Jeffery Winkler

Earth's Core

	The Earth has a crust, upper mantle, lower mantle, outer core, and inner core. The 
outer core is molten. The inner core is crystalline. The geomagnetic field comes from the 
core. The core is an iron-rich alloy. The core temperature is about 5000 K, where 0 K is 
the lowest possible temperature. Everyone hears these facts in school. However, most 
people don't ask the question, "How do scientists know these things?" You don't ask 
people living in the core, like Edgar Rice Burrough, the author of Tarzan, imagined. The 
actual methods show how clever geophysicists, people who study these things are. I'll 
describe some of the methods used to reach these conclusions.

The outer core is molten - There are three pieces of evidence to support this. First of all, a 
type of seismic wave called horizontally polarized shear waves, propagating downward 
through the mantle transmit no measurable energy into the core, showing that the outer 
core is not rigid. We can tell this when we detect the waves when they reach the other 
side of the Earth. Second of all, when there is a large earthquake, it oscillations in the 
Earth that can only be explained if the outer core is not rigid. Third of all, the wobble of 
the Earth's rotational axis is what it would be if the outer core is fluid.

The inner core is crystalline - Seismic free oscillations caused by large earthquakes are 
what they would be if the inner core was rigid. We have never been able to detect a shear 
wave that traveled all the way through the inner core, and come out the other side.

The geomagnetic field comes from the core - It was known the 19th Century from Gauss' 
experiments with spherical harmonics, that almost all of the Earth's magnetic field comes 
from deep inside the Earth. Changes in the magnetic field over time, and electric currents 
measured in the crust, show that the mantle has low electrical conductivity. Also, 
measurements of how the magnetic field has changed over time gives information about 
the fluid velocity at the top of the outer core, about one to ten kilometers per year.

The core is an iron-rich alloy - Laboratory experiments show how the densities, and thus 
the elastic wave velocities, of elements vary with mass. Seismological measurements of 
the elastic wave velocities show that Earth's core has an average atomic number of about 
25, close to that of iron. The other elements close to that mass, such as vanadium or 
chromium, are extremely rare in the Universe, so it is not plausible that they could make 
up a large fraction of our planet. Also, the only known mechanism that could produce 
Earth's magnetic field requires an electrically conducting fluid. Only a metallic liquid, 
such as molten iron alloy, could be involved at these pressures and densities. Also 
metallic meteorites are an iron-nickel alloy. However, we know that the core is not pure 
iron, or pure iron-nickel alloy. It's density is about 10% less than what it would be if that 
were true. We therefore believe that it has a lighter contaminant, such as sulfur, oxygen, 
or hydrogen.

The core temperature is about 5000 Kelvin. Zero Kelvin is the lowest possible 
temperature, and water freezes at 273.16 K. By studying the Earth's free oscillation 
spectrum, using data from earthquakes, and taking gravity into account, you can 
determine how Earth's density changes with distance from the center. By this method, it 
is determined that there is a pressure of 3.6 x 10^6 atmospheres at the center of the solid 
inner core, falling to 1.4 x 10^6 atmospheres at the top of the liquid inner core. The 
temperature at the boundary between the crystalline inner core and the fluid outer core is 
evidently close to the melting temperature of the core material at the appropriate 
pressure. Laboratory experiments reveal that iron and plausible iron alloys melt at about 
4000 - 6000 K at core pressures.