What primary mechanism generates internal heat sources for the liquid oceans found beneath the icy shells of Europa and Enceladus?

Answer

Tidal flexing from their massive parent planets

These moons exist outside the traditional surface Habitable Zone; their internal warmth, sufficient to maintain liquid water, is generated by the gravitational stress (tidal flexing) exerted by Jupiter or Saturn.

What primary mechanism generates internal heat sources for the liquid oceans found beneath the icy shells of Europa and Enceladus?

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