A study examining the deformation of the moon suggests that there’s an extremely soft layer deep inside that’s being kept warm by the gravity of Earth. The work was published in Nature Geoscience last month.
According to data from multiple probes, external forces have been producing changes to the moon’s shape. When these sorts of deformation are caused by the gravitational force of another celestial body, it’s called a tide. The ocean tide at your nearby beach, for example, is thanks to the gravitational force between the moon, the sun, and Earth; seawater is so deformable that you can easily observe its displacement. The moon itself is no exception.
To study the interior of our natural satellite based on the changes caused by the tidal force of Earth, a team led by Yuji Harada from China University of Geosciences combined deformation observations and seismic data with theoretical calculations.
Previous work has shown that the moon’s core is made of metal, while its mantle is made of rock. In order to explain the observed tidal deformation of the moon, the researchers say there’s a soft layer in the deepest part of the lunar mantle (which wraps around the core) that hasn’t cooled and hardened yet.
As tidal forces from Earth put pressure on the moon, it creates seismic waves, Ars Technica explains, which ultimately dissipate as energy is converted to heat deep in the interior of the moon. The partially molten, “ultralow-viscosity zone” at the core-mantle boundary plays a role in that strong tidal heating by helping the waves dissipate. (Viscosity is a measure of tenderness and hardness.)
The team believes that the soft layer deep in the mantle is warming the lunar core with heat that’s being efficiently generated by the tides. The amount of the heat generated inside the low-viscosity layer is balanced with the heat escaping from the layer. In this case, the conversion that happens when stored energy is changed to heat doesn’t occur uniformly in the entire moon -- just intensively in the soft layer, which acts like a thermal blanket on the lunar core.