What mechanism allows moons far outside the traditional Habitable Zone, like Europa, to potentially host subsurface liquid water?

Answer

Tidal heating from gravitational kneading by their giant host planets.

Icy moons far from the star receive energy not from solar radiation, but from the internal heating caused by the gravitational forces exerted by their massive host planets as they orbit.

What mechanism allows moons far outside the traditional Habitable Zone, like Europa, to potentially host subsurface liquid water?
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