| 研究実績の概要 |
The purpose of the research was to investigate the origin of the asymmetry in the lunar crust. In particular, the ratio of magnesium to iron (Mg#) varies longitudinally. As iron fractionates to the liquid phase as the primordial magma ocean crystallizes, we wanted to test if that longitudinal variation could be due to an asymmetric crystallization of the lunar crust.
One of the main driver of this asymmetric could be the influence of a magma ocean on the early Earth, while the Moon's orbit was closer. We computed the difference in surface heating between the lunar farside (with only solar input) and nearside (solar + earthshine). While solar input was kept constant, the input from the Earth is a function both of lunar distance and evolution of Earth's magma ocean.
Once the thermal forcing was characterized, we used it as a boundary condition to an energy balance calculation to estimate crustal growth rate on each hemisphere and predict Mg# variation with depth depending on magma ocean chemistry. This allowed us to generate both crustal thickness difference predictions between hemispheres as a function of orbital evolution history and assumptions about Earth's magma ocean.
Finally, we developed a basin impact model to characterize the degree of mixing of the lunar crust during its early evolution, as what is observed today from satellite observation is the result of long term mixing of the upper crust. We found that it is hard for asymmetric crystallization alone to explain the difference in Mg# distribution, and that differences in impact properties have to be invoked.
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