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Dynamical Origin of Hemispherical Asymmetry in Early Lunar Thermal Evolution

Presentation #119.09 in the session Moon & Earth (Poster)

Published onOct 23, 2023
Dynamical Origin of Hemispherical Asymmetry in Early Lunar Thermal Evolution

The Moon holds its most prominent yet enigmatic global feature: the hemispherical asymmetry between nearside and farside in its geological settings. The KREEP materials, which are rich in heat producing elements, are concentrated in a nearside province on the Moon called Procellarum KREEP Terrane (PKT). Lunar mare volcanism occurred predominantly on the nearside of the Moon between 3.9 and 3.4 Ga and lasted intermittently till about 1.2 Ga with much reduced activities. The mare basalt deposits, mostly on the nearside surface, are geographically largely overlapped with PKT. In particular, the young mare basalts erupted only in the PKT region. The concentration of both KREEP and mare basalts on the nearside is a strong indication that the Moon underwent a hemispherically asymmetric or spherical harmonic degree-1 thermal evolution, and such a degree-1 pattern dominated almost the entire evolutionary course. Furthermore, the high spatial correlation between PKT and mare basalts suggests a strong thermodynamic link between them. In this study, we build 3D thermochemical mantle convection models to investigate the dynamical origin of the Moon’s degree-1 thermal evolution pattern. KREEP and ilmenite-bearing cumulates (IBC) are the final products of lunar magma ocean (LMO) fractional crystallization, and they initially form uniform layers between the anorthosite crust and the olivine mantle. Due to their higher densities, such post-LMO stratification is unstable and gravitational instability would develop to cause mantle cumulate overturn. IBC and KREEP tend to sink into depth, mixing with the underlying mantle, and form a dense, heat producing, ilmenite-rich layer (MIC) above the core-mantle boundary. The MIC may be self-heated to offset its negative compositional buoyancy and rise to the mare basalt source regions to cause nearside volcanism. We extensively explore the model parameters with thermal and rheological properties, density, thickness, and heating rate of IBC, KREEP, and MIC layers. Our modeling effort aims to determine the physical conditions under which 1) a degree-1 KREEP-rich IBC overturn may occur that the trapped KREEP below the crust can aggregate into one hemisphere to form the present PKT on the nearside, and 2) a degree-1 mantle convection may occur that a single MIC upwelling plume rises to cause the nearside mare volcanism. In particular, we will explain the dynamical role of the trapped KREEP in facilitating a degree-1 MIC upwelling and guiding the MIC plume towards the mare basalt source regions beneath the PKT. Our models may provide important implications for future lunar missions.

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