Laboratory Models of Librationally-Driven Flow in Planetary Core and Sub-Surface Oceans.

Many planetary bodies, including Mercury and the moon, undergo forced longitudinal librations. Yet few studies to date have investigated how longitudinal libration, the oscillatory motion of a planet around its rotation axis, couples with its interior planetary fluid dynamics. In the present study, we investigate, via laboratory experiments, the flow in a spherical rotating fluid cavity driven by an axial oscillation of the container. Here we consider the viscous coupling between the solid outer shell and the liquid interior, focussing on libration frequencies less than or equal to the planetary rotation frequency, moderate Ekman numbers (E=10-4 to 10-5) and Rossby numbers between 0.03 and 5. In addition, we model flow in three different core geometry: full sphere; rinner~eq0.6 router; and rinner~eq0.9 router. Direct flow visualizations in the laboratory experiment allows us to identify 3 distinct flow regimes: i) a stable regime dominated by inertial waves; ii) a longitudinal rolls instability regime; and iii) a boundary turbulence regime. The longitudinal rolls are initiated in the vicinity of the equator and are qualitatively similar to Taylor and Taylor-Görtler instabilities. Interior flow visualizations have shown that the longitudinal roll instability remains confined to a layer of fluid near the outer wall. In addition, we do not observe any noticeable effect of the inner core size. Extrapolating our results to planetary conditions suggest that librationaly driven turbulence may exist below the Moon and Mercury's core-mantle boundary (CMB) and Titan and Europa's ice-shell.