This research result supports the above possibility since partially molten rock becomes softer. The previous studies indicated that there is the possibility that a part of the rock at the deepest part inside the lunar mantle may be molten. The research team has found that the observed tidal deformation of the Moon can be well explained if it is assumed that there is an extremely soft layer in the deepest part of the lunar mantle.
One of the analysis results concerning the internal structure of the Moon based upon the seismic data indicates that the satellite is considered to consist mainly of two parts: the "core," the inner portion made up of metal, and the "mantle," the outer portion made up of rock. During the Apollo program, seismic observations (*2) were carried out on the Moon. What the research team focused on is the structure deep inside the Moon. Therefore, the research team performed theoretical calculations to understand what type of internal structure of the Moon leads to the observed change of the lunar shape.
However, models of the internal structure of the Moon as derived from past research could not account for the deformation precisely observed by the above lunar exploration programs. The deformation has already been well known through several geodetic observations (*1). The Moon is no exception we can learn about the interior of our natural satellite from its deformation caused by the tidal force of Earth. Conversely, it means that observing the degree of deformation enables us to learn about the interior, which is normally not directly visible to the naked eye. How much a celestial body can be deformed by tidal force, in this way, depends on its internal structure, and especially on the hardness of its interior. Sea water is so deformable that its desplacement can be easily observed.
For example, the ocean tide on Earth is one tidal phenomenon caused by the gravitational force between the Moon and the Sun, and Earth. The shape of a celestial body being changes by the gravitational force of another body is called tide. How can we know the internal structure of a celestial body far away from us? We can get clues about its internal structure and state by thoroughly investigating how its shape changes due to external forces. When it comes to clarifying how a celestial body like a planet or a natural satellite is born and grows, it is necessary to know as precisely as possible its internal structure and thermal state.
This research provides a chance to reconsider how both Earth and the Moon have been evolving since their births through mutual influence until now. These findings suggest that the interior of the Moon has not yet cooled and hardened, and also that it is still being warmed by the effect of Earth on the Moon. The results were derived by comparing the deformation of the Moon as precisely measured by Kaguya (SELENE, Selenological and Engineering Explorer) and other probes with theoretically calculated estimates.