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Tidally locking a rock or metal moon isn't too hard and the understanding of how a solid moon behaves while tidally locked is relatively well understood. However, can a water moon be tidally locked or would the movement of the water prevent locking?

I don't have any specific parameters for size or orbital distance in mind, just an existence proof ($\exists$) that a tidally locked moon made of water is possible. The surface of the moon should be liquid and as far down as H2O can stay in a liquid phase. Whether it has an ice core or iron/stone core doesn't matter to this question.

Green
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  • Do you mean an Ice moon? If the surface is water then the centre will be ice. – FraserOfSmeg Oct 15 '15 at 14:33
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    What exactly do you mean by a 'water moon'? Do you mean an ice moon with a liquid core, a ball of water in space or an otherwise solid moon with 100% water coverage? @FraserOfSmeg: Jinx. – Joe Bloggs Oct 15 '15 at 14:34
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    No solid core? That would also limit the size of the moon, since at a certain pressure water becomes solid again. – bowlturner Oct 15 '15 at 14:34
  • @bowlturner a iron/stone core or ice core doesn't matter. The moon should have a liquid surface and be liquid for a significant distance below the surface. – Green Oct 15 '15 at 14:36
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    Most moons are going to have an ice covering. Look at titan. Certainly any 'water' not facing the sun will likely be frozen. Tidally locked would likely just need to take into account whether the core is locked a face toward the planet, and that is likely – bowlturner Oct 15 '15 at 14:40
  • @Green it's difficult to know what you mean by vast majority but here's the issue; if you want a moon of any reasonable size (even the size of a small asteroid) then it's going to be impossible to have a lot of water. As you go deeper into the moon it gets colder, extremely quickly (look up temperature variations wrt depth in seas on Earth). So to have liquid water at the surface and deep down you're going to have to break physics. – FraserOfSmeg Oct 15 '15 at 14:40
  • @bowlturner the tidal locking isn't going to be an issue as far as I can tell. Tidal locking refers to the moon being locked with the planet. so the entire moon's surface will see sunlight once per orbit or the main planet (ignoring planetary shadowing). – FraserOfSmeg Oct 15 '15 at 14:43
  • @fraserofsmeg: The temperature drops to about 4 degrees and stays there, as that's the densest water gets to. Anything colder and it floats to form an ice surface. The reason for the solid core would be something entirely different, and moves us into the wonderful territory of different ice phases. – Joe Bloggs Oct 15 '15 at 14:46
  • @FraserOfSmeg there shall be no breaking of physics! The question is amended. – Green Oct 15 '15 at 14:46
  • @JoeBloggs if the only form of ice that existed was indeed hexagonal ice (Ih) as commonly found on Earth you would be quite correct but that isn't actually the case. If the water was really at 4C that deep the first would be Ice VI at (Approx 640 MPa @ 4C) under 1g that's around 60-65 km, if not then the is Ice VII @ (>2.1GPa & < 81C) ~210 km. Ice X (>62 GPa) ~6,135km beyond that Ice XI will form under pretty much any conditions once the pressure passes the 300 GPa (>30,000 km). The insulator to metal transition is predicted to occur at ~1.5-6TPa but such conditions are not available to test. – MttJocy May 22 '16 at 14:59
  • This might prove useful it is the full phase diagram for water including it's more exotic forms. If working with a world involving water under conditions like this it would certainly be worth refering to since is does start to behave very strangely when subjected to extremes of cold or pressure. https://upload.wikimedia.org/wikipedia/commons/0/08/Phase_diagram_of_water.svg – MttJocy May 22 '16 at 15:01
  • @MttJocy: hence the 'wonderful world of different ice phases' comment. Nice numbers though! – Joe Bloggs Jun 08 '16 at 17:34

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Most moons are not going to have much liquid surface. any close enough to the sun to do so are going to be evaporating away, since there won't be much of an atmosphere nor magnetic field to protect it. So one that is liquid to some depth will be constantly shrinking.

Titan has a lot of water, it is frozen on the outside, slowing evaporating, and it is theorized that the ocean floor has Ice VII from pressure. It also has a regular core that everything is floating around.

What constitutes Tidally locked? Tidal locking is the name given to the situation when an object's orbital period matches its rotational period

So it depends on what you measure. If you measure the core rotation, then it is easy to be tidally locked, though it might be hard to make that measurement. However, large bodies of water have currents and if you are measuring the surface or try to find some other fixed point, then no it's not possible.

bowlturner
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  • I'm pretty sure the lack of total tidal locking is what gave Titan and Europa away as having liquid oceans under the ice sheets in the first place (just to bolster your answer) – Joe Bloggs Oct 15 '15 at 14:52
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    @JoeBloggs That's not really correct. Moons experience libration because orbits are not perfect circles but rather ellipses. That's why you get little wobbles over an orbital period. As long as your orbit is not perfectly circular, you will not be "perfectly" tidally locked. – Phiteros Oct 22 '17 at 02:04