Size And Compositional Constraint Of Ganymede's Core From The Condition Of Driving A Dynamo Activity
Ganymede has a intrinsic magnetic field which is generally considered to be originated by self-excited dynamo in the metallic core. The driving of dynamo depends critically on the thermal state and internal structure. However, inferred structure based on gravity data has large uncertainty and this makes the possibility of dynamo activity unclear; variation of the core size and the composition greatly change the heat capacity and hence, it alters the cooling history of the core.Main objective is to explore the conditions for the current active dynamo in Ganymede using numerical simulation on the thermal history, and to confirm the condition of the internal structure that allows the dynamo. We investigated the thermal history with various core size and composition within the solution of MoI, and evaluated the temperature and the heat flux through the core-mantle boundary. Based on two conditions, we evaluated the possibility of dynamo activity and find the “dynamo regime” in the diagram of structure. First condition is that the temperature at CMB must exceed the melting point of a metallic core, second is that the heat flux at CMB must exceed the adiabatic T-grad. Temperature begins to rise due to the decay of long-lived radiogenic elements in the rocky mantle. After few Gyr, radiogenic elements are depleted and temperature turns to decrease. As the rocky mantle cools, heat flux at CMB increases continuously. If the temperature and heat flux at CMB satisfies the conditions, we considered this case is capable of driving a dynamo. We found the “dynamo regime”, which is the specific parameter range of the internal structure which is capable of driving the dynamo, based on the results of simulations with various structures. This regime is characterized as 950 1100 km of core radius and 25 36 wt% of the sulfur content.
AAS/Division for Planetary Sciences Meeting Abstracts #38
- Pub Date:
- December 2006