From the flybys of the spacecraft past Europa, we know something about the overall density of the moon and therefore something about its internal structure too.

We have learned that Europa is differentiated, with heavy stuff in the middle and light stuff at the outside. This sounds elementary but we didn't know that Europa was differentiated until Galileo arrived there. Previously it was thought that Europa was undifferentiated which would indicate that its interior temperatures never were higher than around 273 K. Now we know this is not the case.

http://photojournal.jpl.nasa.gov/

Another cross section view

Note: It is still not known whether Callisto, the outermost Galilean satellite, is differentiated. It might be partially differentiated.

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