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North Pole, South Pole: The Epic Quest to Solve the Great Mystery of Earth's Magnetism

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by Gillian Turner Phd


  This sounds familiar. The theory that a massive asteroid impact preceded the Cretaceous−Tertiary boundary extinctions is well known. The 200-kilometer-in-diameter Chicxulub crater, buried beneath the Yucatán Peninsula and Gulf of Mexico, is thought to be the site of an impact that threw enough dust into the atmosphere to cause a “nuclear winter” and kill off the dinosaurs. Muller’s hypothesis of a chain of events from impacts to core–mantle boundary avalanches, mass extinctions and geomagnetic reversals is seductive in its logic. But is it backed up by observation, theoretical calculation and statistical correlation? Only time and hard work will tell.

  Although from our perspective Earth appears unique, magnetic fields are now recognized as a common feature of many planets. In the 1950s, Jupiter’s strong field was discovered by radio astronomers. In the 1980s, magnetometers carried by the Voyager spacecraft detected similar fields around Saturn, Uranus and Neptune. The sun, Jupiter and Saturn are all thought to have dynamos based not on an iron-rich core like Earth’s, but on hydrogen in a metallic state. The fields of Uranus and Neptune are subtly different and their explanation is not as simple. Few scientists doubt the existence of dynamos, but neither molten iron nor metallic hydrogen seems likely. One suggestion is that water at a high enough pressure may have the necessary properties.

  An artist’s impression of Earth’s magnetic field as if it were visible from space. On the left, the solar wind—a stream of charged particles emanating from the sun—is safely deflected around the planet by the magnetic field. As a result the field is compressed on Earth’s day side, and drawn out into a long tail on the night side. When the solar wind is particularly strong, charged particles sometimes enter the upper atmosphere along field lines near the poles and produce aurora— spectacular light displays.

  Mercury is another intriguing case. In the mid 1970s, the Mariner spacecraft flew by the planet and picked up a weak magnetic field. Like Earth, Mercury is now thought to have an iron-rich liquid outer core, and so may conceivably sustain a similar dynamo. In 2004, NASA dispatched the Messenger spacecraft (named for Mercury Surface,Space Environment, Geochemistry and Ranging) to further investigate Mercury’s magnetism. It has already flown by the planet a number of times, adjusting its path in the process, and will go into orbit in March 2011.

  Venus has posed some curious questions. Despite this planet’s similarity to Earth in size, mass and internal composition, and the near certainty that it has a suitable outer core, all attempts to detect a magnetic field have drawn a blank. Dynamo experts infer that for some reason the all-important convective motion is not happening in Venus’s core. Some suggest Venus may not have an inner core.

  However, perhaps the most interesting bodies in the inner solar system are the moon and Mars. On both, surface rocks seem to be strongly and permanently magnetized. Portions of the Martian surface even seem to display a magnetic barcode—alternating stripes of oppositely magnetized rocks reminiscent of Earth’s seafloor. Mars no longer has an internal magnetic dynamo, but it may have had one in the past. Perhaps an initially molten core cooled and finally froze solid, switching off the Martian magnetic field in the process. If so, did Mars once experience both polarity reversals and plate tectonics? Perhaps Mars is more Earthlike than we have realized.

  What about the planets outside our solar system, which are being discovered by astronomers at an increasing rate? The field of planetary dynamo theory is wide open. As New Zealander David Stevenson, professor of planetary science at the California Institute of Technology, has said, the whole issue is inextricable from “the big questions of how planets form, differentiate and evolve.”

  After centuries of scientific endeavor, it is clear that the planet on which we live is a complex and dynamic body. Mechanical, thermal and electromagnetic interactions occur ceaselessly between the inner core, the outer core, the mantle and the crust. Modern technology such as satellites and computer simulations allow us an ever greater understanding of these processes, and of how they create the unique magnetic field without which life on Earth could not exist. However, direct proof of the geodynamo remains the realm of science fiction. We will almost certainly colonize other planets before making Jules Verne’s fantastic journey to the center of the Earth and learning the whole truth.

