The reason that Europa is warm is because of tides. An example of what happens is shown in this drawing of the Earth and Moon. In this picture, the Earth is pulling on the Moon, and the Moon is pulling on the Earth with gravity. Planets change their shape when they are pulled this way.
In the case of Europa, the push-me, pull-me effect is caused by the pull of Jupiter on one side, and the other moons of Jupiter on the other side. (Europa is an inside moon). The volcanism of Jupiter's moon Io is caused by the very same effect.
If a body is very weak, instead of
getting pushed and pulled out of shape, the tidal forces can actually
tear the body in half, as with comet Shoemaker-Levy 9.
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