It is incredible to think that although we walk on nice cool ground of Earth’s crust, there is a geological concert going on underneath our feet perpetually orchestrated by massive forces and immense transfer of heat where molten metals flow like water. Following a brief account of Earth’s interior, present aim is mainly to discuss some of the interesting enigmas of Earth’s core.
Study of Earth’s Interior
Human’s eternal urge to explore the unknowns such as landing on the Moon or sending satellites to various solar planets, continues with our curiosity to about Earth’s interior.
Scientific understanding of Earth’s inside as layered spherical shells, started from an early part of the 20th century. Seismology provides the best resolution technology of all geophysical probes to map out Earth’s structure and composition. Seismic waves go through Earth’s interior when an earthquake occurs. For example, Tohoku earthquake in Japan in 2011 transmitted seismic waves through Earth’s centre to Chile located on the other side of the globe.
Other methods of investigating Earth’s interior include studies of gravitational and magnetic fields, and surface topography of rocks from Earth’s interior that reached the surface via volcanic eruption.
1) Seismology and Earth’s interior
Transmission of seismic waves occurs via reflection and refraction (bending) of the waves at discontinuities and gradients they encounter. Changes in the material properties of Earth’s interior such as composition, mineral phase and packing structure as well as the temperature and pressure directly affect the nominal velocity (~10 km per second) of seismic wave as it passes through Earth’s centre.
2) Depth of Earth’s interior layers
Based on changes in the velocity of seismic waves, Earth’s layered interior is estimated to consist of an outer solid crust of silicate directly beneath our feet, followed by a highly viscous mantle, a less viscous liquid outer core and a solid inner core. The characteristics of different spherical layers are as follows in increasing order of distance from the surface :
crust (30 km) of solid outer shell on which we live at pressure ~1 atmosphere or 0.0001 GPa and normal air temperature mantle (upper : 720 km and lower : 2,170 km) made up of semi-molten rock, called magma at ~24 GPa and ~ 1,600 C outer core (2,260 km) ofmolten iron and iron-(~5%)nickel alloy at 136 GPa and 3,700 C, and inner core (1,220 km) of solid mass of iron and nickel at 364 GPa and 5,500 C
Study of Earth’s core
Earth was formed around five billion years ago via massive conglomeration and high-velocity bombardment of meteorites and comets. An immense amount of heat was generated. Heavier molten materials like iron and iron-nickel alloy from the meteorites sank into Earth’s core, while lighter silicates, other oxygen compounds and water from comets rose near the surface.
Extreme heat and pressure have made Earth’s core as one of the most inhospitable places in the entire solar system. The inner solid core is smaller than Moon, while Mars could snugly fit inside the outer core.
1) Inner core
A crushing pressure of ~3.64 million atmospheres (364 GPa) compared to 0.0001 GPa on Earth’s surface is exerted on the inner core by gravity from above. As a result, the inner core, mostly iron, remains solid despite being as hot (˃ 5000 C) as Sun’s surface.
Seismologist proved the inner core to be solid by trying to detect transverse shear seismic S-wave which can only penetrate through a solid. However, S-wave is very small to detect. One would, therefore, need a huge amount of data to sift through. Dr.Arwen Deuss of Cambridge University just did that by collecting seismic data from around the globe (47 seismic stations), and eventually observed the tiny S-wave travelling through the inner core, proving it to be solid.
2) Outer core
Unlike the inner core, the outer core is not under enough high pressure to be solid. It consists of molten metals of iron and nickel whose dynamic activity is similar to that happening in the mixing of upper and lower mantle. The turbulence caused by a convection process in the liquid metals, as discussed later on, is believed to create Earth’s magnetic field. The average strength of the magnetic field is highest (25 gauss) inside the outer core, which gradually decreases with distance from the core, and eventually end up ~ 50 times weaker at the surface of Earth..
Enigmas of Earth’s core
Unexplained crystallography at or near Earth’s core
1) Mineral structure of D” layer
The nature of mineral at the boundary between lower mantle and outer core, the so-called D” layer of ~ 300 km thickness at a depth of 2890 km where pressure and temperature are around 125 GPa and 2,200 C respectively, is unclear. The D” layer is expected to be a magnesium silicate mineral with perovskite crystallographic structure. However, seismic data revealed anomalous structure with relative abundance of magnesium silicates.
2) Structure of iron in the inner core
Compressed seismic P-waves which can go through both solid and liquid, are known to travel faster through the inner core by about 3% or ~5 seconds in the direction of Earth’s polar axis (North to South)compared to that along the equatorial plane (East to West). This phenomenon is termed as seismic anisotropy. The anomaly in seismic speeds can be resolved if the inner core, mostly of iron, can be proved to have a texture with the “fast axis” of the crystals mostly oriented in the North-South direction.
