Final answer:
Evidence for a liquid water ocean on Europa includes surface features indicative of an icy crust overlying a mobile liquid layer, an induced magnetic field suggesting a salty ocean, and tidal heating from gravitational interactions with Jupiter maintaining the ocean in a liquid state.
Step-by-step explanation:
The evidence for a liquid water ocean on Europa includes several key observations from spacecraft missions. High-resolution images from the Galileo mission display features on Europa's surface that suggest a layer of ice that has repeatedly been broken and refrozen, consistent with the motion of icebergs and ice floes in a subsurface ocean. These features would have required multiple episodes of ice cracking, movement, and refreezing, implying the presence of liquid water or slush from below the ice to create these complex patterns.
Confirmation of this ocean comes from the detection of Europa's induced magnetic field, which is influenced by its interaction with Jupiter's magnetosphere. The observed magnetic signature suggests that the moon harbors a liquid water ocean beneath its icy crust.
Moreover, tidal heating caused by Jupiter's immense gravitational pull on Europa is thought to generate enough heat to maintain a liquid state for the ocean. This is evidenced by the frictional heating that occurs due to tides generated by Jupiter's gravity, similar to how our Moon's gravity creates tides on Earth.