Gravity field and interior structure of Rhea

Doppler data generated with the Cassini spacecraftʹs radio carrier waves at X- and Ka-bands can be used to determine the quadrupole moments of Rheaʹs gravitational field. The resulting tri-axial field should be consistent with the assumption that Rhea is in tidal and rotational equilibrium. If so, we can construct interior models that are consistent with Rheaʹs mean density of 1236 kg/m3, determined previously from Pioneer and Voyager data, and its axial moment of inertia, to be determined from Cassiniʹs gravity data. Two-zone models consisting of a rocky core overlaid by a deep layer of ice are explored in some detail. While three-zone models consisting of an iron core, or a eutectic mixture of iron and iron sulfide, plus a rocky mantle and an outer layer of ice are possible, Rheaʹs relatively small density suggests that the satellite is not iron rich. Finally, we show that a flyby at the planned altitude of 500 km provides sufficient accuracy for the gravity experiment.