Rather than being dead inside, the Moon still has a warm interior that is due to the effect of the Earth’s gravity on our closest major celestial neighbor, a new study says. The results came after looking at results from the SELENE (SELenological and ENgineering Explorer) spacecraft as well as other missions exploring the Moon.

“I believe that our research results have brought about new questions. For example, how can the bottom of the lunar mantle maintain its softer state for a long time? To answer this question, we would like to further investigate the internal structure and heat-generating mechanism inside the Moon in detail,” stated Yuji Harada, the principal investigator of the research team.

“Another question has come up: How has the conversion from the tidal energy to the heat energy in the soft layer affected the motion of the Moon relative to the Earth, and also the cooling of the Moon?” he added. “We would like to resolve those problems as well so that we can thoroughly understand how the Moon was born and has evolved.”

Clues to the Moon’s interior come from examining how the Earth’s gravity deforms its inside through tidal forces. Models show that tidal changes within the moon are likely due to a “soft layer” deep within the lunar mantle. Scientists learned that the Moon has a core (inner portion, made up of metal) and a mantle (made up of rock) through the Apollo missions, which saw astronauts deploy seismic devices that revealed the interior structure.

“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. This research result supports the above possibility since partially molten rock becomes softer,” the National Astronomical Observatory of Japan stated. “This research has proven for the first time that the deepest part of the lunar mantle is soft, based upon the agreement between observation results and the theoretical calculations.”

Researchers believe the heat occurs in a soft layer that is deep within the mantle, and not throughout the entire Moon. They said that possible future research directions could include why it is only this layer that remains soft, and how tidal energy changes the Moon’s cooling and its relative motion to Earth.

The research was published in Nature Geoscience.

Source: National Astronomical Observatory of Japan