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Question: How do scientists know about the core of the Earth?
Answer: We know about the Earth's core because of what it is, what it did, and what it does. And as a result, all life on Earth can be grateful that the core exists.
The core is the spherical region of the Earth that begins about halfway down to Earth's center. Above the core and below a relatively thin crust is a very thick 'mantle'. Although the core is far from us, and completely inaccessible to our touch, there are many indirect ways to learn about it. Earth's gravity, for example, tells us how massive the Earth is (6 trillion trillion kilograms); that mass implies, given the Earth's size, that each cubic meter of its volume contains, on average, about 5500 kilograms. But rocks on Earth's surface (and in the mantle) are typically only half as dense as that; to boost the average, the core must be made of very dense stuff. Since iron is the most abundant of all heavy elements in the Universe, it is reasonable to propose that our core is mainly iron.
There is other evidence in favor of an iron core. Our earliest deductions that the core exists and is made of iron, for example, came from observations about the way seismic waves pass through Earth's interior. When an earthquake occurs, seismic waves are radiated from it in all directions. But seismographic stations a certain distance away will fail to detect the first waves from the quake; those stations are sitting 'in the shadow of the core', whose material properties caused the waves to deflect away from them. These properties are similar to the properties of iron, measured at high temperatures and pressures.
The Earth's core has not always existed. As the Earth built up into a planet, about 4.5 billion years ago, its interior heated up. At some point things got hot enough that iron, dispersed throughout the interior, began to melt and drip down towards the deepest points of the Earth. As this process of core formation continued, the 'falling' iron released a lot of heat (just as dropping a heavy box converts its gravitational 'potential energy' to kinetic energy, as it falls faster and faster, and then to heat as the box slams into the ground).
The heat released within the Earth by core formation was huge, and the rocks above the core softened and began to flow as a result. This process, with hot, lighter material rising and colder, denser stuff sinking, is called 'convection'. Convection within the mantle brought the Earth to life, geologically: it led to continental drift, earthquakes, mountain-building, and (here's the kicker) volcanic activity. The gases spewing out of the volcanoes, rifts, and hot springs - an immense out-gassing known affectionately as the 'Big Burp' - included large amounts of water vapor and carbon dioxide, giving us watery oceans and an atmosphere! A dense carbon dioxide atmosphere is not what we have now, and would be toxic to humans, but back then it was a great atmosphere to keep Earth warm and encourage the earliest (bacterial) life to evolve. Then, over the next couple of billion years, geologic processes soaked up the CO2, and photosynthesizing organisms gradually added enough oxygen to make life as we know it possible.
The heat from core formation also made the core itself hot enough to convect. Iron is a very good electrical conductor, and as it convects, it generates electrical currents which in turn produce the geomagnetic field! The geomagnetic field extends away from the core, through the mantle and crust, out into space; and it traps charged particles from the Sun and from cosmic rays - in a region of the upper atmosphere called the Van Allen Radiation Belts - that could otherwise rain down on Earth and harm life at Earth's surface.
So, how do we know that the core exists? We know because the earliest atmosphere must have had abundant carbon dioxide, or else life might not have developed on Earth; and we know because the Earth has a strong magnetic field, which has continued to protect life on Earth. Both of these owe their existence to Earth's core.
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