Planetary Scientists studying the rich data set from the Galileo Mission to the outer solar system, the community suffers from an embarrassment of riches, because each of the moons of Jupiter differs in the way that they can reveal more about planets and how they behave. There are four large, moons of Jupiter that in their character and behavior are more like planets than Earth's moon: Io, Europa, Ganymede and Callisto. The last three are icy.

Io's volcanic hyperactivity is well known, but there are mysteries about the temperature of its magmas and its spectacular mountains and what they might reveal about the satellite's interior processes. As for the exterior moon Callisto, how did it acquire an ocean yet not be deeply differentiated? Ganymede's liquid iron core is still generating a magnetic field. This was not predicted beforehand, and thus has much to teach planetary scientists on how magnetic fields are generated in the solar system.