Slow-Cooling Magma Ocean Reconciles the Age of the Moon?

Sketch of the thermal evolution model with its four reservoirs. Image credit: Maurice et al., 2020

The prevailing paradigm for formation of the layered core-mantle-crust structure of a rocky planet or moon is the crystallization of a global magma ocean. The Moon serves as a geologic time capsule for studying this process because it preserves samples that record a nearly 4.5-billion-year rock record. This preservation is due to the absence of plate tectonics and atmospheric erosion on the Moon, whereas on Earth these mechanisms have resulted in recycling and destruction of much of the record of early geologic history. The ages of lunar samples derived from its interior, along with the age of its primary crust, can be used to constrain the timescales associated with the lunar magma ocean (LMO) process. Researchers led by Maxime Maurice of the Technische Universität Berlin, Germany, have updated numerical LMO crystallization models using improved estimates of lunar crust thermal conductivity. Results from their models allowing for less heat conduction (and more insulation) indicate that LMO cooling may have lasted up to approximately 200 million years. Combined with ages of samples crystallized from the LMO, the authors estimate that the Moon formed some 4.425 billion years ago. The authors then demonstrate that the isotopic compositions of mantle-derived samples are consistent with a prolonged cooling period of chemical evolution, contrary to models invoking a rapidly crystallizing LMO. This modeled age of LMO solidification aligns well with isotopic age dating of the Earth, which the authors argue is indicative of the final segregation of Earth’s core after the Moon-forming event. Continuing research will lead to a more complete understanding of the dynamics of the giant-impact theory and the chronology of the lunar formation process. READ MORE