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?
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The planet is cooling off, although very slowly. Heat in the inner planet has three sources:
Original heat present when the planet formed
Heat given off via radioactive decay
Heat given off when liquid outer core iron crystallizes at the inner/outer core boundary.
Eventually the latter two will subside and Earth will slowly cool down permanently.
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.**
Radioactive decay, friction from gravitational influences, and also the earth in in a vacuum.
Although space is cold, there’s not much there that *is* cold too.
Getting rid of heat in space is a bit of an issue as it can only be radiated away, usually you are used to things cooling by conduction which is far better.
Radiators for example, they do radiate heat, but also conduct it into the air around them. Radiators of vehicles particularly need a lot of airflow for them to cool anything.
In space, there’s no icy cold breeze to take the heat away either.
Take a thin, light non-stick frying pan and heat it up, then let it cool. Do the same with a heavy cast iron pan and you’ll find the heavy cast iron holds heat for a lot longer.
Same thing with Earth. It’s very big and heavy, and the core is under immense pressure. It’s cooling, just very slowly.
Radioactive decay, friction from gravitational influences, and also the earth in in a vacuum.
Although space is cold, there’s not much there that *is* cold too.
Getting rid of heat in space is a bit of an issue as it can only be radiated away, usually you are used to things cooling by conduction which is far better.
Radiators for example, they do radiate heat, but also conduct it into the air around them. Radiators of vehicles particularly need a lot of airflow for them to cool anything.
In space, there’s no icy cold breeze to take the heat away either.
Take a thin, light non-stick frying pan and heat it up, then let it cool. Do the same with a heavy cast iron pan and you’ll find the heavy cast iron holds heat for a lot longer.
Same thing with Earth. It’s very big and heavy, and the core is under immense pressure. It’s cooling, just very slowly.
Although scientists are not exactly sure on the balance, about half the energy is coming from radioactive decay and half from leftover energy from when it formed.
Every time a random radioactive atom splits, it releases some energy. The Earth is big, and there is a lot of energy per radioactive atom, so there is a lot of this energy available, which releases over time.
The other interesting part of this is that the Earth is really big. Roughly 6400 km in radius. The center of the core is close to 6000 Kelvin. So on average the temperature would drop 1 Kelvin per Kilometer. In practice, the temperature drops 10 Kelvin per Kilometer in the crust, because it drops much slower in the rest of the Earth (due to convection etc. keeping temperatures more equal). This is a very small temperature gradient, which means heat flows very slowly. Moreover, the Earth being really big means that it stores a lot of energy and it takes a lot of energy to change its temperature even by a single degree. Thus, it takes a long time for the temperature to change.
If you compare the heat flow of the Earth’s interior with the heat coming from the Sun, the Sun dumps about 10,000x as much heat on the surface: 1300 W/m^2 compared to 0.1 W/m^2. Now, this does not mean that the Sun heats the Earth’s core, because the Sun’s heat does not travel to the core. Most of it gets sent back out into space.
It is cooling, thats why its giving off energy. Its just that theres a fuckton of energy in there, and its not giving it off all that fast, relative to the amount.
Although scientists are not exactly sure on the balance, about half the energy is coming from radioactive decay and half from leftover energy from when it formed.
Every time a random radioactive atom splits, it releases some energy. The Earth is big, and there is a lot of energy per radioactive atom, so there is a lot of this energy available, which releases over time.
The other interesting part of this is that the Earth is really big. Roughly 6400 km in radius. The center of the core is close to 6000 Kelvin. So on average the temperature would drop 1 Kelvin per Kilometer. In practice, the temperature drops 10 Kelvin per Kilometer in the crust, because it drops much slower in the rest of the Earth (due to convection etc. keeping temperatures more equal). This is a very small temperature gradient, which means heat flows very slowly. Moreover, the Earth being really big means that it stores a lot of energy and it takes a lot of energy to change its temperature even by a single degree. Thus, it takes a long time for the temperature to change.
If you compare the heat flow of the Earth’s interior with the heat coming from the Sun, the Sun dumps about 10,000x as much heat on the surface: 1300 W/m^2 compared to 0.1 W/m^2. Now, this does not mean that the Sun heats the Earth’s core, because the Sun’s heat does not travel to the core. Most of it gets sent back out into space.
It is cooling, thats why its giving off energy. Its just that theres a fuckton of energy in there, and its not giving it off all that fast, relative to the amount.
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