Effect of Mercury's interior structure on its long-period libration

Mercury experiences long-period forced librations as a result of periodic gravitational perturbations on its orbit from other planetary bodies. These librations can be resonantly amplified when the planetary forcing periods are close to one of the two free libration normal modes. The periods of the free libration modes strongly dependent on the interior structure of the planet and on couplings between the solid shell, outer core, and inner core. Among the internal couplings, the gravitational-pressure coupling between the planet shell and inner core has the largest effect on the forced libration amplitude and free libration periods. Although dissipative couplings — of electromagnetic or viscous origin as well as the viscous relaxation of the inner core —  have a small effect on the free libration periods, they can substantially affect the amplitude of the forced libration.    

In this study, we investigate the effect of Mercury’s interior structure, composition, thermal state, and dynamical shape on the long-period librations, incorporating the effects of internal couplings. We compare our model predictions with the recently derived long-period libration time series obtained from co-registration of MESSENGER laser altimeter observations  (Xiao et al. 2025, this meeting).