Europa Subsurface Ocean Options

[ugordan]

Regarding the very real possibility Europa harbors an ocean underneath the ice, I'm wondering whether there have been any estimates on how long such an ocean might have been sustained (I'm assuming it's still there today). Are we talking about the entire history of Europa, billions of years or a much more recent thing, only a few millions? I know Enceladus, which recently turned out to be much warmer inside than expected, could have been periodically heated, but not on very long timescales.
I'm primarily interested because of the habitability factor, obviously an ocean which freezes out every once and a while would not make for a good incubator to possible life.

Also, supposedly all tidal heating on Europa would cease now, how long would it take for the subsurface to freeze out, that is, what are the thermal conductive properties of the surface ice?

Admittedly, I haven't done much research on the subject and if the question was already asked before, I apologize.

[Guest_BruceMoomaw_*]

Its not so odd, at 4°C water is densest and so tend to sink, so locally where I live we do see upheavels of lake waters when the surface water that have been warmed during the summer get chilled in the autumn and so turn themselves bottom up. This is actually something good, nutrients get brought back and is a reason we have planty of fish in these arctic waters that would have very little life if it wasnt for this fact.

It has been suggested that this very phenomenon might cause significant fluctuations in the thickness of Europa's ice crust over long periods. That peculiar density property of liquid water means that the remaining geothermal heat coming out of Europa's interior will generate convection currents that actually tend to top off at a short distance below the bottom of the ice layer, at the level where the liquid water's temperature has dropped to 4 deg C. The layer of cooler and less dense liquid water above that will actually serve as a layer of insulation to keep the bottom of the ice layer from being warmed -- since, while liquid water is very effective at spreading heat through CONVECTION, it's lousy at CONDUCTING heat. The result will be a slow buildup of trapped heat in the convective layer, with the top of the churning convective layer rising closer and closer to the bottom of the ice layer as the water underneath gets warmer and warmer -- until the convecting water finally does get warm enough that it rises to touch the bottom of the ice layer, at which point it will cause quite dramatic melting of the lower part of that layer, with the trapped heat in the underlying water ocean escaping at that point. This cyclic phenomenon may be a second factor -- along with the gradual fluctuations in the eccentricity of Europa's orbit caused by the other moons, and thus of its tidal heating -- causing periodic changes in the thickness of the ice crust over periods of tens of millions of years.