  And what of the near future? In the past 200 years the strength of Earth’s magnetic field has dropped by some fifteen percent. This rate of change has astonished many scientists. If it is sustained, they argue, we could right now be headed for the next polarity reversal when magnetic north becomes south and south becomes north.

  Sometimes I let my imagination fast-forward: what if we were here to see this happen? Countless migratory species—birds, butterflies, whales, fish, even honeybees—use Earth’s magnetic field lines as invisible way-finders on their great journeys across the planet. In the weakened transitional field, would these creatures increasingly lose their way and end up wandering the world like confused drunkards? If the magnetosphere surrounding the Earth were to collapse, would we humans be entertained by spectacular aurora at equatorial latitudes? Or would we instead be forced to take refuge from the onslaught of the solar wind?

  No one really knows. What we do know is that life on Earth has survived hundreds of reversals of the magnetic poles up to now, and so the next is unlikely to kill us off completely. Someone, although not you or me, will be around to see the field rebuild and the compass needle swing to the south. This time, though, they will understand why.

  Glossary

  amber Fossilized tree resin. When rubbed with fur, amber acquires a negative electric charge, enabling it to attract light objects.

  antiferromagnetism Form of magnetism in which the atomic magnetic moments of a material are ordered so that neighboring moments oppose one another and cancel each other out; the material then has no overall magnetization.

  argon–argon dating method Modification to the potassium−argon method: a radiometric technique for estimating the age of a rock.

  aurora Curtainlike display of colored light occasionally observed in the night sky, most often at high latitudes. Auroras are caused by charged particles emitted from the sun, and guided by Earth’s magnetic field lines into polar latitudes, where they collide with atoms and molecules of the upper atmosphere.

  baked contact Layer of rock heated by contact with hot volcanic lava, which often results in remagnetization in the direction of the magnetic field at the time of the lava’s cooling.

  Chandler Wobble Very small oscillation of Earth’s axis of rotation with respect to Earth’s surface, caused by the planet not being perfectly spherical.

  charge (electric) Intrinsic physical property of some fundamental particles: the electron carries a negative charge (-1.6 x 10-19 C); the proton carries a positive charge (+1.6 x 10-19C).

  Coulomb’s Law See electrostatic force and magnetostatic force.

  compass Pivoted or suspended magnet or magnetized needle, balanced to swing in a horizontal plane so that it settles with its north pole pointing towards magnetic north.

  continental drift Hypothesis that the present arrangement of continents resulted from the break-up of a large land mass and subsequent drift of the fragments over the surface of the Earth. Continental drift is explained by the theory of plate tectonics.

  convection Fluid motion driven by density differences and gravity: less dense material in a fluid rises relative to more dense material. Thermal convection arises because of the tendency of hot material to expand, so becoming less dense and rising through cooler, more dense material. Convective motion that is not necessarily related to temperature differences within the fluid is known as compositional convection.

  crust Rigid, rocky outermost layer of the Earth. Beneath the continents the crust is thirty to forty kilometers thick, but the oceanic crust is only about ten kilometers thick.

  Curie temperature Characteristic temperature above which a ferro -or ferrimagnetic material loses its spontaneous (remanent) magnetization
and becomes paramagnetic.

  current (electric) (Rate of) flow of electric charge.

  declination Angle between magnetic north and true north at any given location; equal to the deviation of the compass from true or geographic north (originally called variation).

  diamagnetism Magnetic property whereby, when an external magnetic field is applied to a substance, a very weak magnetization is induced in it in the opposite direction to the external field; the magnetization is lost when the inducing field is removed.

  dip Old term used for inclination.

  dip needle Magnetized needle, pivoted at its center of mass so it can rotate about a horizontal axis; when aligned in a (magnetic) north–south plane, the needle comes to rest inclined to the horizontal at an angle equal to the inclination.

