Using Models to Understand the Lunar Interior

Credit: NASA/Colorado School of Mines/MIT/JPL/GSFC.

While there is general consensus that the Moon is the product of a large collision between Earth and a Mars-sized planet and subsequent widespread melting and crystallization, details of lunar formation and evolution remain the focus of intense study. In particular, understanding the internal structure of the Moon, such as its composition and density, has been limited to experiments done in laboratories on Earth. Such experiments can be combined with thermodynamic modeling, which uses intrinsic material properties that are independent of a mineral’s mass, such as density and melting temperature, to predict mineral stability. Thermodynamic modeling over a continuous range of pressure-temperature-composition (or P-T-X) values for a given rock composition allows experimental results to be extrapolated to a wider range of conditions. Thermodynamic models can simulate a wide range of conditions in a relatively short time, giving scientists the freedom to test many hypotheses rapidly. However, models are also dependent on the quality and applicability of the experimental datasets used to calibrate the models and have historically been developed using terrestrial, not lunar, compositions and conditions. In a recent study led by Tim Johnson at Curtin University in Perth, Australia, scientists tested the application of thermodynamic models to non-terrestrial environments and explored various crystallization hypotheses for the lunar magma ocean.

The study predicted the density and composition of the lunar crust and mantle and found that results differed depending on the rock type (i.e., lunar basalt, lunar upper mantle) considered. Thus, model results support several current crystallization and melting hypotheses for the Moon. Furthermore, the study compared model predictions with published experimental results for the same conditions and compositions and concluded that thermodynamic models generally agree with experimental data. This result is important because it implies that in future studies, thermodynamic models can be used to quantitatively investigate melting and crystallization processes, which are important in forming and stabilizing crust, on other rocky bodies in addition to the Moon. Thus, thermodynamic models have the potential to help scientists better understand how a planet’s crust formed and ultimately evolved to what we observe today. READ MORE