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Europa Subsurface Ocean
ugordan
post Nov 22 2005, 10:53 AM
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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.


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Guest_Richard Trigaux_*
post Nov 22 2005, 11:46 AM
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There are already a topic about an Europa orbiter and one about the Juno mission project discussing an eventual exploration of this ocean.



To reply your question, as far as I know, the tidal heating of jovian moons was constant over the ages, and thus Europa ocean could be liquid since the formation of this satellite. However in this matter it is difficult to be sure, perhaps some specialists think otherwise.

The situation of Europa (a large moon in a series of four) is different of that of Enceladus and Miranda. Large moons are expected to be more stable, while small moons are expected to be easilly tugged away, and thus undergo relatively brief periods of heating.

About life, it is expected to appear in a rather short time (less than a billion years, and perhaps as short as some millions) so eventually if Europa experienced periodic heating/freezing, life could have reappeared several time. But this is very speculative.
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tasp
post Nov 22 2005, 02:47 PM
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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.
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ugordan
post Nov 22 2005, 03:35 PM
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QUOTE (tasp @ Nov 22 2005, 04:47 PM)
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. 

That's precisely the reason I asked about the thermal properties of the upper ice layer - that's the only way for heat from the interior to escape into space, if the ice is very non-conductive, it could serve as a blanket preserving the warm water beneath.

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

If you don't freeze out, but you do cut out any heating, the organisms wouldn't have any thermal hotspots/vents on the ocean bottom to acquire energy/food from. So, simply keeping the ocean in a liquid state probably doesn't help all that much, but I'm really no expert on biology.


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Rob Pinnegar
post Nov 22 2005, 03:36 PM
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The history of Europa has to be very closely tied to its orbital resonances with Io and Ganymede. These resonances are what keep Europa's orbit around Jupiter slightly elliptical. Without the resonances, tidal dissipation would cause Europa's orbit to become circular (as it did with Triton) and heating would cease.

So the question is really: Have Io, Europa and Ganymede always been in these resonances (perhaps also with Callisto at one time)? If they have, you can bet that Europa's subsurface ocean has always been at least as warm as it is now. I say "at least" because, like all "regular" satellites, the Galileans are slowly moving away from their parent planet, which means that tidal effects would've been stronger in the past when Europa was a bit closer to Jupiter.
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Guest_Richard Trigaux_*
post Nov 22 2005, 05:43 PM
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Europa ice crust is not much thicker than the Antarctic ice shield. The later is already insulating enough to keep a thin layer of layer molten by geothermal heating. So at a very rough guess, the Europa ocean would freeze in at least thousands of years, at last some million of years. This allows it to be sensitive to periods where there would be no tidal heating, the later having a time scale of hundreds of million to billion years. So yes it would freeze, but no we do not know if it had periods without tidal heating. My personal opinion would be that the whole set of Jovian moons evolve slowly all together. If there had be a special event, a moon taking a special orbit, the strong resulting tidal heating would have quickly corrected this. So I rather think that the overal shift in orbits of the whole four moons is correlated, slow, quasistatic*, resulting into a relatively constant tidal heating. An oposite argument is that Ganymede shows clear surface fractures indicating a more localisated tidal heating event, ancient but not at the beginning. Perhaps the moons were set otherwise at the beginning, and when they reached their today orbits, there was a surge of heating. Of heating, not of freezing. We can wonder if Europe began to have liquid water at this epoch (3billion years ago at rough guess, after ganymede craters). This still makes a long continuous period of time.

*quasistatic: a movement slow enough so that at a given moment we can reason as if nothing was moving.
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Bob Shaw
post Nov 22 2005, 06:21 PM
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QUOTE (tasp @ Nov 22 2005, 03:47 PM)
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


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Guest_Richard Trigaux_*
post Nov 22 2005, 08:38 PM
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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...
*


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 4C 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 4C near the surface (under the layers heated by the sun) and as low as 2C 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 4C 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: -50C was found in Antarctica, and -100C 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.
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RNeuhaus
post Nov 22 2005, 09:13 PM
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QUOTE (Richard Trigaux @ Nov 22 2005, 03:38 PM)
But it is a little known phenomenon which governs the temperature layers in an ocean covered with ice: water under 4C 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. smile.gif
QUOTE (Richard Trigaux @ Nov 22 2005, 03:38 PM)
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 4C near the surface (under the layers heated by the sun) and as low as 2C 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 4C 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. smile.gif

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.
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helvick
post Nov 22 2005, 09:26 PM
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QUOTE (RNeuhaus @ Nov 22 2005, 10:13 PM)
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.  smile.gif
*


