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The magnetic field and internal structure of Ganymede


BEFORE the recent fly-bys of Ganymede by the Galileo spacecraft, viable models of the internal structure of Jupiter's largest moon ranged from a uniform mixture of rock and ice to a differentiated body with a rocky core and an icy mantle1. Analysis of the Doppler shift of radio signals from Galileo has now shown that Ganymede is strongly differentiated with a relatively dense core surrounded by a thick shell of ice2. Other instruments have revealed that Ganymede has an intrinsic magnetic field3,4, which is aligned approximately antiparallel to Jupiter's magnetic field. Here we argue that these results imply that Ganymede has an outer silicate core surrounding a liquid (or partially liquid) inner core of iron or iron sulphide, and that the magnetic field is generated by dynamo action within this metallic core. lo also appears to have an intrinsic magnetic field5,6 (antiparallel to that of Jupiter), implying that it too has a metallic core7 in which the field is generated either by dynamo action or by magneto-convection.

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Schubert, G., Zhang, K., Kivelson, M. et al. The magnetic field and internal structure of Ganymede. Nature 384, 544–545 (1996).

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