|MadSci Network: Earth Sciences|
Dear Matthew, The answer to your question is that there is still ample heat in the Earth’s interior to maintain a liquid outer core. The Earth’s core is only partially molten. The outer core is composed of molten iron and nickel, and the inner core is solid. The inner core has grown at the expense of the outer core over the 4.5 billion years of the Earth’s history. Ultimately the core will entirely solidify, but this event will occur only in the far geological future. The Earth has two sources of internal heat. One is the heat of accretion, which is heat converted from gravitational energy as the materials which formed the Earth fell together under gravity in the early Solar System. This heat is now estimated to make up about 20% of the total heat flow from the Earth’s interior. That fact is an indication of how long it has taken the Earth to vent this heat to space. The other source of heat is the decay of radioactive isotopes of uranium, thorium and potassium, incorporated into the Earth at its accretion. This radiogenic heat is the principal explanation of why the Earth’s interior is still so hot after billions of years. Without radiogenic heat the Earth would by now have cooled down to the point where the core would probably be solid. A final factor to consider is the Earth’s size. The Earth is the largest of the rocky planets in the Solar System, and the thickness of the mantle acts as a blanket. Heat is conveyed to the surface by conduction and by convection, the process that drives plate tectonics. The rate of heat loss governed by the Earth’s size and composition, balanced against the production of heat by radiogenic decay in the mantle and core and the remaining heat of accretion, explains why the Earth’s outer core is still molten. 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. I hope this answers your question. Best wishes, David Scarboro
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