Why is the outer core composed of liquid?
The Earth is not a blob of liquid. The thin outer crust sits on the solid mantle, which is 2900 km thick. The liquid outer core beneath the mantle is 2255 km thick, and the solid inner core is 1215 km in diameter. Therefore, and ignoring the thin crust, the liquid outer core comprises only 35% of the Earth’s diameter. But even if the Earth were largely liquid it would still retain the shape of a globe under the force of gravity, albeit it would be more easily deformed by the centrifugal force of its spin and by the tidal pull of the Sun and other massive objects in the Solar System. The situation of a planet composed of volatile materials is realized in the cases of Jupiter, Saturn and the other giant planets. Jupiter is composed mainly of hydrogen and helium, which is a gas in the outer regions of the planet and probably a metallic liquid at the enormous pressures in its interior. Jupiter keeps its shape because the enormous force of gravity pulls all of its matter towards the centre.
