The core flow induced by Mercury’s libration: density stratification and magnetic fields

Since the era of Messenger, many observational constraints on Mercury’s thermal evolution and magnetic field have strengthened the idea that the outermost layer of Mercury’s fluid core is stably stratified. The presence of such a stably stratified zone can significantly alter the core flow compared to the flow in a completely homogeneous fluid core. This is on the one hand because stratified layers can impede fluid motions that are parallel to the density gradient, which in the case of Mercury would mean that radial flows are strongly suppressed. On the other hand this is because a stably stratified layer can support different types of waves that are affected by the buoyancy force.

In this study we have created a numerical model to investigate flow in Mercury’s fluid outer core that includes a radial background stratification. The exact structure of the density profile in the planet’s core is unknown, and we assume profiles based on recent findings of the interior evolution of the planet. We studied core flow that is excited by Mercury’s librations, oscillations of the mean rotation rate due to the solar gravitational torque acting on Mercury’s triaxial shape. Based on the work by Rekier et. al. (2019) we represent the librational forcing by the superposition of three different decoupled motions: a horizontal component, which represents the viscous drag of the core fluid by the librating mantle and spherical inner core, and two radial components that are representing the radial push that the core flow would experience due to the librating triaxial boundaries.

We show that especially the second component has a profound effect, inducing a large non-axisymmetric flow close to the core-mantle boundary. It turns out that even though the origin of said flow is radial, the horizontal component of the flow is far larger than it’s radial counterpart. This indicates that the stratified layer acts to convert radial motions into strong horizontal motions.

We show how the strength and existence of this flow depend on the strength of the stratification of the layer and discuss implications of this flow for the magnetic field.

References:

Rekier, J., Trinh, A., Triana, S. A., & Dehant, V. (2019). Internal energy dissipation in Enceladus's subsurface ocean from tides and libration and the role of inertial waves. Journal of Geophysical Research: Planets, 124, 2198–2212. https://doi.org/10.1029/2019JE005988