月は私たちが思うよりも鉄が多い？～月の直方輝石における弾性波速度測定から月の鉄分量を推定～

2026-04-17 愛媛大学

本研究は、愛媛大学先端研究院地球深部ダイナミクス研究センターなどの研究チームが、月内部に相当する高圧高温条件下で直方輝石の弾性波速度（P波・S波）を測定し、月マントルの鉄含有量を再評価した成果である。実験により、鉄量が増えるほど弾性率が低下することを確認し、そのデータをアポロ計画の地震観測モデルと比較した結果、月の上部マントルは従来想定より鉄を多く含む可能性が高いと判明した。この知見は、月の内部構造だけでなく、地球―月系の形成や進化モデルの再検討にも重要な示唆を与えるものである。

月の構成鉱物の弾性波速度を測定することで、月の内部構造を探る

＜関連情報＞

月の上部マントルの鉄含有量～5.5万気圧1000℃における月マントル組成直方輝石のPならびにＳ波速度測定 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

Yoshihiro Inoue, Yoshio Kono, Steeve Gréaux, Jie-Jun Jing, Sho Kakizawa, Itaru Ohira, Noriyoshi Tsujino, Yuji Higo
Geophysical Research Letters Published: 08 February 2026
DOI:https://doi.org/10.1029/2025GL118120

Abstract

Seismological observations in the lunar mantle, in conjunction with experimental knowledge on the elastic wave velocities and density of lunar mantle minerals, provide important constraints on the composition and mineralogy of the lunar mantle. Here we report elastic wave velocities and density of a lunar orthopyroxene (Mg_0.84Fe_0.13Ca_0.03SiO₃) up to 5.5 GPa and 1,273 K. The result shows that the bulk and shear moduli of orthopyroxene decrease with increasing iron content. Based on the mineral elasticity data, we modeled the P- and S-wave velocities and density of petrologically suggested lunar upper mantle rock composition. The petrological lunar upper mantle rock model shows consistent seismic wave velocities with those observed in the lunar upper mantle whereas markedly lower density. Our modeling suggests an iron-rich (Fe/(Mg + Fe) = 0.20) lunar upper mantle to explain P- and S-wave velocities and density of the lunar upper mantle at 40–740 km depth.

Plain Language Summary

Understanding the composition and mineralogy of the lunar mantle is important to discuss nature and evolution of the Moon. Petrological studies based on high pressure experiments on crystallization of lunar magma ocean suggest that the lunar mantle is mainly composed of olivine and orthopyroxene. On the other hand, seismological observations from the data of Apollo mission is another important information to understand the lunar mantle. Combination of the lunar seismological observations with experimental knowledge on the elastic wave velocities and density of lunar mantle minerals provides important constraints on the composition and mineralogy of the lunar mantle. In this study, we investigated the P- and S-wave velocities and density of a lunar orthopyroxene up to 5.5 GPa and 1,273 K by using ultrasonic technique combined with synchrotron X-ray measurements in a multi-anvil press. Based on our newly obtained orthopyroxene data combined with literature data for olivine, we modeled the P- and S-wave velocities and density of lunar upper mantle rock. The model suggests that lunar mantle rock with 20 mol.% iron content explain the seismological observations of the lunar upper mantle at the depths of 40–740 km.