|MadSci Network: Earth Sciences|
Dear Jennifer, The Earth is very hot inside, as you rightly say, and the temperature rises the deeper you go. In the outer core of the Earth, which is mostly made of liquid iron and nickel, the temperature is estimated to be about 8,000° C. Hot as this is, however, it does not come close to the temperatures reached inside stars. The temperature at the core of the Sun is estimated to be about 15,000,000° C. A point in your question that needs to be defined is what you mean by a “fireball”. Fire implies combustion, which is the chemical combination of oxygen with another material, as happens when wood burns. In this sense the Sun cannot be described as a fireball because its heat is a result not of combustion but of nuclear fusion in its core, in which hydrogen atoms are fused to form helium atoms. The Earth is not like the Sun because it is not made primarily of hydrogen and its heat comes not from nuclear fusion but from nuclear fission, in which radioactive elements inside the Earth, such as uranium and thorium, decay, releasing heat in the process. You will notice that nuclear fission in the Earth is not combustion any more than nuclear fusion inside the Sun is, for oxygen is not involved. Therefore, one answer to your question is that no planet or star can accurately be described as a fireball because they do not burn via combustion, no matter how hot they become. Another way to answer your question is to see how the materials of which the Earth is made behave when exposed to high temperatures. I think that you might, therefore, rephrase your question like this: “If the Earth is so hot inside, why doesn’t it melt?” The answer is that the behaviour of any material, including rocks inside the Earth, is controlled not only by temperature but also by pressure. The Earth is a rocky planet. It is composed primarily of minerals rich in silica making up the crust and mantle, and primarily of an alloy of iron and nickel in the core. The combined conditions of temperature and pressure in the outer core are such that the iron/nickel alloy is liquid. In the inner core, however, even though the temperature is actually higher than in the outer core, the pressure is also greater, and the combined temperature/pressure conditions are such that the iron/nickel alloy is solid! At shallower depths, in the mantle especially, conditions of temperature and pressure are not able to melt the rocks except in some regions where magmas form. Overall, the mantle is hot enough and under such great pressure that the rocks behave like warm plastic, and are able to flow slowly. This allows the mantle to convect very slowly, like water boiling in a saucepan, and it is this motion in the mantle that drives the movement of the crustal plates above (plate tectonics). So, this answer to your question is that the conditions of temperature and pressure inside the Earth’s mantle are, in some limited regions, right for the rock to melt, and that conditions in the outer core are right for the iron/nickel alloy there to be liquid, but that in most of the mantle and in the inner core conditions do not permit melting. A final point, however, is that if you could travel back in time to the very early history of the Earth more than 4.5 billion years ago, scientists think that the Earth went through a phase when it consisted of what is called a “magma ocean”. In this phase the entire Earth was molten because the heat released by the process of accretion under gravity of the materials that formed the Earth was so great that the temperatures throughout the Earth were hot enough to melt everything! In this phase of its history, I would think any passing aliens might have thought the Earth looked like a fireball! Fortunately, enough time has elapsed since the Earth was a magma ocean for it to cool down and (mostly) solidify. I hope this answers your question. Best wishes, David Scarboro
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