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Question:
Regarding pressure at the center of the earth,

http://www.newton.dep.anl.gov/askasci/env99/env002.htm

it seems that the comments by Argonne reviewers that gravity is zero or near zero and that things would float contradicts statements that the pressure is high. If you consider a spherical surface at the center, all the material on the outside of the surface is under no force, i.e., it is floating in a zero gravity field. Therefore the force on any material at the surface is also near zero--in other words no pressure. I would guess that at the maximum estimated temperature at the center, even iron would be a solid, if the pressure estimates of 380 Gpsl's were correct. From seismic surveys it appears the center is in a liquid state which is consistent with iron at 2000 degrees C at a relatively low pressure. What is the model used to calculate the pressure at the center?


Replies:
The force due to gravity is definitely not zero at the center of the earth. You would feel like you were floating because the forces of gravity working on you body from every direction would be balanced. Its a bit like those globes you see for sale in airline magazines and elsewhere that are suspended in air by some magnets in the base and in an overhead arm. The globe is experiencing magnetic force that cancels out the effect gravity and is floating but surely gravity is still around. Just remove the magnets..

As far as the condition of the center of the earth, there seem to be two separate cores, quite creatively named the inner and outer. The outer core is a material that "acts like a liquid in that it will not transmit shear or transverse waves". As to what it looks like, the only thing we can say is that you probably could not pour it. At those temperatures and pressures the words solid and liquid really have a very different meaning.

The inner core does support shear waves and so acts like a solid.

As we cannot generate nearly the temperatures and pressures of the inner and outer core on earth's surface for more than a few milliseconds at a time , the best we can do is some very sophisticated modeling of how materials would react to those conditions.

Robert Avakian



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