4 ms·

Wouldn't the pressure of all the weight of the planet itself trying to press down on a single point eventually lead to a heat equilibrium?

pengaru 4y ago
This isn't my area of expertise, but my high-level lay understanding is the core is spinning at a rate faster than the crust. I suppose when it all slows down and the spin rates from core to crust converge, it will all settle down and cool. But I think that's also when you lose the magnetic field... Would love for someone better versed in earth's geophysics to chime in!
Schroedingersat 4y ago
In addition to that taking a couple billion years, earth is also heated by fission to a fairly substantial degree which slows down equilibrium further.
gizmo686 4y ago
You don't even need gravity. Basic thermal dynamics says that a closed system should reach thermal equilibrium. It is (in my opinion) actually a bit surprising that a massive body in thermodynamic equilibrium has a uniform temperature. Assume the planet is an ideal gas. Applying P=VT, you might intuitively expect temperature to rise as you approach the center, since pressure rises as you do so. However, this analysis ignore the V portion of the equation. Consider a cylindrical cross section going up from the center of the body. Define a series of slices of this cylinder, such that each slice has N particles. Assume that N is sufficiently small that the gravitational and pressure gradient is negligible within each slice. As you descend along the cylinder, you will find that while pressure increases, it is the volume of each slice that decreases, leaving temperature as a constant. All of this assumes Newtonian gravity. I believe if you consider relativistic effects, you find a slight temperature gradiant where temperature decreases as gravitational potential increases.
- thehappypm 4y ago
  A simpler explanation: if the center of the earth-gas was hottest, it would heat up the area around it, which would heat up the area around it, etc. eventually there would be no temperature gradients left!

pengaru 4y ago

This isn't my area of expertise, but my high-level lay understanding is the core is spinning at a rate faster than the crust. I suppose when it all slows down and the spin rates from core to crust converge, it will all settle down and cool. But I think that's also when you lose the magnetic field... Would love for someone better versed in earth's geophysics to chime in!

Schroedingersat 4y ago

In addition to that taking a couple billion years, earth is also heated by fission to a fairly substantial degree which slows down equilibrium further.

gizmo686 4y ago

You don't even need gravity. Basic thermal dynamics says that a closed system should reach thermal equilibrium. It is (in my opinion) actually a bit surprising that a massive body in thermodynamic equilibrium has a uniform temperature. Assume the planet is an ideal gas. Applying P=VT, you might intuitively expect temperature to rise as you approach the center, since pressure rises as you do so. However, this analysis ignore the V portion of the equation. Consider a cylindrical cross section going up from the center of the body. Define a series of slices of this cylinder, such that each slice has N particles. Assume that N is sufficiently small that the gravitational and pressure gradient is negligible within each slice. As you descend along the cylinder, you will find that while pressure increases, it is the volume of each slice that decreases, leaving temperature as a constant. All of this assumes Newtonian gravity. I believe if you consider relativistic effects, you find a slight temperature gradiant where temperature decreases as gravitational potential increases.

thehappypm 4y ago
A simpler explanation: if the center of the earth-gas was hottest, it would heat up the area around it, which would heat up the area around it, etc. eventually there would be no temperature gradients left!