From: shaw@s119.es.llnl.gov (Henry Shaw 510 423-4645)
Newsgroups: sci.geo.meteorology,sci.physics
Subject: Re: pressure at the center of the earth?
Keywords: pressure, earth core, blackhole, truth or consequences
Date: 21 Aug 92 17:10:40 GMT
Followup-To: sci.geo.meteorology
Distribution: na
Organization: Earth_Sciences_Dept.,_LLNL

In article <25627@dog.ee.lbl.gov>, sichase@csa3.lbl.gov (SCOTT I CHASE) writes:
|> In article <sv#ns=b@quantum.on.ca>, ajedgar@quantum.on.ca (Andrew Edgar) writes...
|> >To settle a difference of opinion <grin> I would be most grateful
|> >for an answer and explanation for this question:
|> > 
|> >	"What is the pressure at or near the center of the earth,
|> >	 is it very large or very small?"
|> 
|> The center of the Earth undoubtedly contains a cherry - just like those
|> liquor filled chocolates.   
|> 
|> Back to physics...  The center of the Earth is under tremendous pressure.
|> How could it be any other way?  The liquid core is under so much pressure
|> that geologists believe (though I don't think that the experimental evidence 
|> is entirely conclusive) that there is a solid center to the core - tremendously
|> hot metal which is solidified under the pressure.

The experimental evidence is pretty much entirely conclusive.  The evidence that the inner 
core is solid while the outer core is liquid is the fact that the inner core transmits acoustic 
shear waves (from earthquakes), while the outer core does not.   The ability to transmit shear 
waves requires a non-zero rigidity, which solids have but liquids do not.

You may now be thinking, "if the outer core does not transmit shear waves, then how do they ever 
get past the outer liquid core to the inner core so that we can see that the inner core *does* 
transmit them?"  The answer lies in the fact that compressional waves (which can propagate through 
the liquid outer core) reflect off the inner/outer core boundary.  In doing so, some of the energy 
is converted to a shear wave in the inner core.  This wave propagates through the inner core until 
it reaches the boundary again, where it can generate another compressional wave that continues to 
propagate through the outer core.  We see the results of this compression-shear-compression 
conversion process in the seismic records of large earthquakes.

|> 
|> No planet-sized self-gravitating object could easily be constructed so 
|> that the center was a low-pressure region.  The object would have to have 
|> some kind of supporting structure outside the central region that could
|> withstand the huge pressure from above without transmitting through to the
|> core.  Maybe a cold solid diamond sphere with a central hollow...  
|> 

The density structure of the earth is constrained by both acoustic wave travel times (the sound 
velocity structure) as well as free oscillations of the earth (i.e., the vibrational normal modes 
that are excited by really really big earthquakes).

The density at the center of the earth is approximately 13 g/cc, while the pressure (which you get 
 by integrating -g(z) rho(z) dz, where g(z) is the acceleration due to gravity at a radius z, and 
rho(z) is the density at z) is approximately 3.63 x 10^6 bars. = 3.63 x 10^11 Pa.

While we're on the subject of the interior of the earth, a little known fact among even most 
scientists is that the gravitational acceleration does *not* fall off monotonically with 
decreasing radius within the earth.  This is because the earth has a highly non-uniform internal 
density.  The acceleration actually rises to a maximum of ~10.01 m/s^2 as you go down from 
the surface to a depth of about 250km (the upper/lower mantle bouindary).  It then decreases to 
about 9.93 m/s^2 at about 850km.  The it starts rising again as you approach the core/mantle 
boundary (the core is *much* denser than the mantle).  It maxes out at a value of ~10.7 m/s^2 
at the core/mantle boundary, then decreases monotonically to zero at the center of the earth.


-- 
Henry Shaw, Earth Sciences Dept., LLNL |    All opinions are my own.
Internet: shaw4@llnl.gov               |  
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