Garnet Presence in the Deep Lunar Mantle Confirmed

The current understanding of the Moon's interior reveals a central metallic core, a mantle rich in minerals like olivine and pyroxene, and an outer crust layer. This model has been developed through analyses of lunar samples and seismic data from past Moon missions. Despite extensive studies, the debate about the presence of garnet in the deeper lunar mantle remains unresolved. A critical question posed half a century ago questioned whether laboratory-measured sound velocities in garnet-containing lunar rocks match the seismic profiles of the Moon's interior.

Researchers at Ehime University aimed to address this question by synthesizing garnet-rich lunar rock samples using the multi-anvil press apparatus "ORANGE-2000" at the Geodynamics Research Center. These samples were then taken to SPring-8, a large-scale synchrotron radiation facility in Hyogo Prefecture. At the High Pressure and High Temperature beamline BL04B1, the researchers simulated conditions of the Moon's deep interior and measured sound wave propagation in the garnet-rich samples.

The results indicated that the sound velocities in these garnet-containing samples matched the seismic and density profiles of the Moon's deep interior at depths between 740 and 1260 km. In contrast, samples with little or no garnet failed to explain the observed seismic velocities and densities at these depths.

These findings carry significant implications for our understanding of the Moon's composition, formation, internal temperature, and the now-inactive lunar dynamo.

Research Report:Sound velocities in lunar mantle aggregates at simultaneous high pressures and temperatures: Implications for the presence of garnet in the deep lunar interior