As we all know the Earth has a layered architecture and the layer we see of our planet’s surface is actually the crust, which is also its thinnest layer that ranges from 5 to 70km in depth. The crust is the covering of the earth’s second layer that is the mantle. The mantle, which is also the thickest layer of earth, extends to a depth of about 2890km with pressures at the bottom of the mantle reaching as high as 140 GigaPascals. For comparison, our atmosphere has a pressure of only 0.0001 GigaPascals.
The mantle leads the way to the core of the earth, which in itself is divided into two layers that is the outer core and the inner core. While the inner core is completely solid, the outer core is liquid. While the solid inner core has an estimated radius of 1220km, the liquid outer core extends beyond it to a radius of 3400km. As we can expect that at that depth, pressure and temperature is so high that it even melts iron and as a result everything in there should be in molten form. However, the fact is that despite being at the maximum depth, the inner core is completely solid and the outer core is molten.
To answer the question that why does the earth have a liquid outer core we have to understand that like all elements, iron can exist in any form, that is solid, liquid and gas, depending on the pressure and temperature it is at. However, the iron is a lot more complicated than other metals and it goes back from liquid to solid when the pressure is immense. The inner core experiences a pressure of an estimated 360 GigaPascals and at that pressure, immense temperature, like the one found on the surface of the sun is needed to melt iron. However, the temperature already inside the earth is enough to melt iron, provided there is less pressure.
To prove why earth’s outer core is in liquid state, researchers say that the earth cools over time and its interior temperature drops, while its pressure remains constant. In other words, when the earth was born, it is likely that the entire core was molten. However, as the cooling process started the solid inner core continued to grow and the liquid outer core continued to shrink. In simple words, the earth’s core is liquid, because the temperature there is high enough to melt iron, however, liquid iron only occurs in some places where the pressure is low enough.