That's generally true but it isn't always the case and water she is different! See this handy diagram. Richard is right there is an inverted convection zone in water bodies - this means that the deepest points of cold water bodies are going to remain around 3.9C. Water is weird and is more dense at 3.9C than it is at 2C. Since Ice is less dense again this keeps deep oceans of water liquid.
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Guest_BruceMoomaw_*
post Nov 22 2005, 10:43 PM
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Virtually all the papers on the subject have concluded that -- while the thickness of the ice crust may very well oscillate up and down, over periods of seveal tens of millions of years, between just a few kilometers and up to 50 km or so -- Europa's ocean has never frozen out completely since the moon was formed.

Indeed, this factor was the main reason why scientists before Galileo's data were so hesitant to make a firm prediction as to whether or not an ocean existed at all: if a liquid ocean existed at the start, it would likely sustain its own existence, because the friction from the tidal bulging and flexing of the ocean and the ice shell above it would generate enough heat to keep the ocean liquid -- but if Europa's surface water layer had been frozen full through and thus rigid from the start, its tidal flexing would be so small that it wouldn't generate enough heat to melt itself. Well, now we know from Galileo's induced magnetic field data that -- barring one of the biggest shocks in the history of science -- Europa DOES have a substantial subsurface ocean, and thus always has had one. (I've never seen any paper suggesting that the fluctuations in its degree of tidal heating resulting from the slow orbital shifts of the Galilean moons might, at ANY time in its past history, have generated enough additional heat to melt a fully frozen ice layer on it. I suppose this can't quite be ruled out yet -- after all, calculations indicate that Ganymede got jockeyed into a quite eccentric orbit about a billion years ago which generated enough heat to create its current still-molten central core -- but, as I say, I've never read any such suggestion about Europa.)
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Guest_Richard Trigaux_*
post Nov 23 2005, 07:06 AM
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QUOTE (RNeuhaus @ Nov 22 2005, 09:13 PM)
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 4C, water has another strange property: to the countrary of most bodies, the freezing point is lower at higher pressure. The law is about 1C every 100 bars. So that, in our ocean, at 10kms deep and 1000 bars, the freezing point is -10C 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.
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Guest_Richard Trigaux_*
post Nov 23 2005, 07:22 AM
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Thank you BruceMoomaw to quote papers which more or less confirm what my intuition told me.

When I was speaking of tidal heating, it was about the rocky core, when the papers you quote speak of tidal heating in the ice/water layer itself. Interesting to know that the water layer could sustain itself without the need of any volcanic activity into the rocky core.

However, as far as I heard, scientists are inclined to think that Europa has some volcanic activity like Io. As much? or much less? Difficult to guess now. We can expect that there is at least some hydrothermal circulation, bringing tidal heat out of the core, and rising up into the water layer. Such a situation is interesting, as it would result into "black smokers" hydrothermal vents which could be oasis of life, with abundance of energy to feed chains of ecosystems. But this situation could also result in a sulphuric acid saturated ocean, which would avoid any form of life to exist or even to appear.

If there are hydrothermal or volcanic activity in Europa ocean, we can expect it is turbulent, with plume of hot water rising up swiftly, and then falling again to the bottom when it gets colder than the 4C barrier.

If there is no such activity on Europa, we can on the countrary expect that the ocean is very still, very cold, with an unusual convection pattern due to the inverted law under 4C. But indeed the cold source is at the top, so that a normal convection should occur at least at times. Enventually, with an inverted convection pattern, this very still ocean can be very pure of any dust, and thus reach a surfusion state. In this case, catastrophic mass freezing may occur, explaining the strange upheavals observed in many place on the surface.
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RNeuhaus
post Nov 24 2005, 04:21 AM
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Thanks to Richard and Helvick for the reference. I am still puzzled blink.gif of this odd behavior so this is a new thing for me. I will *study* paper pointed by Helvick and Richard.

Rodolfo
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elakdawalla
post Nov 24 2005, 04:36 AM
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QUOTE (RNeuhaus @ Nov 23 2005, 08:21 PM)
Thanks to Richard and Helvick for the reference. I am still puzzled  blink.gif  of this odd behavior so this is a new thing for me. I will *study* paper pointed by Helvick and Richard.
*

Speaking of papers on Europa, I just noticed that Bob Pappalardo has a veritable treasure trove of papers on Europa (not just his, many people's) at his online class website at http://www.icymoons.com/europaclass.html (cool website address, huh?)

--Emily


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