EPSC Abstracts, Vol. 4, EPSC2009-785, 2009 European Planetary Science Congress, © Author(s) 2009 The Interior Structure of Ganymede and Callisto: Implications from Gravity Data F. Sohl (1), H. Hussmann (1), D. Breuer (1), J. Oberst (1), L. Richter (2), and T. Spohn (1,3) (1) Institute of Planetary Research, German Aerospace Center (DLR), Berlin-Adlershof, Germany, (2) Institute of Space Systems, German Aerospace Center (DLR), Bremen, Germany, (3) University of Muenster, Germany. Abstract. Only a limited amount of gravity field data was collected during close satellite encounters of the Galileo spacecraft in the Jupiter system. The interpretation of these data in terms of interior structure is based on the widely held but unproven assumption of hydrostatic equilibrium. We will discuss physical constraints imposed on present- day interior structure models of Ganymede and Callisto and possible future refinement, as envisaged for the planned EJSM mission to the Jovian system. This should be accomplished by more complete recovery of the static and time- variable parts of the gravitational fields, using complementary observations from JGO and JEO spacecraft, combined with altimetry data of regional topography and global shape, and, possibly, in-situ monitoring of tidally-induced surface distortion. Interiors of Ganymede and Callisto. Ganymede is the largest planetary satellite with a radius of (2631.2 ± 1.7) km. Gravitational and magnetic field observations by the Galileo spacecraft together with spectral data of the surface suggest that Ganymede's interior is composed of water ice and rock-metal components in nearly equal amounts by mass and strongly differentiated [1]. Its intrinsic magnetic field is most likely maintained by dynamo action in a liquid Fe-FeS core. Shown in Fig. 1 are model density distributions that satisfy Ganymede's mean density and moment-of-inertia constraints of (1942.0 ± 4.8) kg m -3 and 0.3115 ± 0.0028, respectively. The models suggest Ganymede's interior to be composed of an iron-rich core surrounded by a silicate rock mantle and overlain by an ice shell. The latter may contain a briny subsurface water ocean sandwiched between a high-pressure water ice layer and the outermost ice layer [2,3]. Fig. 1: Radial density distribution for three core compositions of Ganymede. From top to bottom: pure Fe; Fe-FeS (50:50 by wt.); pure FeS core. Adapted from [3]. Callisto is (2410 ± 1.7) km in radius and less massive than Ganymede but similarly composed. The gravity data collected during several close Galileo flybys suggest only partial internal differentiation [4,5], augmented by a density increase with depth due to pressure-induced water ice phase transitions [6]. The magnetic data are consistent with a briny subsurface water ocean present at around 150 km depth [7,8]. Incomplete separation of ice and rock component may have resulted in deviation of Callisto's interior from hydrostaticity. Depending on the timescale of satellite formation [9], the primordial rock concentration may have been retained at shallow depth, followed by a rock-depleted water ice/liquid shell and a mixture of rock and ice below, with the rock concentration increasing with depth up to the close-pack limit [10].