From: PointedEars@web.de   
      
   Thomas 'PointedEars' Lahn wrote:   
   > Popping Mad wrote:   
   >> then why is it that the deeper you go into a plant, like Jupiter for   
   >> example, the pressure increases and the center of a star, when it   
   >> forms, ignites into nuclear fusion?   
   >   
   > Consider the terrestrial atmosphere and hydrosphere first.  There is   
   > atmospheric pressure on the terrestrial surface because we are living at the   
   > bottom of an "ocean" of air: The air above you and the planet attract each   
   > other gravitationally (in Newton's theory of gravitation), and each layer of   
   > air pushes down on the layer below it and compresses it (thus the   
   > atmospheric pressure and density decreases with increasing altitude).   
   >   
   >   [In fact, you can arrive very closely at the standard atmospheric pressure   
   >    by estimating the mass of the terrestrial atmosphere from the density of   
   >    air, and approximating it as a thin uniform spherical shell (so that you   
   >    can calculate as if it were a cuboid) out of nitrogen and oxygen in the   
   >    known proportions (say 79 % nitrogen and 21 % oxygen, ignoring trace   
   >    gases), with a thickness of ca. 100 km, and calculating the pressure that   
   >    the atmosphere therefore exerts as the gravitational force divided by the   
   >    surface area.]   
      
   This is another paragraph that can be improved ':-)   
      
     [In fact, you can arrive very closely at the standard atmospheric pressure   
      by approximating the atmosphere as a thin uniform spherical shell out of   
      nitrogen and oxygen in the known proportions (say 79 % nitrogen and 21 %   
      oxygen, ignoring trace gases), with a thickness of ca. 10 km.  This   
      approximation allows you to calculate its volume as if it were a   
      rectangular cuboid.  You can estimate the mass of the terrestrial   
      atmosphere from the density of air and this estimate of its volume.  Then   
      you can calculate the pressure that the atmosphere therefore exerts as   
      the gravitational force divided by the surface area.]   
      
   > [...]   
   > It is the same with gas giants, only their atmospheres are much deeper (it   
   > is assumed, now also from gravitational measurements, that they have a core   
   > out of metallic hydrogen, but they are still huge), and the substances are   
   > all *gaseous* (despite the low temperature), thus *compressible*, so the   
   > atmospheric pressure and temperature towards the core can increase even   
   > further (when a gaseous substance is compressed, momentum is imparted on its   
   > freely moving particles, so they move faster which we understand as a higher   
   > temperature; cf. the equation of state of an ideal gas).   
      
   And this one ':-)   
      
   It is the same with gas giants as with ocean water, only the substances out   
   of which their atmospheres consist are all *gaseous* (despite the low   
   temperature), thus *compressible*.  When a gaseous substance is compressed,   
   momentum is imparted on its freely moving particles, so those move faster.   
   We understand that as the gas having a higher temperature; cf. the equation   
   of state of an ideal gas.   
      
   It is assumed, now also from gravitational measurements with space probes,   
   that the gas giants have a core out of metallic hydrogen.  So they do not   
   only consist of their atmospheres.  But they are still huge, and so their   
   atmospheres are much deeper than the terrestrial oceans, and the atmospheric   
   pressure and temperature towards the core can increase much further.  In   
   fact, it is the high pressure that allows exotic states of matter like   
   metallic hydrogen to exist.   
      
   HTH.   
   --   
   PointedEars   
      
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