Tide-Driven Magma Ocean Convection as the Origin of the Lunar Crustal Dichotomy

The Lunar crustal dichotomy, expressed in farside-nearside differences in crustal thickness, volcanism and surface composition, does not yet have a well-established origin. Multiple mechanisms proposed in the literature can explain some aspects of the dichotomy; however no single model is able to fully explain the entirety of its observed features. We hypothesize that all aspects of the dichotomy are related and originate from the solidification of the Lunar Magma Ocean (LMO). Given that the dichotomy is aligned in reference to Earth, we investigate if Earth’s tidal influence on the LMO, when the Moon was in proximity to the Roche limit, can explain this dichotomy.

We investigate this hypothesis with numerical models of lunar evolution, using a modified version of StagYY (Tackley, 2008) that includes three-dimensional gravity accounting for tidal effects. We model the LMO solidification starting from a fully molten Moon, followed by the onset of solid-state mantle convection.  

Our models show that an asymmetric degree-two convection pattern can emerge during the early stages of the LMO solidification. This tide-driven magma ocean convection is characterized by two large plumes on the nearside and farside, with downwelling in the perpendicular plane at the poles. The nearside plume upwells faster than the farside plume given the asymmetries in the tidal forces between each side of the Moon. This convection pattern inhibits both the solidification of the LMO, and the compaction of the solid fraction, resulting in a convecting mush. Melt segregates towards the sides of the plume heads, where velocities are lowest, forming a low crystallinity magma ocean that is continuously replenished by decompression melting of the mostly solidified mantle that rises through the plumes. The LMO solidifies near the surface as material travels towards the perpendicular plane and subducts, creating a barrier that isolates the two hemispheres. Differences in the timing of melt segregation and the rate of decompressive melting eventually create significant hemispheric chemical contrasts, which ultimately can lead to all observed aspects of the crustal dichotomy of the Moon.

Reference

Tackley, P. J. (2008). Modelling compressible mantle convection with large viscosity contrasts in a three-dimensional spherical shell using the yin-yang grid. Physics of the Earth and Planetary Interiors, 171(1-4), 7-18.