The Core of Earth's Structure

    It’s a hot day, and you need a refreshing drink. You order a nice, big cup of boba, like the one pictured here on the right. It’s a nice sight - the milk tea and the tapioca pearls all in one tasty drink. Now, imagine that cup of boba is our Earth, but instead of being made up of tea and tapioca, it consists of a crust, mantle, and core. The fact that Earth is structured this way may be common knowledge, but how exactly this structure came to be may be a mystery to some.

    To start with, Earth hasn’t always looked the way it does now. Early on in Earth’s history, our planet was hot enough to be partially molten. This heat came from a couple of different sources: impacting material and the decay of radioactive elements. Radioactive elements are atoms that are unstable, and tend to decay to a more stable form. For example, over time, uranium can decay to thorium, potassium to argon, and aluminum to magnesium. These decay processes, among others, can generate significant amounts of heat inside the Earth. Aluminum in particular was a substantial heat source during Earth’s formation. This specific isotope, referred to as aluminum-26, has a short half life, meaning that it decays relatively quickly. This means that all of the aluminum-26 was used up when it was heating Earth, and actually doesn’t exist anymore!  

    Our other main heat source, impacting material, is very different in nature. Think of what happens when you clap your hands together. Depending on how hard you do it, your hands will get warm (and maybe even sting a little bit). Now, instead of your hands, imagine a massive planetary body, like an asteroid or planetesimal, crashing into Earth at an extreme speed. Events like these occurred often as the planets in our Solar System formed, and Earth was no exception. These collisions generated large amounts of heat - enough to trigger localized melting. 

    All of this heat and melting made it easier for the now molten material within the Earth to move around and separate from one another. This process is called planetary differentiation, and it’s the reason Earth is structured the way it is. Denser, molten material was able to travel toward the center of the Earth, while the less dense material rose toward the surface. This is why Earth’s core is primarily made up of iron and nickel - two relatively dense metals. In contrast, the crust and mantle are made up of minerals called silicates, which are much lighter. To go back to the boba analogy, the iron and nickel would be our tapioca balls sinking to the bottom of the cup. The tapioca is denser than the overlying milk tea, just like how Earth’s core is denser than the lighter minerals that comprise the crust and mantle. Earth is exactly like boba! Just maybe a little bit harder to drink.

    This process doesn’t just apply to Earth, though. Similar processes took place on the other planets in our Solar System, as well as on their moons, and on a variety of asteroids. This reorganization of material gives planetary bodies their characteristic layering, and can tell us a bit about what’s going on in their interiors - even though we may not be able to see them up close. Fortunately, we’re able to use meteorites that fall to Earth to learn more about these other planetary bodies - whether it be Mars, the Moon, or some far-off asteroid! For example, the meteorite pictured below is composed nearly entirely of iron and nickel, meaning that it was likely part of the core of a planetary body that had broken apart at some point. This meteorite, and the thousands of others that have been discovered across Earth, can tell us a lot about the planets and asteroids that we share our Solar System with!