The existence of a partially molten layer between the Moon’s rocky mantle and its solid metallic core seems more likely after examining its changing shape and gravity.

Researchers at NASA’s Goddard Space Flight Center and the University of Arizona analyzed new data describing the Moon’s stiffness under the gravitational influence of the Earth and the Sun and concluded that its mass is unlikely to be solid throughout. Most likely the Moon’s mantle has a thick, viscous region that rises and falls like our tides.

“Modeling of the interior suggests that these values ​​may correspond only to the low viscosity zone (LVZ) at the base of the lunar mantle,” the researchers wrote in their published paper.

The idea of ​​this non-rigid layer has been discussed by researchers for decades, but until now the available data did not allow us to state with certainty whether this layer actually exists.

Under the influence of the gravitational attraction of the Earth and the Sun, the Moon experiences a tidal effect, not in the sense of oceans, but in the sense of physical deformations of the Moon’s shape and gravitational field. For this study, the team used new data obtained by NASA’s Gravity and Internal Processes Laboratory (GRAIL) and the Lunar Orbiter Reconnaissance Vehicle. These measurements allowed researchers to estimate changes in lunar tides on an annual basis for the first time.

Computer models that describe the nature of deep rocks as the Moon orbits the Earth show that the layer beneath the solid mantle must be at least somewhat viscous for these numbers to be accurate.

This raises additional questions: How did this region get there? So what makes him hot? More research is needed to know for sure, but the team behind the study believes the mineral ilmenite, an oxide of titanium and iron, may be involved.

“The existence of the LVZ in the lower part of the lunar mantle can most easily be explained by partial melting of the ilmenite-rich layer, making the Moon similar to Mars, where partial melting has recently been detected based on seismic data analysis,” the researchers wrote. wrote.

As with Earth exploration, deciding what lies hundreds and thousands of kilometers below the surface requires some guesswork, but these are all very educated guesses based on what we know about the Moon and the planets.

We know that the mantle above this LVZ region consists mostly of the mineral olivine, and that this has a very long history spanning several billion years. If we can establish a permanent base on the Moon in the coming years, seismic data from the Moon’s surface will be able to give us more information about what is happening below the surface.

“The existence of this region has profound consequences for the thermal state and evolution of the Moon,” the researchers write. The study was published in the journal AGU Advances.