Origin and evolutionGanymede probably formed by an accretionin 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.
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