  dipole See electric dipole and magnetic dipole.

  dipole field (1) Magnetic (or electric) field due to a magnetic (or electric) dipole. It can be visualized as a series of magnetic (or electric) field lines looping from the north pole (positive charge) to the south pole (negative charge). (2) Part of Earth’s magnetic field that can be modeled by a (tilted) geocentric dipole.

  disc dynamo Mechanical dynamo consisting of a metal disc rotating in a magnetic field; a current is induced between the center and the rim of the disc and may be fed into a circuit. In a self-sustaining disc dynamo, the induced current is used to generate a magnetic field that reinforces the original magnetic field.

  diurnal (daily) variation Variations in the direction and intensity of Earth’s magnetic field, with a period of twenty-four hours; due mainly to heating of the atmosphere by the sun.

  dynamo Original term for an electromagnetic generator: a device that converts mechanical energy into electrical energy through the process of electromagnetic induction.

  effluvium/effluvia Imaginary fluid/s once thought to be responsible for the transmission of electric and magnetic forces.

  electric dipole Positive and negative charges of equal magnitude separated by a small distance.

  electric field In general, a region of space in which an electric effect can be observed; technically, the electric field at any point in space is the electrostatic force acting on a unit positive charge at that point.

  electric motor Device that converts electrical energy into mechanical energy— for example, to turn the rotor of a machine.

  electromagnetic induction Process of inducing an electric current by moving a magnet close to a conducting circuit, or changing the magnetic field through it.

  electromagnetic wave Wave consisting of magnetic and electric fields that oscillate perpendicular to each other and to the direction of propagation. The spectrum of electromagnetic waves ranges from radio waves (low frequency, long wavelength) to gamma rays (high frequency, short wavelength); all electromagnetic waves travel at the speed of light, about 300 million meters per second.

  electrostatic force Force that exists between two electric charges: it is proportional to the product of the charges, and inversely proportional to the square of the distance between them. Like charges repel, opposite charges attract (Coulomb’s Law).

  equator, geomagnetic The great circle on the surface of the Earth that lies 90 degrees from both the geomagnetic north and south poles.

  excursion, geomagnetic Major change in the direction of Earth’s magnetic field, usually lasting only a few thousands of years and often observed only regionally, after which the field returns to its previous stable polarity.

  ferrimagnetism Spontaneous magnetic property of some natural minerals such as magnetite and titanomagnetite, certain alloys and ferrites, which may result in a strong stable remanent magnetization, with a characteristic Curie temperature. In ferrimagnetic minerals the atomic magnetic moments are ordered in two opposing directions, as in an antiferromagnet, but in unequal proportions so there is a residual magnetization.

  ferromagnetism Magnetic property occurring in metals such as iron, cobalt and nickel, due to the spontaneous alignment of atomic magnetic moments, resulting in a strong, stable remanent magnetization. Above the Curie temperature, the spontaneous alignment is lost and the material becomes paramagnetic.

  geocentric axial dipole See geocentric axial dipole field.

  geocentric axial dipole field Magnetic field that would result from a dipole at the center of the Earth, aligned with the rotation axis. The declination of such a field would be zero at all locations on the Earth; both inclination and intensity would depend on latitude.

  geographic poles The two locations where Earth’s rotation axis intersects the Earth’s surface; all lines of longitude (meridians) converge at the geographic poles.

  geomagnetic field reversal Reversal in orientation of Earth’s magnetic field, such that north and south geomagnetic poles change positions. Also known as polarity reversal.

  geomagnetic polarity timescale (GPTS) Dated sequence of geomagnetic polarity reversals, derived mainly from marine magnetic anomaly profiles, radiometrically dated rocks and sedimentary sequences.

  geomagnetic poles The locations where the axis of the geocentric dipole that best fits Earth’s magnetic field intersects the Earth’s surface; the north and south geomagnetic poles are antipodal.

  gravitational force Force of attraction between two bodies that results solely from their masses. It is proportional to the product of the masses of the bodies, and inversely proportional to the square of the distance between them (Newton’s Law).

