Io

Io is the closest to Jupiter of its large moons.  It orbits just outside Jupiter's ring system and undergoes extreme geological distortions, due to tidal interactions between Jupiter and the other three large moons.  This distortion - as much as 100 meter (330 feet) rise and fall in the land surface - generates internal frictional heat.  This in turn causes the rocky interior of Io to melt and expand.  The surface of Io is exposed to the cold of space and most of it is therefore cool and solid.  The molten interior breaks through the surface in places creating highly active volcanoes, which constantly remodel the surface through lava flow and ash dumping.  Some parts of Io's surface are maintained at temperatures favorable for life and protected below the surface from radiation, by the fluffy outfall from the volcanoes, primitive life may be able survive.  However, it is unlikely that any water remains on Io and this is essential for life to prosper.  It is therefore unlikely that life could have started on Io, but conditions may be right for life that has been brought to Io on space debris.  Survival in stasis or at a very slow rate of metabolism may be possible,.
 

Galileo Takes a Closer Look at Io
Io Fly-by animation
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