Constraining Ganymede's Interior Structure and Bulk Composition from Its Present-Day Magnetic Field Strength
Abstract
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 (∼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.
Related
- Ganymede’s surface composition from 0.5 to 5.3 µm using combined VLT and JWST datasets and the resulting lessons learned
- Interior Magnetic Fields in Magnetars and Radio Pulsars
- Constraining Primordial Magnetic Fields with Weak Lensing
- From IFS Maps to 3D Structure I: Constraining Geometry in the Circumgalactic Medium
- Potential Cryovolcanic Regions on Ganymede: A Priority Target for JUICE
BibTeX
@misc{kimura2026constraining,
title = {{Constraining Ganymede's Interior Structure and Bulk Composition from Its Present-Day Magnetic Field Strength}},
author = {Jun Kimura and Ryohei Kawakami and Taro Matsuo and Sho Sasaki},
howpublished = {arXiv},
year = {2026},
month = nov,
doi = {10.48550/arxiv.2611.05727},
eprint = {2611.05727},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2611.05727},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Constraining Ganymede's Interior Structure and Bulk Composition from Its Present-Day Magnetic Field Strength
AU - Jun Kimura
AU - Ryohei Kawakami
AU - Taro Matsuo
AU - Sho Sasaki
PY - 2026
DA - 2026/11/03
PB - arXiv
DO - 10.48550/arxiv.2611.05727
UR - https://arxiv.org/abs/2611.05727
AB - 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 (∼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.
ER -