How do we know Earth has an iron core? Experts think that the Earth has an iron core because of the way seismic waves pass through the Earth, its overall density, the abundance of iron, our magnetic field, and laboratory experiments.
Before we begin, scientists do not know with absolute certainty that the Earth has an iron core. An iron core just seems to be the most likely explanation based on all of the available evidence. Nobody has managed to drill down deep enough to sample the core directly. In fact, with current technology, it would not be possible even if someone wanted to. The heat and pressure are far too high, and the core is far beyond anything humans can physically reach.
There are several layers to the Earth. The outer layer is the crust. The continental crust is what we stand on, while the oceanic crust lies beneath the sea. Below that are the upper and lower mantle, which are made up of rock that can flow very slowly despite being solid. Heat inside the Earth causes convection currents in the mantle, and those currents help move the tectonic plates. Then there is the outer core, which is liquid and is thought to be mostly molten iron and nickel. Inside that is the inner core, which is a solid ball roughly 2,440 km in diameter. It is also thought to be made mostly of iron and nickel.
There are several reasons why scientists believe the core of the Earth is made of iron and nickel. The first is the way seismic waves travel through the planet. When an earthquake happens, it sends shock waves through the Earth. Two of the main kinds are P waves, or primary waves, and S waves, or secondary waves. P waves are longitudinal, very fast, and can travel through solids, liquids, and gases. S waves are transverse, slower, and can only travel through solids. Scientists noticed that S waves do not pass through the middle of the Earth. Instead, they leave a large shadow zone on the far side of the planet. That suggested that part of the Earth’s interior must be liquid, because S waves cannot travel through liquid. P waves do pass through that region, but they slow down and bend, which also suggests a liquid outer core. More detailed study of the way P waves move through the very center of the Earth suggests that inside that liquid layer there is a solid inner core. The speeds and paths of those waves fit very well with a core made mostly of iron.
The second reason is that the Earth is too dense to be made only of rock. Scientists can estimate the Earth’s mass by measuring its gravitational pull. When they compare that mass to the Earth’s size, they find that the average density is too high for the planet to be made only of crust and mantle rock. There must be some much denser material inside it, and iron is one of the most likely candidates.
The third reason is that iron is a very common element. In large stars, heavier and heavier elements are made until iron is reached. After that, the star can no longer produce energy in the same way, and violent stellar events scatter those elements into space. Iron is common in meteorites, and it makes up a large part of the Earth’s mass. Scientists also realized that when the early Earth was much hotter and more molten, heavy elements like iron would have tended to sink toward the center while lighter rocky materials stayed closer to the surface.
The fourth reason is that scientists can test these ideas in laboratories. They can subject iron and iron-rich mixtures to enormous pressures and temperatures similar to those deep inside the Earth. When they do that, the material behaves in ways that match what is expected from the Earth’s core. That does not prove it directly, but it adds more support to the idea.
The fifth, and probably most convincing, reason is that our planet has a magnetic field. A strong global magnetic field suggests that there is some kind of internal dynamo operating inside the Earth. The movement of electrically conductive liquid iron in the outer core, helped by the Earth’s rotation, is exactly the kind of process that could generate such a field. Due to all of these reasons, scientists are fairly sure that our planet’s core is made mostly of iron. They can also use similar methods to work out what the cores of other planets are probably made of. And this is what I learned today.
Sources
https://www.scientificamerican.com/article/why-is-the-earths-core-so
https://en.wikipedia.org/wiki/Earth%27s_inner_core
https://en.wikipedia.org/wiki/Earth%27s_outer_core
https://en.wikipedia.org/wiki/Internal_structure_of_Earth
https://www.sciencefocus.com/planet-earth/what-is-at-earths-core
https://en.wikipedia.org/wiki/Seismic_wave
Photo by Pixabay: https://www.pexels.com/photo/earth-wallpaper-41953/