[continued from previous message]   
      
      the atmosphere therefore exerts as the gravitational force divided by the   
      surface area.]   
      
   Then when you go underwater, like in a lake and the actual (water) ocean,   
   the water and the rest of the planet also attract each other   
   gravitationally.  So now the underwater pressure is produced not only by the   
   atmosphere, but also by the water column above your head.  The deeper you   
   go, the larger that column, So the underwater pressure increases with depth.   
    But water as a liquid (with particles bound to each other) is barely   
   compressible, so the water temperature does not increase (much) with depth   
   (see below).   
      
     [This is in fact how buoyancy arises: The pressure by a fluid at the   
      bottom of a immersed object or substance is greater than at the top, and   
      equal on the sides.  So if the object's/substance's mass density is less   
      than that of the fluid that it displaces, it will be pushed up, and it   
      can even float on that fluid (like a boat or ship where the submerged   
      part consists mostly of air).   
      
      If the substance is also a fluid, layers with less mass density will   
      arrange themselves to be above layers with greater mass density.  That is   
      also how, for example, water ice is found at the top and not the bottom   
      of a frozen lake or river, allowing the animals requiring the lake or   
      river to survive the winter: the lake or river never completely freezes   
      because water below 4 °C moves to the top and freezes there, and the ice   
      at the top prevents further heat loss of the water below it.]   
      
   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).   
      
   Now you can understand how stars form.  In fact, if the mass of Jupiter   
   would be approximately 82 times its current mass, the pressure and   
   temperature in its interior would be high enough that nuclear fusion becomes   
   possible and it would become a star.  (Don't worry, there is not enough   
   matter in the Sol System or its vicinity left for that to happen.  The mass   
   of Sol alone constitutes ca. 99.8 % of the mass of the Sol System :))   
      
   HTH.   
      
   --   
   PointedEars   
      
   Twitter: @PointedEars2   
   Please do not cc me. / Bitte keine Kopien per E-Mail.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)