THE IMPACT OF VARIUS INTERNAL STRUCTURE MODELS ON GANYMEDE’s ROTATIONAL STATE

The rotational motion of a celestial body is influenced by its internal structure, shape, and the gravitational torques exerted by external massive bodies. The European Space Agency's JUpiter ICy moon Explorer (JUICE) mission aims to explore the Jupiter system, focusing primarily on Ganymede. After performing a series of flybys of Ganymede, Europa, and Callisto, JUICE will enter its main scientific phase, during which it will orbit Ganymede for several months. Throughout this phase, the spacecraft will gather extensive data, including gravity and altimetry measurements, surface images, and magnetic field observations. By combining these data sets, researchers will gain valuable insights into Ganymede's orientation.

This analysis is based on a coupled numerical integration of both the orbital dynamics of the Jupiter system and Ganymede's rotational dynamics. The integration tracks a set of Euler angles for each layer of Ganymede, capturing how they evolve over time. The model of Ganymede’s orientation incorporates not only the gravitational torques from external bodies but also the internal coupling torques between the moon's different layers, including viscoelastic, gravitational, and inertial torques.

Viscoelastic torque arises from the differential angular velocities between adjacent layers, leading to shear forces at their boundaries due to viscosity. Gravitational torque is caused by the misalignment of the principal axes of inertia between the layers, which works to realign them [1]. Inertial coupling torque results from interactions between the solid and fluid layers, where the solid boundaries constrain the motion of the fluid layer. In this dynamic system, the physical properties, shape, and dimensions of each layer are crucial to Ganymede's librations.

This study examines how variations in Ganymede's internal structure affect its orientation. The current model assumes a three-layer structure [2]: a solid inner core (with an overlying high-pressure ice layer), an icy outer shell, and a subsurface ocean layer. By varying the physical properties, shape, and dimensions of these layers, we assess their impact on Ganymede's libration. The analysis highlights the sensitivity of Ganymede’s orientation to changes in its internal structure model.

The upcoming JUICE mission will provide real data on the amplitude of Ganymede’s libration. Any libration effects smaller than the mission's detection threshold will be undetectable by the spacecraft. Therefore, the sensitivity analysis presented here is essential for forecasting the mission's data analysis capabilities and understanding which features of Ganymede’s libration may be measurable.

 

[1] Van Hoolst, T., Rambaux, N., Karatekin, Ö., Dehant, V., & Rivoldini, A. (2008). The librations, shape, and icy shell of Europa. Icarus, 195(1), 386-399. https://doi.org/10.1016/j.icarus.2007.12.011

[2] Gomez Casajus, L., Ermakov, A. I., Zannoni, M., Keane, J. T., Stevenson, D., Buccino, D. R., et al. (2022). Gravity field of Ganymede after the Juno Extended Mission. Geophysical Research Letters, 49, https://doi.org/10.1029/2022GL099475