UT researchers recreate moon’s magma ocean

Jennifer Liu

New research from the Jackson School of Geosciences illuminates the history of the moon’s landscape. 

In a paper published in Geophysical Research Letters last month, a team of researchers from UT’s Jackson School of Geosciences discovered why the moon’s crust is composed of one mineral.

One thing that the Earth’s surface and the moon’s surface have in common is the abundance of plagioclase, an aluminum and calcium-rich silicate mineral, said Nicholas Dygert, an assistant professor of petrology and geochemistry at the University of Tennessee.

The moon’s rust is especially interesting, Dygert added, because it is made up of 98 percent plagioclase. It is extremely rare for igneous rocks to be made of more than 90 percent of any individual mineral.

“The question about the viscosity of lunar materials under relevant pressure-temperature of the moon’s formation has been around for a while,” said Jung-Fu Lin, associate professor of geological sciences and a researcher in this experiment. 

There wasn’t a way to investigate the issue until a synchrotron, or high-energy electron X-ray facility became available at Argonne National Lab, Lin said. But when it did, the researchers set out to recreate the magmatic melt that covered the moon’s surface. 

To accomplish this, Lin said rocks believed to represent the moon’s surface were placed in a device able to generate high pressure and temperature conditions to melt the lunar rock. The synchrotron X-ray source was then used to measure the magma’s viscosity, or resistance to flow. 

The viscosity of the lunar magma ocean was found to be very low, promoting efficient fractionation, or the separation of low and high density crystals during the process of crystallization in the formation of the moon’s crust, Dygert said. This revealed that the moon had a stratified crust with layers of different mineral compositions in the past.

Dygert said this fractionation was initially inefficient, causing the oldest lunar crust to be impure or containing minerals other than plagioclase. This suggests that our present-day lunar surface resulted from crustal overturn, where the impure crust was replaced by buoyant and hot deposits of pure plagioclase. 

Another possibility is that the old crust was blasted away from asteroidal impacts, he added.

This discovery was surprising and also yielded some other interesting results, according to Dygert.

“Making unexpected discoveries is why experiments are important,” Dygert said. 

Although the research may have provided an answer for the composition of the moon’s crust, Dygert said the next step is investigating the viscosity of Mercury’s magma ocean, with help of NASA’s MESSENGER mission. The MESSENGER (MErcury Surface, Space ENvironment, GEochemistry and Ranging) mission’s goal is to determine why Mercury is significantly denser and more mineral-rich than other planets, according to NASA’s website.

“Thanks to NASA’s recent MESSENGER mission we have good estimates of Mercury’s crustal composition for the first time,” Dygert said. “The composition of Mercury is really exotic, and it will be fun to run experiments on this unexplored system”.