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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.

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  • $\begingroup$ Do you mean an Ice moon? If the surface is water then the centre will be ice. $\endgroup$
    – FraserOfSmeg
    Commented Oct 15, 2015 at 14:33
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    $\begingroup$ 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. $\endgroup$
    – Joe Bloggs
    Commented Oct 15, 2015 at 14:34
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    $\begingroup$ No solid core? That would also limit the size of the moon, since at a certain pressure water becomes solid again. $\endgroup$
    – bowlturner
    Commented Oct 15, 2015 at 14:34
  • $\begingroup$ @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. $\endgroup$
    – Green
    Commented Oct 15, 2015 at 14:36
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    $\begingroup$ 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 $\endgroup$
    – bowlturner
    Commented Oct 15, 2015 at 14:40

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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.

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  • $\begingroup$ 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) $\endgroup$
    – Joe Bloggs
    Commented Oct 15, 2015 at 14:52
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    $\begingroup$ @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. $\endgroup$
    – Phiteros
    Commented Oct 22, 2017 at 2:04

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