School of Geosciences
Moon's Crust Underwent Resurfacing After Forming from Magma Ocean
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.
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.
an assistant professor of petrology and geochemistry
at the University of Tennessee Knoxville.
He conducted the moon magma research
as a postdoc at the University of Texas at
Austin Jackson School of Geosciences.
Nick Dygert collaborated with,
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 purity is due to plagioclase floating to the surface of the magma ocean over hundreds of millions of years and solidifying into the Moon's crust.
This theory hinges on the
magma ocean having a specific viscosity, a term related to the
magma's "gooiness," that would allow plagioclase to separate
from other dense minerals it crystallized with and rise to the
falling through a magma sample.
The researchers calculated
the viscosity of the magma sample
by measuring how quickly the sphere sunk
through the magma.
The feat involved using a high-pressure apparatus called a synchrotron to shoot 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, somewhere between that of olive oil and corn syrup at room temperature, a value that would have supported plagioclase flotation.
However, it would have also led to mixing of plagioclase with the magma, a process that would trap other minerals in between the plagioclase crystals, creating an impure crust on the lunar surface.
Because satellite-based investigations demonstrate that a significant portion of the crust on the Moon's surface is pure, a secondary process must have resurfaced the Moon, exposing a deeper, younger, purer layer of crust.
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 crust could
have also been eroded away by asteroids slamming into the Moon's
The experimental apparatus
in the synchrotron facility.
The researchers used the machine
to recreate moon magma in the lab.