MODELS OF THE INTERNAL STRUCTURE OF IO

A model of Io that consists of a the crust, mantle, and core is used t o estimate the density distribution in the mantle and the core size by correlating calculated values of Io's mass and moment of inertia with experimental data. Models of Io's chemistry and inner structure based on geophysical characteristics show that this Jovian satellite has a massive, presumably Fe-FeS core and a mantle consisting of anhydrous s ilicates. The probable sizes of the core are 530-660 km for an Fe core and 810-1000 km for an FeS core (6-10% and 12-22% of Io's mass, respe ctively). Models of Ganymede's internal structure involving an Fe core with a radius 500-760 km (3-9% of the total mass) art: constructed. G anymede is the second body in the outer solar system that is known to have a core. Previous models of the structure of the Galilean satellit es involving an FeS core surrounded by ice are disproved. On the basis of available data on the density distribution in the Jovian satellite s, the Fe/Si atomic ratio is estimated to be 0.45-0.55 for Io and 0.36 -0.53 for Ganymede. The fact that both Io and Ganymede have Fe-FeS cor es favors a model in which the satellite swarm was heated by a hot gia nt planet. In addition, this adds support to the concept that the plan ets and satellites formed at two centers at least: (1) the Sun and ter restrial planets with a Fe/Si ratio dnecreasing from Mercury to Mars a nd (2) Jupiter and the Galilean satellites with a decreasing average d ensity (and apparently Fe/Si ratio) from Io and Callisto.