Plot of the rocking of Ganymede’s magnetic field

This chart plots the excursion of a pair of auroral belts on Jupiter's moon Ganymede. Their motion provides insight into the moon's interior. Ganymede has a magnetic field produced by an iron core but because Ganymede is close to Jupiter, it is also embedded in Jupiter's own magnetic field.

When Jupiter's magnetic field changes, the aurorae on Ganymede also change, "rocking" back and forth. This amount of rocking is inhibited if the moon has a subsurface ocean. By watching the rocking motion of the two aurorae, scientists were able to determine that a large amount of saltwater exists beneath Ganymede's crust, affecting its magnetic field.