  Halleyan line Original name for an isogonic contour, or (imaginary) line of constant declination on the Earth’s surface.

  hematite A naturally occurring antiferromagnetic form of ferric oxide Fe2O3, mined as the main ore of iron.

  homogeneous dynamo Dynamo process involving currents in a continuous medium, rather than in wires of a circuit.

  inclination Angle of direction of the magnetic field below the horizontal; originally called dip. Equal to the angle of a dip needle below the horizontal.

  induced magnetization Magnetization acquired by a material in the presence of an applied magnetic field, which is lost when the material is removed from the inducing field. This is the only form of magnetization found in paramagnetic and/or diamagnetic minerals. See also remanent magnetization.

  inner core Innermost part of the Earth; a solid sphere about 1200 kilometers in radius, thought to be predominantly iron-nickel alloy.

  International Geomagnetic Reference Field (IGRF) Mathematical model of Earth’s geomagnetic field, based on spherical harmonic analysis; endorsed by the International Association for Geomagnetism and Aeronomy and updated every five years.

  inverse square law Any law of physics in which a property decreases in proportion to the inverse square of distance. For example, if the distance is doubled, the property decreases to one-quarter of its original size; if the distance is tripled, the property goes down by a factor of nine; and so on. The force of gravity between two bodies, the electrostatic force between two charges and the magnetic force between two poles are all inverse square laws: each depends on the inverse square of the separation —of the bodies, charges or poles.

  latitude Angle, measured at the center of the Earth, between a location on the surface and one on the same meridian (line of longitude) at the equator.

  line of force A line drawn in a magnetic or electric field so that at every point it gives the direction of the field at that point.

  lodestone Naturally occurring, strongly magnetized rock, rich in magnetite or titanomagnetite.

  longitude The angle, measured at the center of the Earth, between the point where the meridian through a location crosses the equator and where the prime meridian crosses the equator.

  magnetic anomaly Difference between the measured magnetic field at a location and that expected from the main geomagnetic field. The difference usually arises from remanent or induced magnetization in local rocks.

  magnetic dipole In theory, magnetic north and south poles of equal strength, separated by a small dist
ance. In practice, a bar magnet, a uniformly magnetized sphere or terrella, and a circular coil carrying a steady current each produce a dipole field.

  magnetic equator Imaginary line on the surface of the Earth along which the inclination is zero.

  magnetic field In general, a region of space in which a magnetic effect can be observed. Technically, the magnetic field at any point is related to the force experienced by a magnetic pole or a moving electric charge at that point.

  magnetic moment Measure of the strength of a magnetic dipole.

  magnetic poles (of Earth) The points on Earth’s surface at which the magnetic field is vertical—that is, the inclination is +90° at the north magnetic pole and −90° at the south magnetic pole. The magnetic poles are, in general, not antipodal.

  magnetic storm Irregular small-scale variations of Earth’s magnetic field, particularly at high latitudes; associated with solar activity and often synchronous with auroras.

  magnetite Naturally occurring mineral, an oxide of iron, Fe3O4, possessing strong ferrimagnetic properties.

  magnetohydrodynamics Study of the movement of electrically conductive fluids in magnetic fields; the dynamics are determined by Maxwell’s equations and the Navier-Stokes equations.

  magnetosphere Volume surrounding the Earth inside which the geomagnetic field is confined by the pressure of the solar wind.

  magnetostatic force The force between two static magnetic poles. It is proportional to the product of the pole strengths, and inversely proportional to the square of the distance between them; like poles repel, opposite poles attract (Coulomb’s Law).

  mantle The layer between the Earth’s outer core and the crust; about 2900 kilometers thick, it makes up more than 70 percent of the Earth’s volume. The mantle is almost solid and largely composed of silicate minerals.

 

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