apart. As it runs out of hydrogen, the Sun will begin to fuse helium and then heavier and
heavier elements in a desperate attempt to keep itself from collapsing. This change happens
at the very centre of the Sun. The temperature of the Sun will begin to rise and to radiate
this energy the star expands. But this process of energy release due to elemental fusion can
only proceed up to the stage of elemental iron; nuclei heavier than iron are endothermic
and can only be produced by supplying energy. This is what makes the nuclear furnace run
out of fuel and causes the centre of the star to collapse inwards due to gravity. The centre
will not, however, collapse completely because of the inability of the Sun’s gravity to
counteract the repulsive force of the star’s electrons, but if the Sun had been 1.44 times or
more massive (the Chandrasekhar limit) than it is, then its gravitational pull would
overcome the electron repulsion and collapse it even further. The result would be a neutron
star or a black hole and the simultaneous release of an enormous amount of energy in a
spectacular explosion called a supernova which would be visible for billions of light years in
all directions. The predicted fate of our Sun which is a main sequence star is, however, less
spectacular. While its centre shrinks with increasing depletion of nuclear fuel, the hightemperature region will move to the interface between the “burned-out core” and the
outer layers that still contain enough hydrogen to maintain a nuclear fire. As Nobel
Laureate George Gamow describes it, “the internal structure of the Sun will be transformed
from a so-called point-source model to a shell-source model in which the energy is liberated
in a thin spherical shell that separates the burned-out core from the rest of the solar body.
As more and more hydrogen is consumed, this shell will expand outward from the centre
with the result that the star’s size will increase (to some 200 times its present size) and its
luminosity will also increase by a factor of between 10 and 20, making the oceans on the
earth boil violently.” Due to this expansion of the outer layers of the star, the energy
produced in the core of the star is spread over a much larger surface area, resulting in a
lower surface temperature and a shift in the star's visible light output towards the red –
hence red giant, even though the colour usually is orange. This expansion of the Sun to its
red-giant phase with its atmosphere engulfing the current orbits of the inner planets of the
Solar System — Mercury, Venus and Earth - will certainly kill off all life on Earth. The
ultimate fate of all these planets could be a fiery death, but based on recent cosmological
evidence, the possibility cannot be excluded that some close-in planets may not be entirely
destroyed during a stellar evolution.
{Ref:
www.rps.psu.edu/probing/earth.html;
www.physorg.com/news105637304.html;
www.nature.com/nature/journal/v480/n7378/full/480460a.html; George Gamow & John
M.Cleveland: “Physics- Foundations and Frontiers, 3rd Edn., Prentice-Hall, 1989}
vg kumar das (15 June 2012)
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