Europa 

Further from Jupiter than Io, less tidal heating (remember Ftidal ~ 1/r3). No hot volcanos, higher ice content.

View from Voyager, then later Galileo, shows extremely smooth ice plains and large fractures (cracks) in the ice.



Closeup of ice fractures:
What does the fact that the surface is smooth (very few craters) and cracked tell us about the age of Europa's surface? What does this mean for the interior?
 
 

Breakup of Europa ice pack:


These Europa images lead to a possible picture of Europa where a few kilometer thick ice crust overlies a liquid water ocean a few hundred kilometers deep.

What would keep the ocean from solidifying?


Liquid ocean --> Life?? Utter speculation at this point. (See Arthur C. Clarke's 2010).


Other models say, no, it's a mix of hard and "soft" ice. As always, more work is needed...


 

Europa 3D images
(image source and more information here)

 
Pwyll crater (from Galileo)