Reasons that planetary bodies are warm include
- 1.) leftover heat from it's formation
- 2.) radioactive material found in the body
- 3.) outside forces on the body as a whole, such as those which cause tides
- 4.) warming by the sun
Reason #3 applies to Europa. An example of what happens is shown in this drawing of the Earth and Moon. In this picture, the gravitational force of the Earth is pulling on the Moon, and the gravitational force of the Moon is pulling on the Earth. The Moon pulls more strongly on the side of the Earth facing the Moon than on the side facing away from the Moon. Because planets are not perfectly rigid, they 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 rigid or is not held together well, instead of
getting pushed and pulled out of shape, the tidal forces can actually
tear the body in half, as with comet
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