Let's start with a simple calculation. The mean radius of Europa has been estimated at 1,560.8 km. The mean depth of the ocean has been estimated at 100km. Earth's ocean by comparison has a mean depth of 4 km, but covers a much bigger sphere. How do the volumes of these two bodies of water compare?
Volume of Europa=(4/3)*3.14*(1,560.8^3)=1.59×10^10
Volume of Europa excluding outer 100KM of water= (4/3)*3.14*(1,460.8^3)= 1.30×10^10
Volume of Europa's water=1.6×10^10- 1.3×10^10= roughly 2.8 billion cubic kilometers of water.
The volume of earth's oceans is estimated at 1.3 billion cubic kilometers.
So Europa hypothetically has about twice as much liquid water by volume as earth. So a big ocean.
Now let's think for a minute about that ocean. 100,000 meters deep. The gravity on Europa is a gentle .134g, so I'm guessing the water pressure 100K down is similar to that on earth 13,000 meters down. Deeper than the earth's ocean actually gets, but comparable to the pressure at the bottom of the Marianas Trench, something over 1000 atmospheres.
The bottom is important when thinking about life, because unlike earth, where most of the life lives on the surface, where the light is, on Europa the proposed energy source is hydrothermal vents, on the bottom. There are lots of ramifications to this. One, as I discussed before, hydrothermal vents are unlikely to produce enough usable energy to keep a whole ocean twice the size of Earth's own filled with life. Life will live where the energy is, as close to the vents as can be tolerated.
Second, unlike on Earth where there is serious stratification of the oceans because the top is warm and the bottom is cold, on Europa, the heat comes from below. Hot water rises, and cold water sinks. If you heat a pot of water from below, you quickly get powerful convection currents roiling the waters. Europa likely has powerful upwellings, downwellings, sidewellings and so on. Then, once you have all that motion on a rotating body, you get the Coriolis effect which leads to such things as cyclones, whirlpools and gyres. Add to that tides from interactions with Jupiter and the other moons, and all that water isn't just sitting still down there under the ice. It is racing too and fro and hither and yon. Which makes possible another type of energy collection on Europa; windmills (or hydro electric, or something such). Anchor yourself to ground or ice, let the water swirl by you and allow it to turn some part of you. Use that energy to fix carbon (or something) and you are an autotroph. Then other Europans can come along and eat you, and you are supporting a community, without relying on chemicals from the vents.
I admit it is a bit far fetched, but it is the only way I can think of to support life away from the hydrothermal vents in Europa's ocean. Thoughts?
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Seems pretty farfetched, simply because I can't think of any plant or animal on earth that captures tidal, current, or wind energy. Life in/around geothermal vents seems much more likely, and in fact Europa seems to provide an interesting test case on how inevitability life is, given conditions that could theoretically support it.
Now that I look at it, it seems that the water pressure at the ice/water boundary layer would be extremely high - on the order of 100 atmospheres. This makes the sorts of "moon pool" colonies imagined by the Artemis society pretty unlikely, as I'm pretty sure nobody's lived at more than 30 atmospheres or so for any extended period.
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