The Moon’s early years were difficult. It was created from a piece of Earth that was severed after a planetary collision, and during its early years, it was engulfed in a turbulent ocean of molten magma on a global scale before cooling and becoming the calm surface we see today.
In order to simulate the magmatic melt that once shaped the lunar surface, a research team led by The University of Texas at Austin Jackson School of Geosciences went to the lab and discovered fresh information on how the current moonscape came to be. Their research demonstrates that the Moon’s crust was initially created by cooling rock that floated to the surface of the magma ocean.
The scientists also discovered that one of the major puzzles of the development of the lunar body how it could produce a crust made of only one mineral cannot be explained by the crust’s initial construction and must have been the consequence of a subsequent occurrence.
The results were published on Nov. 21 in the Journal for Geophysical Research: Planets.
“It’s fascinating to me that there could be a body as big as the Moon that was completely molten,” said Nick Dygert, an assistant professor at the University of Tennessee, Knoxville who led the research while a postdoctoral researcher in the Jackson School’s Department of Geological Sciences. “That we can run these simple experiments, in these tiny little capsules here on Earth and make first-order predictions about how such a large body would have evolved is one of the really exciting things about mineral physics.”
Dygert collaborated with Jackson School Associate Professor Jung-Fu Lin, Professor James Gardner and Ph.D. student Edward Marshall, as well as Yoshio Kono, a beamline scientist at the Geophysical Laboratory at the Carnegie Institution of Washington.
It’s fascinating to me that there could be a body as big as the Moon that was completely molten. That we can run these simple experiments, in these tiny little capsules here on Earth and make first-order predictions about how such a large body would have evolved is one of the really exciting things about mineral physics.
Nick Dygert
98 percent of the mineral plagioclase makes up significant sections of the lunar crust. The discovery casts doubt on the widely accepted hypothesis that the purity of the Moon’s surface is the result of plagioclase rising to the top of the magma ocean and hardening into the Moon’s crust over hundreds of millions of years.
This hypothesis is predicated on the existence of a particular viscosity in the magma ocean, which would permit plagioclase to separate from other dense minerals it crystallized with and climb to the top.
Dygert decided to test the plausibility of this theory by measuring the viscosity of lunar magma directly. By using a synchrotron facility’s high-pressure apparatus, which emits a focused beam of high-energy X-rays, scientists were able to recreate the molten substance in a lab setting. They then timed how long it took a melt-resistant spherical to descend into the lava.
“Previously, there had not been any laboratory data to support models,” said Lin. “So this is really the first time we have reliable laboratory experimental results to understand how the Moon’s crust and interior formed.”
According to the experiment, the viscosity of the magma melt was extremely low, falling halfway between that of corn syrup and olive oil at room temperature. This viscosity would have allowed plagioclase to float.
In addition, combining plagioclase with the magma would have trapped additional minerals in the spaces between the crystals of plagioclase, resulting in an imperfect crust on the lunar surface.
A subsequent process must have resurfaced the Moon, exposing a deeper, younger, purer layer of flotation crust because satellite-based investigations show that a sizeable amount of the crust on the Moon’s surface is pure.
Dygert said the results support a “crustal overturn” on the lunar surface where the old mixed crust was replaced with young, buoyant, hot deposits of pure plagioclase. The older cruse could have also been eroded away by asteroids slamming into the Moon’s surface.
Dygert said the study’s results exemplify how small-scale experiments can lead to large-scale understanding of geological processes that build planetary bodies in our solar system and others.
“I view the Moon as a planetary lab,” Dygert said. “It’s so small that it cooled quickly, and there’s no atmosphere or plate tectonics to wipe out the earliest processes of planetary evolution. The concepts described here could be applicable to just about any planet.”
