QUOTE (Bob Shaw @ Nov 22 2005, 06:21 PM)
One aspect of the temperature regime which will be fascinating will be the way that warmth turns to cold at various points below the surface - I wonder whether there's an upper zone which might be substantially warmer than a mid-zone, for example, allowing for at least temporary colonisation and - of course - some luvverly dead bodies for us to sniff...
On Earth, the inner circulation of oceans is dominated by very cold water flowing from the ice shields, and this explains why the bottom of the ocean is very cold (4 and even 2°). Occasionally evaporation at the surface can also create currents of more salty (heavier) water, as the one which occurs in the Gibraltar straight. (On Europe there is of course no evaporation, by melting/thawing of ice near the top of the ocean may also produce differences in salinity).
But it is a little known phenomenon which governs the temperature layers in an ocean covered with ice: water under 4°C becomes less dense. So that, in some circumstances, convection can be reverted: heat goes down, and cold gets up!. This is commonly observed in permafrost and is expected to explain many strange features on Mars. So the main phenomenon governing the repartition of heat in the depths of Earth ocean (where sun heat never goes) is this: a small gradient of temp, from about 4°C near the surface (under the layers heated by the sun) and as low as 2°C near the bottom. (this difference is mainly due to temperature). And if we heat this water a little, this heat sticks to the bottom!! Of course if there is an intense heat source, like black smokers, it will produce plumes going up, until it is diluted at less than 4°C and falls again.
This law of the repartition of heat in an ice-capped water layer is as much fundamental, I think, that the law which makes air temp getting lower in an exponential way with altitude. To set a complete profile of Europa ocean, we need this law and the pressure as a function of depth.
Of course we must account also with diluted salts and gasses, which may modify the water fusion temp. And in some cases, (this was evoked for Titan) this temp can become very low: -50°C was found in Antarctica, and -100°C is possible. In this case, it would be bye-bye life!!
however there are some interesting phenomenon we can expect are taking place in Europa oceans:
-small but continuous geothermal heat leakage, which may produce slow but constant convection currents. These currents would be governed indirectly by the convective movements in the ice crust itself.
-occasionnal or punctual intense heat (volcanoes, hydrothermal vents) which may produce large uprises of water. These uprises would fall after, perhaps more violently than they rose.
This must also explain the strange features at the surface: in many places, the ice look like broken with a giand showel, and refrozen after. I think only a very violent phenomenon can produce this. It could be violent outgassing, called limnic eruption, where water suddenly bubble and release huge amounts of dissolved gasses. But it could be also caused by the tidal movement of ices. Or maybe the convection patterns in ices produce occasionnal catastrophic fractures.