Eli5: How does the core of the Earth (or any planets) not cool and still give off tons of heat energy? Where is this energy coming from?

Question

Eli5: How does the core of the Earth (or any planets) not cool and still give off tons of heat energy? Where is this energy coming from?

In: 202

Answer 1

It’s a function of volume and surface area.

Think about it this way:

If you’ve got a cup of normal-sized ice cubes, they’ll melt in a handful of minutes.

If you’ve got a cup with a cup with a single, big ice cube in it, it’ll last much much longer. Hours maybe. Even if it’s technically the same amount of ice.

And if you’ve got a bunch of crushed ice or snow, it’ll melt in an instant. Once again, even if it’s the same amount of ice, just crushed up.

This is because the only part of a thing that can melt (or cool, if it’s hot) is the part on the outside. So the more outside a thing has relative to the total volume (the more surface area it has), the faster it will warm up or cool down.

For example, back before refrigeration, people used to harvest ice for the summer months. They’d take enormous ice blocks, pile them up in huge insulated “ice houses” and cover them in saw dust to insulate them further. But mainly because these huge piles of ice had a lot more “inside” (volume) than “outside” (surface area) they would last for *months or years,* even though it was really just a big ice cube slowly melting.

Intuitively, we think that if a block of ice is twice as big, it’ll take twice as long to melt, but this isn’t true. It’ll take much longer.

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Now if we talk about the Earth:

Yes, there’s radioactive elements slowly decaying deep inside it, and this creates heat.

And yes, the tidal pull of the moon stretches and squishes the Earth somewhat, generating heat. No idea off the top of my head how significant either of these things are.

But the main reason it’s still hot is simply that it started out really hot a long time ago, and it’s really really really big. It has enormous volume relative to its surface area, and it takes an enormous amount of time for all of that heat to slowly slowly bleed away. And meanwhile, the cooler outer layers act like insulation, further slowing the process.

**TL:DR: A red hot ball of iron a thousand miles in diameter, insulated by thousands of miles of solid rock, will stay hot for billions of years. Because it’s big.**