Prescott College — For the Liberal Arts, the Environment & Social Justice
Critical Review: Is Earth's Outer Core Liquid
or a High Density Plasma?
Daniel S. Helman
Sustainability Education, Prescott College, 220 Grove Ave., Prescott, AZ 86301
[email protected]
Abstract
This work explores composition and melting point data (or lack of data) related to
Earth's outer core, and also the possibility of a high-density plasma composition, plus
gives a background on Inge Lehmann's seismological observation demonstrating a
solid inner core.
Implications related to geomagnetic field dynamics are also
presented, as are evidence in the rock record for changes to the geomagnetic field over
time, including the intersection (if any) of such changes and the origin and
development of life on the planet.
Keywords
geodynamo, plasma, metallic liquid, solid-density plasma, plasma channel, origin of
life, geomagnetic field
Inner and Outer: How Do We Know?
Here are the words of Inge Lehmann, who made this discovery:
Evidently, there was a reflection of the waves in the interior of the earth that
caused them to emerge at a shorter epicentral distance. It was shown in a
simple example how this could happen. I considered a globe in which a hard
mantle surrounded a softer core, the radius of which I took to be five ninths
of the surrounding sphere. The velocity of the longitudinal waves was 10
km/s in the mantle and 8 km/s in the core. It was then a simple matter to
calculate the time curves arising from an earthquake that took place at the
surface of the globe. The P curve that resulted from waves confined to the
mantle ended at 112° distance from the epicenter. P' consisted of two
brances, as observed in the New Zealand earthquake. When the variation of
the travel time was considered in relation to the angle of incidence, an
estimate of the intensity could be obtained. In this way it was found that the
intensity of the waves corresponding to the upper branch of the P' curve
would be small. This was in accordance with the fact that it had been
difficult to observe the upper branch.
No rays emerged at epicentral distances between 112° and 154° (Figure 6). I
then placed a smaller core inside the first core and let the velocity in it be
larger so that a reflection would occur when the rays through the larger core
met it. After a choice of velocities in the inner core was made, a time curve
was obtained (Figure 7), part of which appeared in the interval where there
had not been any rays before. The existence of a small solid core in the
innermost part of the earth was seen to result in waves emerging at
distances where it had not been possible to predict their presence.
Gutenberg accepted the idea. He and Charles Richter (California Institute of
Technology, Pasadena) placed a small core inside the earth and adjusted the
radius of this small core until the calculated time curves agreed with the
waves observed. Jeffreys was slower to accept the inner core. JeffreysBullen time curves had been completed in 1935. In 1939, a new edition was
published in which the inner core had been accepted.
...
Inge Lehmann was born in 1888 and received her degree in mathematics in
1920. She later became chief of the Seismological Department of the
Geodetic Institute of Denmark, which was established in 1928. As described
in this article, she aided in setting up seismic stations in Greenland and
Copenhagen. Her studies of the travel times of a special phase led to the
discovery of the inner core of the earth in 1936. [1]
Is the Outer Core a Liquid or a Plasma?
Consider the following quote in light of how a plasma is defined: “The best hard sphere model for
the core is a transition metal liquid whose valence electrons form an electron sea bathing and
charge-compensating the spherical ionic cores.” [2] The term metallic (or metal) liquid also
appears in text related to the core of Jupoter: “Jupiter's magnetic field is caused by convective
dynamo motion of electrically conducting fluid hydrogen. The data imply that Jupiter should
become metallic at 140 gigapascals.” [3] But is this state any different from a plasma? Yes, since
“a characteristic property of any liquid is its tendency to cohere and to maintain a fixed volume in
the absence of confinement.” [2] The outer core of the Earth is certainly in a confined state, so
perhaps this distinction between metallic liquid and plasma is moot.
