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G. Schubert, W.B. Moore (UCLA), J.D. Anderson, R.A. Jacobson, E.L. Lau (JPL)
Radio Doppler data generated by the Deep Space Network (DSN) from four encounters of the Galileo spacecraft with Io, Jupiter's innermost Galilean satellite, can be used to infer Io's gravitational quadrupole moments. By combining the four flybys into a single solution for the gravity field, both tidal and rotational components are separately determined, thereby establishing that Io is in global hydrostatic equilibrium. Consequently, viable interior models must satisfy constraints imposed by the mean radius R~=~1821.6~±~0.5~km, the mean density \rho~=~3527.8~±~2.9~kg/m3, and the normalized axial moment of inertia C/MR2~=~0.37685~±~0.00035. With equilibrium now established for Io, the gravity data and the mean radius determine Io's principal axes (c~<~b~<~a) as a~=~1830.0~±~0.5~km, b~=~1819.2~±~0.5~km, c~=~1815.6~±~0.5~km. Io almost certainly has a metallic core with a radius between 550 and 900~km for an Fe-FeS core or between 350 and 650~km for an Fe core. Io is also likely to have a crust and a partially molten asthenosphere, but their thicknesses cannot be separately or uniquely determined from the above constraints.
Supported by NASA through the Galileo Project at JPL and the Planetary Geology and Geophysics Program.
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