Deep Down, The Moon Is Still Warm

Contrary to expectations, scientists have found that the Moon has a warm inner core.

AsianScientist (Aug. 13, 2014) – Researchers have found that there is an extremely soft layer deep inside the Moon that has been kept warm by the gravity of the Earth. This research has been published in the journal Nature Geoscience.

But how do scientists know the internal structure of a celestial body far away from us? One way is to study the shape, or tide, of the celestial body as it changes in response to gravitational forces exerted by other nearby bodies. For example, the ocean tide on the Earth is a tidal phenomenon caused by the gravitational force between the Moon, the Sun, and the Earth. How much a celestial body can be deformed by tidal force depends on its internal structure, especially the hardness of its interior. What this means is that observing the degree of deformation enables us to learn about the interior, which is normally not directly visible to the naked eye.

The deformation of the Moon as it revolves around the Earth.  For illustration purposes, the deformations have been exaggerated. Credit: National Astronomical Observatory of Japan.
The deformation of the Moon as it revolves around the Earth. For illustration purposes, the deformations have been exaggerated. Credit: National Astronomical Observatory of Japan.

Deformations have long been tracked via mirrors placed on the Moon’s surface during space missions such as the Apollo program. However, models of the internal structure of the Moon as derived from past research could not fully account for the deformation observed.

In the present study, a research team led by Dr. Yuji Harada from the China University of Geosciences performed theoretical calculations to simulate the properties of the moon that would give rise to the observed deformations.

Previous research showed that the Moon consists mainly of two parts: the “core”, the inner portion made up of metal, and the “mantle”, the outer portion made up of rock. The research team 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.

Furthermore, they also showed that tides generated in this soft layer efficiently generate heat. The conversion of energy from tidal deformation to heat was not found to occur uniformly across the entire Moon, but only intensively in the soft layer. This implies that that the soft layer acts as a thermal blanket, wrapped around the core of the Moon and keeping it warm.

“A smaller celestial body like the Moon cools faster than a larger one like the Earth does. In fact, we had thought that volcanic activities on the Moon had already come to a halt. Therefore, the Moon had been believed to be cool and hard, even in its deeper parts,” said Professor Junichi Haruyama of Institute of Space and Aeronautical Science, Japan Aerospace Exploration Agency, co-author of the study.

“However, this research tells us that the Moon has not yet cooled and hardened, but is still warm. It even implies that we have to reconsider the question as follows: How have the Earth and the Moon influenced each other since their births? That means this research not only shows us the actual state of the deep interior of the Moon, but also gives us a clue for learning about the history of the system including both the Earth and the Moon.”

The article can be found at: Harada et al. (2014) Strong Tidal Heating in an Ultralow-Viscosity Zone at the Core–Mantle Boundary of the Moon.

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Source: National Astronomical Observatory of Japan.
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