Scientists have described the role of the Earth in the ancient volcanism of the Moon

The moon could retain internal heat due to the gravity of the neighboring Earth. Billions of years ago, the satellite was much closer to our planet, and its orbit was more elongated. Because of this, the earth's gravity periodically deformed the lunar interior, creating an additional source of energy. A new computer model has shown that heat could be concentrated in two regions at the boundary of the core and mantle and support the geological activity of the satellite. How the Earth could have influenced the volcanic history of the Moon is described in the Izvestia article.

Why did volcanic rocks appear on the Moon?

The moon formed about 4.5 billion years ago, probably after the collision of the young Earth with a large celestial body— the protoplanet Theia. As a result of the impact, a huge amount of matter was thrown into space, from which the satellite was subsequently formed. Due to its relatively small size, the Moon should have cooled down quite quickly and gradually lost its internal geological activity.

However, samples of lunar soil brought to Earth by the Chinese Chang'e-5 mission raised a question for researchers: some basalts from the Ocean of Storms formed about 2 billion years ago. This means that at that time, conditions were still present inside the satellite for the melting of rocks and the release of magma to the surface.

One of the possible sources of heat is the radioactive decay of elements contained in the lunar interior. But, according to scientists, by that time it was no longer enough to explain the late volcanic activity. The impacts of large asteroids also could not fully explain what was happening: the period of intense bombardment of the Moon ended long before the formation of these rocks.

A group of researchers led by Miao Chen from Chengdu University of Technology has proposed another mechanism — tidal heating caused by Earth's gravity. The results of the work are published in the Planet magazine. Scientists have built a model of the Moon's internal structure and calculated how the impact of our planet could distribute heat in its mantle.

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How the earth's gravity warmed up the lunar interior

Tidal heating occurs when the gravity of a large celestial body stretches and compresses another object. If its orbit has an elongated shape, the impact force changes periodically: the satellite undergoes deformations, and some of the mechanical energy is converted into heat. This process is especially noticeable on Jupiter's moon Io, where tidal forces support intense volcanic activity.

In the early history of the Earth—Moon system, the conditions for such warming were much more pronounced than they are now. According to the calculations used by the authors, about 3.2 billion years ago, the distance between the planet and its satellite was about 70% of the modern one, and the lunar orbital period was about 16 Earth days. In addition, the Moon's orbit was more elongated, so the Earth's gravitational influence changed more strongly during the satellite's movement.

Previous models of tidal heating often simplified the internal structure of the Moon, considering it as an almost homogeneous body. The authors of the new work took into account that the viscosity of matter in different parts of the mantle is not the same. The region with a very low viscosity at the core-mantle boundary was of particular importance: according to calculations, it could enhance energy dissipation and create local heating zones.

The researchers used three-dimensional modeling to trace how tidal forces affected the inner layers of the satellite and how the released energy affected the movement of matter in the mantle. This approach allowed us to evaluate not only the overall effect of Earth's gravity, but also the heat distribution in specific areas of the lunar interior.

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Where was the heat concentrated inside the moon

The simulation revealed two symmetrical regions of enhanced tidal heating at the core-mantle boundary. The authors called them zones of abnormal tidal heating — THAZ. Despite the fact that together they occupied about 2.5% of the mantle volume, they accounted for a significant part of the released energy.

According to the researchers, about 3.2 billion years ago, the heating capacity in these areas could reach 150 GW. For comparison, the local heating capacity from radioactive decay in the same zone was estimated at about 34.9 GW. Thus, tidal action could become a more powerful source of heat there than radioactive processes.

Heating, in turn, could support the updrafts of matter — mantle plumes. They transferred energy from deep layers to the upper part of the mantle and contributed to the partial melting of rocks. The authors suggest that this process helped to move titanium-rich materials formed during the early stages of cooling of the lunar magmatic ocean. Their melting could have been involved in the formation of ancient basalt rocks.

The model also offers an explanation for the continued existence of the lunar magnetic field. Additional heating at the core-mantle boundary could slow down core cooling and support the processes necessary for the operation of the ancient lunar dynamo. According to the authors, this mechanism is consistent with data on the existence of the Moon's magnetic field for more than 500 million years.

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What does the new model explain about the history of the Moon

Researchers believe that tidal heating could have influenced the formation of both ancient and relatively young lunar basalts. In particular, the model links the movement of matter in the mantle with the formation of titanium-rich rocks about 3.7 billion years old, samples of which were brought to Earth by the Apollo 11 and Apollo 17 expeditions.

Separately, the authors examined basalts about 2 billion years old, discovered by the Chang'e-5 mission. According to the proposed explanation, intense heating in an earlier period could have contributed to the movement of part of the mantle substance closer to the surface. Some of its reserves remained there and later became a source of magma, even when the tidal effects of the Earth had already significantly weakened.

As the Moon moved away from the Earth, the gravitational forces that caused the tidal heating became weaker. In the model, this explains the gradual extinction of the corresponding processes and the cessation of large-scale volcanic activity associated with the movement of deep-seated materials. At the same time, the authors note that the distribution of volcanic rocks on the surface of the satellite depended not only on processes in the mantle, but also on the features of the lunar crust.

The study has limitations: the authors used a simplified scheme for the interaction of tidal heating and the movement of matter in the mantle. In particular, the model does not fully account for the cooling associated with the melting of rocks and the movement of magma. Therefore, the proposed mechanism remains a theoretical explanation that needs to be verified using new calculations, geophysical observations, and analysis of lunar samples.