Is it a combination of subsurface radioactive decay and tidal flexing from Jupiter?

Wiki on Ganymede

Had you meant to say Europa?

No.

Jupiter, Io, Europa, Ganymede, and Callisto all experience gravity from each other, and this gravitational pull causes the planets to sort of flex and move. This keeps the interiors nice and warm, and that keeps the water in liquid form. I'm not sure, but I think that since Ganymede, Europa, and Io have a 1:2:4 resonance, so I think that effects the tidal heating even more. Anyone else know for sure?

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Yes, I read that somewhere, an online article, I forget where, but that bit about the 1:2:4 orbital resonance between Io, Europa, and Ganymede sounds familiar.

As for Callisto's ocean, I think the general consenus is that it's interior is kept hot by radioactive decay, as it is too far out from Jupiter and the other satellites to receive sufficient tidal flexing.