<P> In addition to the intrinsic magnetic moment, Ganymede has an induced dipole magnetic field . Its existence is connected with the variation of the Jovian magnetic field near Ganymede . The induced moment is directed radially to or from Jupiter following the direction of the varying part of the planetary magnetic field . The induced magnetic moment is an order of magnitude weaker than the intrinsic one . The field strength of the induced field at the magnetic equator is about 60 nT--half of that of the ambient Jovian field . The induced magnetic field of Ganymede is similar to those of Callisto and Europa, indicating that Ganymede also has a subsurface water ocean with a high electrical conductivity . </P> <P> Given that Ganymede is completely differentiated and has a metallic core, its intrinsic magnetic field is probably generated in a similar fashion to the Earth's: as a result of conducting material moving in the interior . The magnetic field detected around Ganymede is likely to be caused by compositional convection in the core, if the magnetic field is the product of dynamo action, or magnetoconvection . </P> <P> Despite the presence of an iron core, Ganymede's magnetosphere remains enigmatic, particularly given that similar bodies lack the feature . Some research has suggested that, given its relatively small size, the core ought to have sufficiently cooled to the point where fluid motions, hence a magnetic field would not be sustained . One explanation is that the same orbital resonances proposed to have disrupted the surface also allowed the magnetic field to persist: with Ganymede's eccentricity pumped and tidal heating of the mantle increased during such resonances, reducing heat flow from the core, leaving it fluid and convective . Another explanation is a remnant magnetization of silicate rocks in the mantle, which is possible if the satellite had a more significant dynamo - generated field in the past . </P> <P> Ganymede probably formed by an accretion in Jupiter's subnebula, a disk of gas and dust surrounding Jupiter after its formation . The accretion of Ganymede probably took about 10,000 years, much shorter than the 100,000 years estimated for Callisto . The Jovian subnebula may have been relatively "gas - starved" when the Galilean satellites formed; this would have allowed for the lengthy accretion times required for Callisto . In contrast Ganymede formed closer to Jupiter, where the subnebula was denser, which explains its shorter formation timescale . This relatively fast formation prevented the escape of accretional heat, which may have led to ice melt and differentiation: the separation of the rocks and ice . The rocks settled to the center, forming the core . In this respect, Ganymede is different from Callisto, which apparently failed to melt and differentiate early due to loss of the accretional heat during its slower formation . This hypothesis explains why the two Jovian moons look so dissimilar, despite their similar mass and composition . Alternative theories explain Ganymede's greater internal heating on the basis of tidal flexing or more intense pummeling by impactors during the Late Heavy Bombardment . In the latter case, modeling suggests that differentiation would become a runaway process at Ganymede but not Callisto . </P>

The largest moon in the solar system is ganymede a moon of jupiter