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@abetterjulie Ganymede's surface gravity is 0.146g, which is plenty to keep whatever is on the ground where it is. 1L of liquid water weighs 146g.

@abetterjulie Regardless of gravity, material in space retains its full inertial mass -- that is, it takes the same amount of energy to move it, minus the gravity you're not working against. So something that weighs 1KG here on Earth, and something that weighs 1KG on Ganymede, will require different amounts of energy input to move.

@mos_8502 One more thing---so if I'm inventing a watery moon for a story, I can handwave the math because gravity and inertial mass? It's not a story for like a big publisher or anything, it's just a little game I'm working on.

@abetterjulie Well the thing is, the amount of gravity you experience on the surface of a terrestrial body like a moon is a function of two variables -- the mass of the body, and the distance from the centre of mass to the surface. So if your watery moon happens to have a very dense core, say lots of lead and heavy metals, it could have a much higher surface gravity than you would expect from its size.

@mos_8502 Okay, so I was looking at Ganymede to get ideas, and it apparently has a large mass, magnetic field, but no atmosphere. And the no atmosphere part led to the 'how does that stay' question.

@abetterjulie Aha. Retaining an atmosphere is more to do with blocking solar wind than gravity. Earth has its atmosphere because the core is spinning, creating a rotating magnetic field that blocks the solar wind. Mars lacks this, and so the atmosphere is literally blown away.

@mos_8502 Am I crazy or isn't that part of how you make power? Spinning magnets or something? I swear I've heard that before.

@abetterjulie Well yes, when a magnetic field rotates, anything conductive will experience an induced electric potential. Detecting that on Earth is hard because we fill the air with so much EM noise, but it's there.