Presentation Information
[PPS01-P18]Coupled thermal and dynamo evolution of Ganymede: Implications for core size and composition
Ryohei Kawakami1, *Jun Kimura1, Taro Matsuo1, Sho Sasaki1 (1.The University of Osaka)
Keywords:
Icy body,Evolution,Ganymede,Magnetic field,Interior
Ganymede is the only satellite in the Solar System that possesses a global intrinsic magnetic field generated by a dynamo in the metallic core. Measurements by Galileo spacecraft revealed that the field is well approximated by a dipole of ~720 nT at the equatorial surface, and the small moment-of-inertia (MoI) factor indicates a strongly differentiated interior consisting of a metallic core, a rocky mantle, and an outer hydrosphere. Sustaining a core dynamo requires long-term cooling of a (partly) molten core and the development of thermochemical convection. These processes are controlled by the radiogenic heat budget and transport efficiency in the mantle and by the composition-dependent melting relations of the core. However, under the limited observational constraints currently available, not only the layer thicknesses but also the bulk composition of Ganymede remains highly uncertain. Such uncertainties lead to substantially different thermal histories and solidification pathways, and hence different dynamo mechanisms. Determining interior structures consistent with both gravity and magnetic observations therefore requires explicit consideration of core crystallization for a range of plausible compositions.
Here we developed a theoretical model that couples thermal evolution with magnetic-field generation while incorporating core solidification processes. We first constructed interior structure models consistent with the observed MoI by assuming various chondritic bulk compositions (H, L, LL, CI, CM, CO, CV, CK). For an hydrosphere density of 1200 kg m-3, allowable solutions yield core radii of 550 - 850 km and sulfur contents of 14 - 36 wt.%, while structures based on the H chondrite composition fail to satisfy the gravity constraint. For a denser hydrosphere of 1300 kg m-3, the permitted ranges become 600 - 850 km and 10 - 22 wt.%, whereas CI, CM, CV, and CK compositions are excluded because they predict excessively large MoI.
Thermal evolution calculations including fully molten, Fe-snow, and FeS-layer regimes in the core show that dynamo action is possible in all cases and can produce magnetic intensities comparable to the present observation. When the additional requirement of matching the observed surface field strength is imposed, the admissible solutions are significantly restricted. The models that remain viable are: (i) core radius 830 +/- 20km with 12.9 wt.% sulfur and an hydrosphere density of 1300 kg m-3 (H chondrite), (ii) 800 +/- 20 km with 16.0 wt.% and 1200 kg m-3 (L chondrite), and (iii) 850 +/- 20 km with 35.0 wt.% and 1200 kg m-3 (CI chondrite). Consequently, the wide uncertainty previously ranging core radii 550 - 1150 km and sulfur contents of 0 - 36 wt.% can be reduced to 800 - 850 km and 12.9 - 35.0 wt.%, and the H, L, or CI types are preferable for bulk composition.
Future exploration will provide multiple independent tests of these predictions. The JUICE spacecraft is scheduled to arrive in the Jovian system in 2031 and to enter orbit around Ganymede in 2034. High-precision gravity measurements will refine the MoI factor, magnetometer observations will characterize the magnetic morphology and its temporal variability, and laser altimetry will quantify tidal deformation. Together, these data are expected to determine whether a subsurface ocean exists and to constrain its depth, thickness, and electrical conductivity. Estimating the presence and thickness/depth of the ocean will in turn constrain the mean density of the hydrosphere, providing a direct means to verify the interior structures derived here. Moreover, improved determinations of magnetic multipole components and their time dependence may reveal the present state of the core dynamo, including the mode of crystallization. Confronting these forthcoming measurements with the model results will further narrow the permissible parameter space of the interior or provide observational support for the solutions identified here.
Here we developed a theoretical model that couples thermal evolution with magnetic-field generation while incorporating core solidification processes. We first constructed interior structure models consistent with the observed MoI by assuming various chondritic bulk compositions (H, L, LL, CI, CM, CO, CV, CK). For an hydrosphere density of 1200 kg m-3, allowable solutions yield core radii of 550 - 850 km and sulfur contents of 14 - 36 wt.%, while structures based on the H chondrite composition fail to satisfy the gravity constraint. For a denser hydrosphere of 1300 kg m-3, the permitted ranges become 600 - 850 km and 10 - 22 wt.%, whereas CI, CM, CV, and CK compositions are excluded because they predict excessively large MoI.
Thermal evolution calculations including fully molten, Fe-snow, and FeS-layer regimes in the core show that dynamo action is possible in all cases and can produce magnetic intensities comparable to the present observation. When the additional requirement of matching the observed surface field strength is imposed, the admissible solutions are significantly restricted. The models that remain viable are: (i) core radius 830 +/- 20km with 12.9 wt.% sulfur and an hydrosphere density of 1300 kg m-3 (H chondrite), (ii) 800 +/- 20 km with 16.0 wt.% and 1200 kg m-3 (L chondrite), and (iii) 850 +/- 20 km with 35.0 wt.% and 1200 kg m-3 (CI chondrite). Consequently, the wide uncertainty previously ranging core radii 550 - 1150 km and sulfur contents of 0 - 36 wt.% can be reduced to 800 - 850 km and 12.9 - 35.0 wt.%, and the H, L, or CI types are preferable for bulk composition.
Future exploration will provide multiple independent tests of these predictions. The JUICE spacecraft is scheduled to arrive in the Jovian system in 2031 and to enter orbit around Ganymede in 2034. High-precision gravity measurements will refine the MoI factor, magnetometer observations will characterize the magnetic morphology and its temporal variability, and laser altimetry will quantify tidal deformation. Together, these data are expected to determine whether a subsurface ocean exists and to constrain its depth, thickness, and electrical conductivity. Estimating the presence and thickness/depth of the ocean will in turn constrain the mean density of the hydrosphere, providing a direct means to verify the interior structures derived here. Moreover, improved determinations of magnetic multipole components and their time dependence may reveal the present state of the core dynamo, including the mode of crystallization. Confronting these forthcoming measurements with the model results will further narrow the permissible parameter space of the interior or provide observational support for the solutions identified here.