QUESTION:
Have we ever been on Jupiter's moons?

ANSWER from Kevin H. Baines:
No, we have not yet visited any of Jupiter's moons with a lander. So far,
only the moon, Mars, and Venus have had landers reach their surfaces (and of
course now we have visited the Jovian atmosphere). The idea of landing
on one of Jupiter's moons is one which we could obtain without much difficulty
(except for the high cost!). Europa is an excellent candidate as it is 
relatively
smooth (it's icy surface has "cracks", but few if any craters). One problem is
the extremely high radiation surrounding Jupiter. This makes Io not a tenable
target (as well as it's active volcanism which could bury the lander if the
spacecraft landed too near an active volcano!). Europa is about 800,000 km
away from Jupiter (about twice as far as Io), where the radiation belts aren't
so strong.