Pressure-dependent rheology and the Lunar crustal dichotomy

Abstract:

For over 60 years it has been known that the Moon's near and far sides are very different. More recently, data from the GRAIL mission has shown that there is a strong hemispheric dichotomy: the crust on the far side is far thicker, on average, than that on the near side. [1]
This has led to the exploration of several possible mechanisms, including radiant heating from the Earth, asymmetric radiogenic heating, and tidal heating.
There exist other unresolved issues in the study of the formation of the Lunar crust. For example, radioisotopic dating suggests the crust took around 200 Ma to form, whereas existing models in which the crust formed by early flotation of light plagioclase crystals from the magma ocean suggest a timescale of 10-20 Ma. [2]

We consider a simple one-dimensional model with pressure- and temperature- dependent silicate viscosity, with a stagnant lid divided into two hemispheres overlying a well-mixed silicate interior and ferrous core. By maintaining this well-mixed interior, the silicate visco sity is reduced to a function of potential temperature and pressure. If this viscosity increases sufficiently rapidly with pressure, an symmetric state is unstable to antisymmetric perturbations in stagnant-lid thickness. This instability saturates only when a significant dichotomy has developed in the stagnant lid.

The crustal thickness can be considered closely related to that of the stagnant lid, by having a crust that forms from compaction of the stagnant lid, rather than the more orthodox flotation crust. In addition to resolving timescale issues, our model could provide the mechanism behind the crustal dichotomy.

[1] 'The Crust of the Moon as Seen by GRAIL', Mark A. Wieczorek et al., Science 339, 671 (2013)

[2] 'The lunar magma ocean: Reconciling the solidification process with lunar petrology and geochronology', Linda T. Elkins-Tanton et al., Earth and Planetary Science Letters 304 (2011)