|MadSci Network: Earth Sciences|
Since we obviously have no direct way to sample the earth's core, geophysicists have had to deduce what is made of based on seismic properties, density measurements, and high temperature/high pressure experiments.
Seismic properties: When an earthquake occurs, a number of different types of energy are released, including what seismologists call P waves and S waves. P waves are compressional and can pass through solids and liquids, whereas S waves are shearing and can only pass through solids. Through measuring how these waves travel through the earth and out the other side, a seismic wave shadow zone was discovered in about 1910. The shadow zone resulted from S waves being stopped entirely by the liquid core and P waves being bent (refracted) by the liquid. From the lack of S waves and a great slowing of the P wave velocity (by about 40%) it was deduced that the outer core is made of liquid. The shadow zone also defined the diameter of the core.
The core was deduced to contain a solid center based on two seismic attributes: actual seismic reflections were seen from the liquid outer core/solid inner core boundary in 1936; and the travel time of P waves waves that go through the center of the core are appreciably faster then those that skirt the outside, indicating the center contains some dense, solid material that the waves travel faster through.
An accurate measurement of the size of the solid inner core was not established until the 1960's when nuclear test explosions provided the precise time and location control of the energy source needed for the precision data collecting and calculations.
Density measurements: The earth's average density is easily calculted from the strength of its gravity field. The crust of the earth is quite light, about 2.6 to 3.0 grams/cm3, forcing the interior of the earth to be quite heavy to compensate, about 10 to 13.5 grams/cm3. Assuming the core needs to be composed of a common material (since there is so much of it) an iron/nickel mixture was proposed based on experiments and meteorites (this mix matches the composition of iron meteorites thought to have come from the cores of destroyed planets). For the density to match the proposed compositional mix, however, it can not be entirely liquid, but needs an even higher density solid inner core.
Hope this answered your question,
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