Composition Data
An iron-nickel inner core is a supposition based on observational data: “The reason that the inner
core was considered to be composed of nickel-iron metal is that nickel and iron were thought to be
essentially inseparable from one another by natural processes in the alloy of meteorites, and
heavier elements are not sufficiently abundant, relative to iron, to constitute a mass as large as the
inner core.” [4]
Geodynamo
Here is how Busse (1975) has put it: “the toroidal field in the core is of the same order of
magnitude as the poloidal field. This result is consistent with the basic assumption of theory that
the Lorentz force is small compared to the Coriolis force.” [5] New model parameters are a
source of studies, as with the following that takes an iron silicon composition. “The high electrical
conductivity increases the magnetic decay time of the inner core by a factor of more than three,
lengthening the magnetic diffusion time to 10 kyr and making it more likely that the inner core
stabilises the geodynamo and reduces the frequency of reversals.” [6] Modeling geomagnetic
reversals is one grail. Note that plasma channels, such as those seen in plasma sphere toys, do not
seem to have been included in models. They might find some evidence in the placement of
mantle hot spots. The term solid may also be a misnomer, since there are reports of solid-density
plasma having been created artificially for study, an interesting case, since “its thermodynamic
and transport properties are challenging to measure.” [7]
Changes Over Time
“Palaeomagnetic and palaeointensity data from rocks formed near the boundary of the Proterozoic
and Archaean eons, some 2.5 Gyr ago, show many hallmarks of the more recent geomagnetic
field. Reversals are recorded, palaeosecular variation data indicate a dipole-dominated
morphology and available palaeointensity values are similar to those from younger rocks. The
picture before 2.8 Gyr ago is much less clear [but, using analytical techniques on single crystal
inclusions yields] 3.2-Gyr-old field strengths that are within 50 per cent of the present-day value,
indicating that a viable magnetosphere sheltered the early Earth’s atmosphere from solar wind
erosion.” [8]
Life
How lfe originates on planetary bodies is an ongoing discussion. Preventing a solar wind erosion
of the early Earth's atmosphere was perhaps important in some of the global geochemistry that led
either to proto-ecosystem expansion or the biome itself. Organisms exhibit magnetic fields as a
consequence of metabolism and other functions that move ionic fluids from one place to another
inside. Perhaps hydrothermal vent proto-cell chemical gardens were influenced by the
geomagnetic field—setting up yet another gradient upon which to build an interesting protometabolism.
Discussion
There are a few things that have been hinted at herein, and these have been meant to raise
questions related to the following: (1) the difference between a plasma and a metallic liquid; (2)
the difference between a solid and a solid-density plasma; and (3) the possibility of plasma
channels in the outer (or inner) core. The hope is that this will spur discussion and lead to
additional models of the geodynamo, and especially of how geomagnetic reversals might arise,
and also to highlight the parameters within which origin-of-life research takes place.
References
[1] Lehmann, I. (1987). Seismology in the days of old. Eos, Transactions American Geophysical Union, 68(3), 33-35.
[2] Helffrich, G. (2014). Outer core compositional layering and constraints on core liquid transport properties. Earth and Planetary
Science Letters, 391, 256-262.
[3] Nellis, W. J., Weir, S. T., & Mitchell, A. C. (1996). Metallization and electrical conductivity of hydrogen in Jupiter. Science:
New Series, 273, 936-937.
[4] Herndon, J. M. (1996). Substructure of the inner core of the Earth. Proceedings of the National Academy of Sciences, 93(2),
646-648.
[5] Busse, F. H. (1975). A model of the geodynamo. Geophysical Journal International, 42(2), 437-459.
[6] Pozzo, M., Davies, C., Gubbins, D., & Alfè, D. (2014). Thermal and electrical conductivity of solid iron and iron–silicon
mixtures at Earth's core conditions. Earth and Planetary Science Letters, 393, 159-164.
[7] Vinko, S. M., Ciricosta, O., Cho, B. I., Engelhorn, K., Chung, H. K., Brown, C. R. D., ... & Hajkova, V. (2012). Creation and
diagnosis of a solid-density plasma with an X-ray free-electron laser. Nature, 482(7383), 59-62.
[8] Tarduno, J. A., Cottrell, R. D., Watkeys, M. K., & Bauch, D. (2007). Geomagnetic field strength 3.2 billion years ago recorded
by single silicate crystals. Nature, 446(7136), 657-660.