11:30 AM - 11:45 AM
[PPS01-10] Constraint for size and composition of Ganymede's core: Evaluation of solidification regime and dynamo activity
Keywords:Ganymede, magnetic field, thermal evolution, icy satellite
In 1996, magnetometer observations for the Jovian moon Ganymede by the Galileo spacecraft found a global intrinsic magnetic field. This field can be well approximated by a dipole at the moon's center with a strength of approximately 720 nT at an equatorial surface, suggesting that Ganymede possesses a dynamo activity in the metallic core. A small value of the moment of inertia factor (0.3156 ± 0.0125) also supports the strongly differentiated interior including the central metallic core. For a core dynamo to generate and sustain a magnetic field, the following conditions must be required: (1) the core must be (at least partly) molten, (2) the core must cool at a sufficiently high rate, and (3) the flow velocity in the molten core (i.e., the magnetic Reynolds number) must be large enough to sustain the field. Within a constraint only from the moment of inertia factor, expected interior structure has a large variation of each size according to the assumed density for each layer. Interior thermal evolution is strongly influenced by the size and material properties of both the rocky mantle, which contains radiogenic heat sources, and the metallic core, which hosts the dynamo. Assuming a core density for a Fe-FeS composition, the core radius could range from 660 to 1110 km within a range of the core density between 5,500 and 8,000 kg/m3. Thus, investigating thermal evolution across the full range of plausible interior structures to identify the conditions under which a magnetic field can be generated.
In this study, we construct 1-D numerical model to describe the interior thermal evolution based on the modified mixing length theory to consider heat conduction and convection including a solidification in the core. 4.6 Gyr simulations for various interior structures inferred by the moment of inertia factor are conducted, and we evaluate the present-day thermal state according to the three conditions required for magnetic field generation by a core dynamo. Initial state assumes a fully molten core with the melting point of the assumed core density (i.e., sulfur content), and assumes that the mantle temperature is 300 K at the upper boundary and linearly increases toward the core-mantle boundary. In the mantle, the decay heats from long-lived radioactive isotopes with CI chondritic abundances are considered as heat source. In the core, various styles of solidification can occur according to the internal temperature structure and sulfur content. If the core sulfur content is below the eutectic composition of the Fe-FeS system, a solid Fe inner core grows from the center. On the other hand, if the sulfur content exceeds the eutectic composition, the formation of a solid FeS outer core growing inward from the core–mantle boundary. The latent heat and gravitational energy released during these solidification processes are included as heat sources within the core. In addition, our model considers the effects of sulfur content and temperature on both the thermal and electrical conductivities of the core.
Our results indicate that if the core sulfur content is 0 wt.%, the core must be entirely solidified at present because the higher melting point of the core leads to higher initial temperatures in both the core and mantle, enhancing the mantle's cooling efficiency and accelerating core solidification. For the core sulfur contents between 8 and 32 wt.%, the core remains molten but does not exhibit sufficiently vigorous convection. At 36 wt.% sulfur, the core remains molten and convective; however, the magnetic Reynolds number is too low to sustain a magnetic field. It is because higher sulfur contents reduce thermal conductivity, lowering the adiabatic temperature gradient at the top of the fluid core and facilitating convection, while lowering electrical conductivity decreases the magnetic Reynolds number, making it more difficult to sustain a dynamo-generated magnetic field.
In this study, we construct 1-D numerical model to describe the interior thermal evolution based on the modified mixing length theory to consider heat conduction and convection including a solidification in the core. 4.6 Gyr simulations for various interior structures inferred by the moment of inertia factor are conducted, and we evaluate the present-day thermal state according to the three conditions required for magnetic field generation by a core dynamo. Initial state assumes a fully molten core with the melting point of the assumed core density (i.e., sulfur content), and assumes that the mantle temperature is 300 K at the upper boundary and linearly increases toward the core-mantle boundary. In the mantle, the decay heats from long-lived radioactive isotopes with CI chondritic abundances are considered as heat source. In the core, various styles of solidification can occur according to the internal temperature structure and sulfur content. If the core sulfur content is below the eutectic composition of the Fe-FeS system, a solid Fe inner core grows from the center. On the other hand, if the sulfur content exceeds the eutectic composition, the formation of a solid FeS outer core growing inward from the core–mantle boundary. The latent heat and gravitational energy released during these solidification processes are included as heat sources within the core. In addition, our model considers the effects of sulfur content and temperature on both the thermal and electrical conductivities of the core.
Our results indicate that if the core sulfur content is 0 wt.%, the core must be entirely solidified at present because the higher melting point of the core leads to higher initial temperatures in both the core and mantle, enhancing the mantle's cooling efficiency and accelerating core solidification. For the core sulfur contents between 8 and 32 wt.%, the core remains molten but does not exhibit sufficiently vigorous convection. At 36 wt.% sulfur, the core remains molten and convective; however, the magnetic Reynolds number is too low to sustain a magnetic field. It is because higher sulfur contents reduce thermal conductivity, lowering the adiabatic temperature gradient at the top of the fluid core and facilitating convection, while lowering electrical conductivity decreases the magnetic Reynolds number, making it more difficult to sustain a dynamo-generated magnetic field.