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_*]

The short version of the "cold low-density upper ocean layer" theory about Europa can be found at http://www.lpi.usra.edu/meetings/lpsc2002/pdf/1824.pdf . The longer version, published in the April 2004 "Icarus", is at http://www.lpl.arizona.edu/~rlorenz/europa.pdf .

However, a caution: one thing which I'd forgotten about is that the theorists say that such a phenomenon can NOT occur if either the pressure or the salinity of Europa's ocean rises to levels which, from what we now know, seem very likely for it (an ice layer more than 23 km thick, or a salinity level about equal to that of Earth's ocean). In either case, the density of liquid water simply continues to rise with decreasing temperature until the exact freezing point is reached, and there cannot be a thin surface "stratosphere" of low-density near-freezing liquid water. So it starts to look probable after all that Europa's cycles of thin and thick ice crust probably are entirely due to changes in its level of tidal heating instead.

[ljk4-1]

Two Large Lakes Discovered Under Antarctic Ice

http://www.livescience.com/othernews/06012...ctic_lakes.html

Antarctica has at least 145 small lakes buried under its ice and one large one
called Vostok. Now scientists have found the second and third largest known
bodies of subsurface liquid water there.