Earth’s mysterious magnetic phenomena
1) Importance of Earth’s magnetic field for our survival
Scientists are fascinated by the fact that Earth’s outer core is responsible for Earth’s magnetic i.e. geomagnetic field which is vital to the evolution of life on Earth. The magnetic bubble surrounding the Earth called magnetosphere contains the necessary magnetic field to deflect away harmful plasma storms and deadly charged particles from solar radiation. This action protects us from being fired out of existence. Gradual perish of atmosphere on Mar, for example, is due to loss of magnetic field which has probably led the Red Planet to become a dead world.
Our insufficient knowledge on the creation of planetary magnetism has left us pondering about the unanswered questions on the mechanics of origin and survival for billions of years of Earth’s magnetic field. It is even more puzzling that the smallest planet Mercury has a magnetic field, while both Mars and Venus have none. 2) Criteria of geomagnetic field source
Earth’s magnetic i.e. geomagnetic field must be continuously fed with energy for its survival over billions of years. Life on Earth would, otherwise, decay and disappear by interacting with the solar radiation. Mechanisms are sought to explain perpetual generation of geomagnetic field, and an energy source to power it all the time.
3) Possible mechanisms of geomagnetism
The traditional belief that a giant magnet lying between the north and south poles inside the Earth creates geomagnetism is a myth. This is because the temperature inside the Earth is far above the Curie point of 770 C for iron, making it impossible for the core to remain magnetized.
A most plausible alternative mechanism similar to a dynamo operation is briefly discussed next whereby mechanical energy is converted into electrical energy.
4) Geodynamo theory of Earth’s magnetism (geomagnetism)
Most scientists agree that Earth’s magnetic field arises from the turbulence of liquid metal of mostly iron in the outer core. This movement originates from convection of heat left over from the birth of the Earth.
Molten iron conducts electricity which, as expected, is surrounded by magnetic fields. Geomagnetism is created by the circulating electric currents in Earth's molten outer core via self-sustaining geodynamo which generates magnetic fields assisted by a “seed” field. The prerequisites for geodynamo can be summarised as
a) fluid with good electrical conductivity, namely molten iron or iron-nickel alloy.
b) high temperature to keep the metal in liquid state,
c) enough energy difference in terms of temperature and pressure to provide convection movement of the molten metal with sufficient speed and appropriate flow pattern assisted by Coriolis Force from Earth’s rotation, and finally
d) presence of “seed” magnetic field such as provided by the Sun to start the process.
All these conditions are met in the outer core where the temperature is ~4000 C. The turbulence by convection of electrically conductive liquid iron generates magnetic field. Also, electric current via magnetic induction process is generated from Sun’s “seed” magnetic field. This newly created electric current in turn produces a magnetic field that interacts with the fluid motion to create a secondary magnetic field. Together, the two fields are stronger than the original. Thus as long as there is sufficient liquid turbulence in the outer core, cycle continues establishing a self-sustaining loop of geodynamo, and hence perpetual generation of Earth’s magnetic field with one pole up in Canada and another down in Antarctica.
5) Magnetic flip/reversal
Scientists believe that over the past 400 years, there has been a steady decline in Earth’s magnetic field, probably due to changes in the churning motion of the liquid outer core. Since the first measurement reported some 180 years ago, magnetic field strength has waned 10-15%. This would lead to shrinkage of magnetosphere and hence less shielding from harmful solar radiation. There are already little bit of the radiation leaking through near the poles that causes the aurora.
There had been reports of delicate sensors in Hubble space telescope, satellites and computer in space shuttles malfunctioning when passing over the Atlantic. It soon became clear that this type of incidents dubbed as “South Atlantic Anomaly” (SAA) was tightly clustered around the centre of South America and South Atlantic. The region of anomaly is growing all the time, and it is speculated that in just over 200 years, the region could cover the entire Southern Hemisphere.
SAA is suspected to be much more than just an inconvenience to satellite operators. It may be the first indication of a profound change in Earth’s magnetic field. On mapping out Earth’s magnetic field, scientists have discovered that the magnetic field under SAA patches has actually flipped with magnetic north pointing south and vice versa. If the patches continue to deepen and spread, it is just a hunch that Earth’s entire magnetic field could reach a tipping point and flip/reverse. Possible journey to the centre of Earth
Fascinated by the mysteries surrounding Earth’s interior, scientists for centuries have been dreaming of reaching Earth’s centre some 6400 km (~ 4000 miles) beneath our feet. The deepest depth so far managed by drilling through Earth’s crust is 12 km at Kola Superdeep Borehole in Russia, which is merely 0.2% of the distance to the centre of the Earth, and only 40% of the thickness of Earth’s crust.
Temperature at the bottom of the Borehole was already ~ 180C which would extend to about 4000 C along with the pressure reaching very high on further descending down. Drilling through these extreme hostile conditions, much worse than on the Sun, is simply not possible, unlike Jules Verne’s imaginary story.
