Evidence of Ocean on Europa

The watery centre of Europa appears to be caused by the huge tidal forces caused by orbiting a humongous lump of matter like Jupiter.
Similar mechanisms are thought to cause the tremendous volcanism on the nearby (rocky) moon of Io.

The absence of any external power source (i.e. sunlight) to drive this biosphere was thought to be a problem until the discovery of black smokers a.k.a hydrothermal vents in the deep oceans of Earth.

If anybody has a very large drill, go and check it out.
Wrap up warm, though.

[Ed. note (Gz) - Minor spelling change for better integration 3/17/02]

Semi-recent simulations of impacts on Europa's surface are indicative that the rock-solid ice shell must be at minimum 3-4 kilometres thick (otherwise there should be no central peak crater, contradicting the fact that we do see them), with no calculated upper bound in sight.

Another analysis, this time of large plateaus on Europa, proclaims a minimum 6 kilometres of ice.

One model to explain the big pits and domes on the surface is that convection currents in the liquid water cause very slow convection currents in the solid water. This would require at least 20 kilometres of ice.

Considering that there are about 100 kilometres of water (solid or liquid) above the rocky core, how much in-between really is made up of ice? How thick could it be while still allowing life to survive? Does Europa have all the other characteristics that one would need to get life?