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Our Natural Satellite May Have Partly Molten Insides

internal structure of the moon

Image credit: Artist’s conception of the internal structure of the moon / National Astronomical Observatory of Japan.

A recent study focusing on the deformation of the moon demonstrates that there is a highly soft layer deep inside and it is being kept warm by Earth’s gravity. The results are published in recent Nature Geoscience.

The researchers collected data from multiple probes and found that external forces have been inducing changes to the shape of moon. When such deformation is caused by the gravitational force of another celestial body, it is called a tide. We have experienced this kind of phenomenon in our life often. For example, the ocean tide at a beach results from the gravitational force between the Earth, the sun and the moon. The same rules apply for the moon itself.

To investigate the interior of the moon based on the deformation resulted from the tidal force of Earth, a group of scientists led by Yuji Harada at China University of Geosciences combined deformation observations and seismic data with theoretical calculations.

It has been revealed previously that the core of the moon is made of metal, while the mantle of rock. To explain the observed tidal deformation of the moon, the scientists note that there is a soft layer in the deep inside of the lunar mantle (which wraps around the core of the moon) that has not cooled and hardened yet.

Seismic waves are created when the tidal forces from Earth put pressure on the moon, explains Ars Technica, which eventually dissipates as the energy converts to heat deep inside the moon. This partially molten, “ultralow-viscosity zone” at the core-mantle boundary plays a vital role to help the waves dissipate in the strong tidal heating. (Viscosity is measure of a fluid’s resistance to gradual deformation by stress).

middle NAOJ

The research group believes that this soft layer deep in the moon mantle keeps warming the lunar core using the heat that is being efficiently produced by the tides. The amount of heat produced inside the “ultralow-viscosity” layer is balanced with the heat escaping from the layer. In this case, the conversion occurs as the stored energy is transformed to heat. However, such conversion does not happen uniformly in the whole moon, instead, it just occurs intensively in the soft layer, which functions as a thermal blanket wrapping the core of the moon.

Source: National Astronomical Observatory of Japan

Images: NAOJ

Journal reference: Harada, Yuji, et al. “Strong tidal heating in an ultralow-viscosity zone at the core-mantle boundary of the Moon.” Nature Geoscience 7.8 (2014): 569-572.