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.

[tasp]

I don't imagine I'm an expert on this, but wouldn't Europa freeze from the top down, and warm (from tidal effects) from the bottom up?

This would keep the 'interesting' part of Europa, the bottom of the water layer, the last to freeze and the first to thaw.

If the heating and cooling periods (if there is variation) aren't too intermittent, we shouldn't freeze the 'potential life zone'.

IIRC, microbes have been found deep under the sea floor on earth, so perhaps potential Europan life forms have a much larger volume of Europa to live in than we think.

[Bob Shaw]

IIRC, microbes have been found deep  under the sea floor on earth, so perhaps potential Europan life forms have a much larger volume of Europa to live in than we think.

I tend to agree - whatever rocky material is in the interior might well be colonised - but so also might the ice, especially above plumes from hot spots, leading to energy sources from outside as well as from within. The diapirs (as I'd interpret them) on Europa, plus the 'blue' ice look very interesting to me!

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...

Let me rephrase that last bit...

Bob Shaw

[Guest_Richard Trigaux_*]

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...

Interesting question.
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.

[RNeuhaus]

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!.

Out of thermodynamic law. The cold body is always denser than the hot body due to the shrinking process. Thus, the heat goes up and the cold goes down.

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.

The heat never goes to bottom! otherwise, the cold water looks for deeper waters. There is a common Atlantic cold water that flows deep water from North to South and the warm water flows from the South to North of Atlantic Ocean. National Geographic has reported the discovery of underwater currents in an edition of this or last year.

As far as my memory does not fail, the deep ocean water, the temperature is below than 0 degree centigrades approaching to -1.5 degree centigrade. Due to the high salinity of deep water, the high salinity avoids the forzen process of deep water.

Rodolfo

[Guest_Richard Trigaux_*]

As far as my memory does not fail, the deep ocean water, the temperature is below than 0 degree centigrades approaching to -1.5 degree centigrade. Due to the high salinity of deep water, the high salinity avoids the forzen process of deep water.

Rodolfo

In more of having a reverted dilatation behaviour at less than 4°C, water has another strange property: to the countrary of most bodies, the freezing point is lower at higher pressure. The law is about 1°C every 100 bars. So that, in our ocean, at 10kms deep and 1000 bars, the freezing point is -10°C for pure water. At more pressure, the temp law becomes normal (freezing point raises with pressure). On Europa, we have similar conditions of pressure than in our oceans, so this inverted laws must play. But there are still large uncertainties on the dissolved bodies into this ocean. Candidates are gasses (carbon dioxyd, methane, ammonia...) salts and even a high content in sulphuric acid was quoted. These bodies could be a great concentrations and completelly change the behaviour of the liquid.


The overall conclusion of all this is that Europa ocean must be nearby entirely as cold as possible, near the freezing point.

I learned all this in faculty, thermodynamics lessons.