Low density material migrating to Earth’s center because it’s zero gravity there

[Jegroeg]

Moderation note: The first 5 posts of this thread were moved from the Strange Claims thread 13598 and the Earth Science thread 19656, because those threads are long and old, so new discussion is better served in a new thread

 

I have not been present for a few years, but something has been bothering me about the earth core debate.

I can easily discount the centrifical effect for creating the possibility for hydrogen migrating to the core. However, I can conceive of hydrogen migrating there. Low density hydrogen gas in a higher density medium will migrate counter to a gravitational attraction, that is, up into the air. I am not sure of the relative density of liquid or solid hydrogen in relation to high density media.

 

OOPS. Point is: The center of the Earth approaches ZERO gravity.

[modest]

The lower density thing (hydrogen) in a solution of higher density thing (liquid rock or metal) will always want to move from lower gravitational potential to higher gravitational potential where potential is a negative value—the most negative at the center and progressively less negative toward the surface.

 

Another way of looking at it: it takes work to pull a balloon filled with hydrogen closer to the center of the earth. All the way down to the center of the earth it would take work to pull it down. Because you are displacing something more dense you are working against gravity to get it down there.

 

~modest

[Jegroeg]

It has occurred to me that this discussion has ignored gravity WITHIN A SPHERE. The very center of the planet would experience ZERO gravity, as mass surrounds it in all directions. Low density material, like hydrogen and helium, in whatever phase, would migrate in a direction away from the relative gravity well of the surrounding planetary mass.

Just a thought. Physicists can work out the details.

[stereologist]

This discussion has not ignored that issue. The migration as you call it would not work since there are other factors involved such as temperature, chemistry, and pressure.

[CraigD]

Welcome back to Hypography, Jegreog! :naughty: It’s been a few years.

It has occurred to me that this discussion has ignored gravity WITHIN A SPHERE. The very center of the planet would experience ZERO gravity, as mass surrounds it in all directions. Low density material, like hydrogen and helium, in whatever phase, would migrate in a direction away from the relative gravity well of the surrounding planetary mass.

Intuitively, I can see why one might think that this might happen, and why, in a body somewhat like a hollow sphere, bodies inside it might be gravitationally attracted toward a “gravity well of the surrounding planetary mass”. However, this was shown not to happen about 300 years ago, via Newton’s famous proof of the shell theorem.

 

Though the details of this theorem requires a bit of calculus (though regardless of your mathematical skills, is worth reading about via the above wikipedia article or any of many other books or websites), the consequences are simple: inside a hollow sphere, regardless of the mass surrounding the hollow spherical volume, the net force of gravity is everywhere zero. The gravitational force at any given point in a non-hollow spherical body, therefore, is determined only by the mass “under” (closer to the body’s center of mass than) it.

 

So, regardless of the density of layers of the Earth or any other body, denser material will, if able to move, move toward its center, displacing less dense material toward its surface.

Just a thought. Physicists can work out the details.

Quite a good thought – though one for which physicists have worked out the details a long time ago.