Is the lunar upper mantle more iron-rich than expected?

A research group including Associate Professor Steeve Gréaux of the Geodynamics Research Center at the Advanced Research Institute, Ehime University, announced on April 17 that the lunar upper mantle is highly likely to contain more iron than previously believed. This finding was achieved by measuring the elastic wave velocities (P-wave and S-wave velocities) of orthopyroxene, a key component of the lunar mantle, under high-pressure and high-temperature conditions corresponding to the lunar interior. This discovery provides crucial clues for understanding the evolution of the Earth-Moon system. The results were published in the February 8 issue of Geophysical Research Letters.

The Moon is thought to have formed approximately 4.5 billion years ago when a Mars-sized protoplanet named Theia collided with the young Earth. Immediately after its birth, the Moon was in a high-temperature, molten state known as a "magma ocean." As it subsequently cooled, layers with differing mineral compositions and iron contents crystallized, forming the present-day internal structure.

Unlike Earth, the Moon exhibits almost no plate tectonics. Consequently, its internal structure is believed to have preserved its original state from the time of formation relatively well. Studying the lunar interior is therefore expected to lead to a better understanding of the composition of the early Earth and the evolution of the Earth-Moon system.

Information regarding the lunar interior has primarily been estimated based on moonquake data obtained by seismometers deployed during NASA's Apollo program. However, to link seismic wave velocities to specific mineral compositions, it is necessary to measure the elastic wave velocities of the minerals constituting the lunar mantle under high-pressure and high-temperature conditions, and data on these physical properties had been insufficient.

In this study, the research group utilized a multi-anvil apparatus installed at the large synchrotron radiation facility SPring-8 to measure the P-wave and S-wave velocities, as well as the density, of orthopyroxene, a primary mineral of the lunar mantle, under high-pressure and high-temperature conditions up to 55,000 atmospheres and 1,000℃.

Next, they combined the obtained elasticity data with existing data on iron-rich olivine to calculate the seismic wave velocities and densities for rock models of the lunar upper mantle.

The results revealed that a mantle composition containing approximately 20 mol% iron is required to explain the seismic observation data of the lunar upper mantle (at depths of 40 to 740 kilometers). This indicates that the lunar mantle may be more iron-rich than conventional models suggest.

This finding implies that Theia, the celestial body that caused the giant impact, may have been denser and more iron-rich than previously assumed. It also suggests that the early Moon may have experienced more active igneous activity and internal dynamics.

Gréaux noted: "Although iron is a common constituent element in minerals, the difficulty of synthesizing iron-bearing minerals in laboratory experiments means that elasticity data for iron-rich minerals is currently scarce. Moving forward, we plan to expand the technology we developed to a wider variety of minerals and apply the insights gained to estimate the internal structures and dynamics of other celestial bodies."

Journal Information
Publication: Geophysical Research Letters
Title: P- and S-wave Velocity Measurement of Lunar Orthopyroxene up to 5.5 GPa and 1,273 K: Implication for the Iron Content of the Lunar Upper Mantle
DOI: 10.1029/2025GL118120