What makes Earth’s core hot? Why isn’t it just a cold blob made up of metal and minerals?

287 views

What makes Earth’s core hot? Why isn’t it just a cold blob made up of metal and minerals?

In: Earth Science

Two major reasons:

1. The earth was formed as a result of accretion. Small particles gradually attracted each other due to gravitational forces and formed bigger objects. This process accumulated a lot of heat; when two objects collide, heat is generated. And as a majority of the heat was retained and never lost, the earth’s core remained hot.
2. However, a majority of the heat (>90%) is produced through the **decaying of radioactive elements**. The nucleus of unstable elements like U238 (Uranium) gradually break down, releasing energy.

It will become a cold blob made up of metals and minerals eventually, it’s just so phenomenally large that it takes a very very long time to cool down.

The Earth was formed quite a long time ago by a bunch of space rocks smashing together. When space rocks smash together, they heat up, because collisions produce heat (search youtube for “slapping a chicken until it’s cooked” for an amusing demonstration of this). Get a lot of space rocks smashing together over a long period of time and the resulting big space lump gets so massive and so hot it rearranges itself into a roughly spherical shape under gravity and separates out into layers based on density.

Hot things cool down. You can observe this by for example boiling some water, pouring it into a mug and just waiting. It just takes a really long time for big things to cool down. Takes even longer when the big thing has the equivalent of a bunch of nuclear generators inside it generating extra heat.

Two reasons:

1. Before Earth came together, it was a bunch of rocks far apart. In order to move towards each other, the rocks borrowed motion energy from gravity. When they ran into each other, the motion energy turned into heat energy, just like rubbing your hands together turns motion into heat. Part of the heat in Earth’s core is leftover heat from that process, which is still there because the Earth hasn’t had time to cool down yet. In fact, compasses work because the Earth’s core turns itself into a magnet as it cools. Compasses don’t work on Mars because Mars is smaller, and so has already cooled off enough that its core isn’t partly melted.
2. Rocks have a bit of radioactive stuff in them, and radioactive stuff warms itself up by breaking apart big atoms into smaller ones. Really radioactive stuff like some kinds of plutonium can get hot enough to glow even when it’s in small pieces, but for ordinary rocks the warming is so tiny that you don’t notice it unless you have a whole Earth worth of rocks in one place. Without slightly radioactive rocks warming themselves up, the core would be a lot colder than it is.

There are many different factors for this:

1. Surface area to volume ratio: The bigger an object, the slower it cools because it has only so much surface area from which heat can dissipate. The earth, just by virtue of its size, still retains heat from its formation 4 billion years ago.
2. Once the earth cooled enough, its outer layer solidified into the crust (rock), which is an excellent insulator and made it even harder for heat to escape, effectively acting as a blanket.
3. Earth is surrounded by a vacuum where you can only lose heat via radiation. This is extremely inefficient and takes an insanely long amount of time. It’s also the same reason why people don’t freeze instantly when exposed to outer space.
4. The tidal force that the moon exerts on the earth converts earth’s rotational energy into heat and warms the interior. This process is called tidal heating. A good example would be Jupiter’s moon Io. When the moon was young, an earth day was just 6 hours long, meaning it was spinning 4x as fast. Now, all these years later, all that energy has been converted, and continues to be converted, into heat.
5. The earth has a lot of radioactive elements in the core which decay and release energy, warming up the other layers.

Now not all of these factors contribute equally and perhaps some play a much bigger role than others, but yeah, because of all of them, even after all these years, the earth’s core is still hot.

It’s hard for non-hot things to lose heat in the vacuum of space. Even with the so-very-cold 3 K deep space background. Because only radiation (glowing) works, not convection (not much there to move) or conduction (not much there to touch). And glow brightness, radiative loss, goes as the *forth* power of temperature. Half the temperature means 16 times less loss (2⁴). So cooling spacesuits is a challenge, because people generate heat, that needs to be gotten rid of, but not like a glowing red-hot temperature, that would radiate quickly away (suits boil off water instead). One non-realism in The Expanse is spacecraft not having *bloody giant* radiators to cool off their fusion engines.

The crust and mantle insulate the core, keeping Earth’s surface non-hot, keeping radiative loss low. If you exposed the core to space, it would initially cool down 10000x faster, being 10x hotter. Earth is cooling faster lately, because of more tectonics and thinner crust, since the breakup of supercontinent Pangaea. Like a mantle degree per 10 million years.

The heat loss to space is partially compensated for by new heat from radioactive decay of big atomic nuclei assembled in exploding star(s?) before the solar system formed. Before radioactivity was discovered, people were puzzled why the initially molten Earth hadn’t frozen yet, and so thought the Earth must not be very old. Earth was initially molten because slamming together big rocks makes for white-hot splashing – thus burnt-toast dinosaurs. With 4.5 billion years of half lives past, radioactive heating isn’t what it used to be.

The Sun will expand and melt the Earth again, before it has time to freeze solid. And then it will vaporize it.