The Moon has been the cornerstone of our understanding of terrestrial planet formation and early evolution since the Apollo investigations 40 years ago. Geochemical studies of returned samples combined with geophysical experiments such as laser ranging, magnetic induction, heat flow, gravity, and seismology have all contributed to the currently accepted large impact formation and subsequent magma ocean differentiation model of the Moon’s earliest history. This paradigm has successfully illuminated our understanding of early planetary evolution for decades.

Of all the bodies in the solar system, the Moon is uniquely accessible for both orbit- and ground-based geophysical studies, and the recent increase in both domestic and international lunar missions emphasizes this fact. The geophysical experiments deployed on the lunar surface during Apollo remain the benchmark for ground-based studies on other planets. As such, ongoing analysis of this unique data set continues to yield new information relevant to the Moon’s formation and evolution, and encourages the development of data analysis techniques that can be applied to future planetary geophysical data.

Our work focuses on the Apollo seismic data. The Apollo Passive Seismic Experiment consisted of a network of four seismometers deployed on the lunar surface between 1969 and 1972. Data from these instruments were recorded continuously until late 1977. Several types of seismic signals were recorded, including natural impacts (meteoroids), artificial impacts (booster rockets from the Apollo spacecraft, and the landers themselves), shallow moonquakes (natural events occurring in the upper 50 to 220 km of the Moon), and deep moonquakes (natural events occurring between 700 and 1000 km depth).

and the volatile content of the mantle remain controversial. Advancing our understanding of the Moon’s interior is critical for addressing these details. The Moon’s lack of Earth-like plate tectonics means that a record of early planetary differentiation has been preserved. With that in mind, future ground-based missions can build on the legacy of Apollo by designing instruments capable of addressing deficiencies in the existing lunar data.

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Recently, we performed an analysis of the deep moonquake data to identify seismic energy reflecting off the Moon’s core, and used this information to confirm the presence of solid, molten, and partially molten layers. The size, composition, and present state of the layers of the lunar interior are relevant to many topics concerning the formation and early evolution of the Moon. Using the core as an example, its size acts as a constraint in impact formation models. Its composition is used to infer the origin of present-day remnant magnetization observed in lunar samples (e.g. an iron core may once have provided the early dynamo necessary to support magnetic field generation). Its current state (molten vs. solid) is relevant to thermal evolution models.

Although general models of the processes that contributed to the formation of the present-day lunar interior are robust, details such as the fate of the gravitationally unstable stratification in the mantle, the origin and depth of lateral inhomogeneities,

The lunar core is about 60 percent liquid by volume, according to a new study by MSFC’s Renee Weber.