17:15 〜 18:45
[PPS01-P10] Additional constraint for the interior structure of Ganymede based on a core-dynamo generation.
Ganymede, the largest moon in our Solar System, is a unique moon with respect to its intrinsic global magnetic field which had been discovered by the Galileo spacecraft in 1996. The magnetic field, approximately antiparallel to Jupiter's magnetic field, exhibits an equatorial surface strength of around 720 nT. Analysis of observational data suggests that the permanent dipole component aligns with this magnetic field, implying dynamo activity within the metallic core. The existence of the metallic core is supported from a small value of the moment inertia factor of 0.3105+/-0.0028 from the gravitational measurements by the Galileo spacecraft, and of the recently updated value of 0.3156+/-0.0125 together with the Juno's measurement. Driving processes of the dynamo activity are strongly linked with a thermal state and its evolution. To induce thermal and/or compositional convection in the core, balance between a heat extraction a heating is essential. If a substantial heat source is absent in the metallic core, the removal of heat by the overlying mantle becomes a key to dynamo activity. Current internal state is the consequence of a long–term evolution governed by a balance between internal heating and cooling, which is controlled by the volume of each layer. Small value of the moment of inertia and its bulk density (1.936 g/cc) of Ganymede indicates a three-layered structure comprising an outermost water layer, a rocky mantle, and a metallic core. However, the only constraint currently available on the interior structure of Ganymede is the moment of inertia, and thus a large uncertainties remain. For example, the inferred radius of the metallic core fluctuates between 600 to 1150 km, depending on the endmember compositions in the iron (Fe)–iron sulfide (FeS) system. Such structural variations are expected to exert a profound influence on the thermal history and magnetic field evolution.
This study aims to confirm that the current thermal state of Ganymede's interior is compatible with an ongoing dynamo activity, and to identify the structural condition that can drive the dynamo using numerical calculations. Previous work (Kimura+ 2009, Icarus) performed numerical investigation based on a similar framework and found a specific range of the core radius and composition of 1000-1100 km with sulfur content of 25-36 wt%. This study revisits it using CI chondritic abundances for radioactive isotopes, modified values for the thermal properties for the metallic core and the updated value for the moment of inertia factor. We employed a 1D method utilizing the mixing length theory (MLT) to estimate the convective heat flux in the sub–solidus regime, and solved heat transfer equation from the core-mantle boundary (CMB) to the surface. To assess the generation of thermal convection in the core, we employed the following two simple conditions. The first condition is that the temperature at the CMB is higher than the melting point of the assumed core composition, implying that the core is at least partially molten. The second condition is that the heat flux through the CMB must exceed that which can be carried by conduction along an adiabat. Additionally, the magnetic Reynolds number is calculated to evaluate a sustainability of the dynamo.
This study aims to confirm that the current thermal state of Ganymede's interior is compatible with an ongoing dynamo activity, and to identify the structural condition that can drive the dynamo using numerical calculations. Previous work (Kimura+ 2009, Icarus) performed numerical investigation based on a similar framework and found a specific range of the core radius and composition of 1000-1100 km with sulfur content of 25-36 wt%. This study revisits it using CI chondritic abundances for radioactive isotopes, modified values for the thermal properties for the metallic core and the updated value for the moment of inertia factor. We employed a 1D method utilizing the mixing length theory (MLT) to estimate the convective heat flux in the sub–solidus regime, and solved heat transfer equation from the core-mantle boundary (CMB) to the surface. To assess the generation of thermal convection in the core, we employed the following two simple conditions. The first condition is that the temperature at the CMB is higher than the melting point of the assumed core composition, implying that the core is at least partially molten. The second condition is that the heat flux through the CMB must exceed that which can be carried by conduction along an adiabat. Additionally, the magnetic Reynolds number is calculated to evaluate a sustainability of the dynamo.