Constraining Ganymede's Interior Structure and Bulk Composition from Its Present-Day Magnetic Field Strength
Auteurs : Jun Kimura, Ryohei Kawakami, Taro Matsuo, Sho Sasaki
Résumé scientifique
Ganymede is the only Solar System satellite with a global magnetic field driven by a core dynamo. Although its small moment-of-inertia (MoI) factor indicates a differentiated structure of a metallic core, rocky mantle, and outer hydrosphere, its exact bulk composition and layer thicknesses remain highly uncertain. We developed a comprehensive, long-term thermochemical evolution model that couples thermal history with magnetic field generation. Building upon previous theoretical frameworks, this study systematically evaluates how diverse chondritic precursor compositions constrain Ganymede's internal thermochemical evolution and core solidification processes. Based on interior structures initially permitted by a wide range of chondritic compositions and geodetic constraints, we applied the observed equatorial surface magnetic field strength ($\sim$719 nT) as a dynamic filter. Our evolutionary simulations demonstrate that while magnetic dynamos can operate under entire melting, Fe-snow, and FeS-layer regimes, the requirement to sustain the observed field strength strongly restricts the viable interior models. Satisfying this constraint preferentially requires a relatively large core radius and a specific, non-eutectic bulk core sulfur concentration. Consequently, this coupled thermochemical approach highlights the main novelty of this study: characterizing the hydrosphere's density serves as an independent constraint to break the structural degeneracy of Ganymede's bulk composition. Our model indicates that L and CI chondrites yield parameter spaces capable of sustaining comparable field strengths under a low-density hydrosphere scenario (1,200 kg m-3), whereas the H chondrite composition can satisfy this criterion under a high-density scenario (1,300 kg m-3). These predictions provide a critical framework to be tested by the JUICE mission arriving in 2031.
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