However, this set back did not deter scientists to consider alternative routes. Simulation programmes are developed to investigate the conditions of Earth’s core in the laboratory.
Laboratory simulation of the extreme environment of Earth’s core
1) Resolution of D” layer structure
The research team of Prof.Kei Hirose of Tokyo Inst. of Technology succeeded the seemingly impossible task of reaching the extreme conditions (i.e. 125 GPa and over 2200C) of the D” layer using a laser-heated diamond-anvil cell. A sample (25μm thick) of magnesium silicate was sandwiched between the flattened tips of two opposing diamond anvils. The crystal structure was studied in-situ by Synchrotron x-ray diffraction method.
An unknown structure was observed with ~ 1% higher density than the expected perovskite structure. Dubbed as “postperovskite” mineral, this new material can explain the puzzle about the type of mineral in D” layer. Postperovskite has crystal structure similar to mica. Its electrical conductivity is about four orders of magnitude higher than the corresponding perovskite form.
2) Creation of inner core conditions
Flushed with success, Hirose continued to push harder to attain conditions of the inner core. He succeeded in pressing an iron sample to as high as 377 GPa at over 5400 C. Results showed that at these conditions similar to inner core, iron has a hexagonal closed-packed (HCP) structure with the atoms bonded at highdensity.
Speculation is that these HCP iron crystals in the core are huge up to 10 km in height. They are preferentially aligned like a forest with the c-axis parallel to Earth’s rotational axis. This unidirectional alignment can explain the observed seismic anisotropy of the inner core mentioned earlier.
Laboratory study of Earth’s magnetic field
1) Possibility of creating geodynamo in the lab
Scientists try to develop a system mimicking Earth’s dynamo. But the problem lies with the difficulty of modelling down the immense size of Earth’s core, and also the heat or the speed at which it spins. For example, a 100-metre sphere filled with liquid metal would have to rotate insanely fast at more than 100,000 revolutions per minute (or 100,000 miles per hour at its equator) to achieve true dynamic similarity with Earth’s core. An estimated 100 kilowatts of thermal power would be needed to drive the kind of turbulence expected in Earth’s core. A lesser rate of revolution would need more thermal power to drive the necessary degree of convection. Undaunted by the reality of the whole thing being beyond normal comprehension, scientists are still trying to create geodynamo in the laboratory.
It is universally agreed that since iron melts at high temperature (1538 C), an alternative metal such as sodium with melting point at 97.7 C would be a better candidate to manage in the lab as the working fluid. Liquid sodium, like iron, strongly interacts with magnetic field.
For container designs, the best suite of sodium experiments uses spherical shapes to mimic the round shape of Earth’s geodynamo. Prof. Lathrop and his team at the University of Maryland, have thus designed two stainless steel spheres nestled one within the other. The inner sphere of 1 metre across represents Earth’s solid inner core, while a 3-metre tall outer sphere represents outer core.
The space between the two is filled with 12 tons of liquid sodium mimicking the liquid iron in Earth’s outer core. It is hoped that spinning independently at 4 and 12 revolutions per second for the outer and inner spheres respectively, will create its own dynamo to provide a self-sustaining magnetic field with Earth’s natural magnetism as a “seed field” to kick start the process. The world awaits the results with great anticipation once the machine is powered in near future.
New alternative design using superheated state of matter known as plasma instead of liquid sodium is being developed at Wisconsin University, Madison. This would upgrade the study of geodynamo by several notches.
2) Thoughts on magnetic reversals
Earth’s magnetic field has never reversed while people are around to record it. Studies of rock samples definitely point out magnetic reversal occurring many hundreds of times over the past billion years. The last time reversal took place was 780,000 years ago when Homo erectus was still learning how to make stone tools.
It appears that every 100,000 to million years, the north-south orientation of the magnetosphere reverses, and is often preceded by an overall weakening of magnetic field. Based on present detection of SAA along with the report of a steady decline in field strength, one can speculate that we are heading for a magnetic reversal.
Such magnetic flip, even if imminent, is not something that would occur overnight. Magnetic reversal can take hundreds or thousands of years to mature from start to finish. During this period one can only speculate that the magnetic field would be pretty confused with perhaps magnetic poles wandering to the equator taking with them the spectacular Northern Lights.
After dreaming of reaching the centre of the Earth, scientists are now uncovering a bizarre and alien world via simulation programmes. Unlike anything else we experience on Earth’s surface, the extreme conditions in the interior are unique in the solar system. Inside the Earth, the outer core of the size of Mars has perpetual violent storms in a sea of molten iron, while the solid inner core of the size of the Moon consists of giant forest of iron crystals. A unique magnetosphere created by the outer core, protects us from burning out by solar radiation. With all this, one can envisage Earth’s interior as a planet buried within a planet of Earth we know.