From: sylvia@email.invalid   
      
   NASA has a mission to the Jovian system, to study Europa. That moon is   
   interesting because it appears to have liquid water under an icy   
   surface. The heat need to keep the water liquid comes from the   
   stretching and compression Europa experiences during its orbit around   
   Jupiter, the orbit not been exactly circular.   
      
   So much, so simple.   
      
   Some thought made me realise that although the tidal forces on Europa   
   mean that it is not exactly spherical, its two bulges cannot remain   
   perfectly aligned with Jupiter, because Europa's angular velocity   
   relative to Jupiter is higher at periapsis than at apoapsis. The result   
   is that the nearer bulge is sometimes ahead, and sometimes behind,   
   relative to Europa's orbital motion, resulting in a net force backwards   
   along the orbit, or forward along the orbit.   
      
   Again, certainly stuff that's already well known.   
      
   As far as I can see, the energy that is being dissipated as heat inside   
   Europa has to come from changes to Europa's orbit. Further, if Europa   
   were either perfectly rigid, or perfectly elastic, there would be no   
   energy transfer, and consequently no change to the orbit.   
      
   It would make no difference if Jupiter itself were perfectly rigid, so   
   the transfer cannot involve tides on Jupiter generated by Europa.   
      
   So the existence of the orbital energy transfer depends on Europa being   
   neither perfectly rigid nor perfectly elastic.   
      
   What escapes me is the mechanism.   
      
   Any thoughts?   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)