Dissipation in fluid planeraty layers induced by libration and precession

Planets and moons are subject to gravitational torques from the parentb odies about which they orbit. These torques lead to temporal changes in the orientation of their spin axes and to fluctuations in their rotation rates. In planetary terminology, these correspond to precession and libration, respectively. For a planet or a moon that has a global internal fluid layer - either a liquid iron core or a subsurface ocean underneath an ice shell - the periodic forcing caused by precession and libration of the solid regions induce a complex flow within these fluid layers.

When the amplitude of the boundary forcing is small, the flow remains in a laminar regime; the adjustment takes the form of thin viscous boundary layers and internal shear layers. When the amplitude of the forcing is sufficiently large, the flow becomes turbulent (see animation below). Viscous dissipation associated with these flow takes rotational and orbital energy away. Likewise, dissipation associated with electromagnetic coupling between the fluid and its solid region also lead to dissipation.

Animation: Turbulent flow near the outer boundary of a fluid shell induced by viscous coupling of its librating shell. Red = eastward flow; Blue = westward flow. (Credit: Ian MacPherson.)

Some of my papers on this topic