Nearly three decades after Galileo first revealed Ganymede's intrinsic magnetic field, the giant moon of Jupiter remains one of the Solar System's most compelling destinations. Future observations could determine just how closely its interior resembles the processes operating inside Earth—and how radically different two worlds can be while still sharing some remarkable physical characteristics.

When people think about moons, they usually picture small rocky companions circling much larger planets. Ganymede does not fit that picture.

Nearly three decades after Galileo first revealed Ganymede's intrinsic magnetic field, the giant moon of Jupiter remains one of the Solar System's most compelling destinations.…