December 3, 2018
The Earth’s moon had a rough start in life. Formed from a chunk of the Earth that was lopped off during a planetary collision, it spent its early years covered by a roiling global ocean of molten magma before cooling and forming the serene surface we know today.
A research team led by the Jackson School of Geosciences took to the lab to recreate the magmatic melt that once formed the lunar surface and uncovered new insights on how the modern moonscape came to be. Their study shows that the moon’s crust initially formed from rock floating to the surface of the magma ocean and cooling.
However, the team also found that one of the great mysteries of the lunar body’s formation — how it could develop a crust composed largely of just one mineral — cannot be explained by the initial crust formation and must have been the result of some secondary event. The results were published on Nov. 21, 2017, in Geophysical Research Letters.
The research was led by Nick Dygert, an assistant professor at the University of Tennessee, Knoxville, who conducted
the research while a postdoctoral researcher in the Jackson School’s Department of Geological Sciences.
Large portions of the moon’s crust are made up almost entirely of a single mineral. In these sections, 98 percent of the crust is plagioclase. According to the prevailing theory, which this study calls into question, the crustal purity is due to plagioclase floating to the surface of the magma ocean over hundreds of millions of years and solidifying. This theory hinges on the magma ocean having a specific viscosity that would allow plagioclase to separate from other dense minerals and rise to the top.
Dygert decided to test the plausibility of the theory by measuring the viscosity of lunar magma directly. The feat involved shooting a concentrated beam of high-energy X-rays into a sample of mineral powders and flash melting them into magma. The researchers then measured the time it took for a melt-resistant sphere to sink through the magma.
The experiment found that the magma melt had a very low viscosity that would have supported plagioclase flotation, but would have also led to mixing of plagioclase with the magma, a process that would trap other minerals in-between the plagioclase crystals and create an impure lunar surface. Dygert said the results support a “crustal overturn” on the lunar surface where the old mixed crust was replaced with deposits of pure plagioclase.