150 Gigawatts Inside the Moon May Explain Its Late Volcanoes

Earth's gravity may have helped prolong the Moon's volcanic life by concentrating heat deep underground, according to a new study investigating why our neighbour stayed active so late. ^[1]

Three-dimensional simulations suggest that tidal forces could have heated two small regions near the boundary between the Moon's core and mantle. The researchers propose that this concentrated energy could have helped drive hot mantle material upward, carrying heat toward shallower layers. ^[1]

The puzzle comes from rocks returned by China's Chang'e-5 mission. A 2021 study dated fragments of lunar basalt, rock formed from erupted lava, to 2.03 billion years ago. That extended the range of measured lunar basalt ages by about 800 million to 900 million years. The Moon had kept producing magma far longer than the older dated samples showed. ^[2]

The new work offers a possible source of some of that extra heat. Miao Chen, Da Wang and Yun Liu, whose study appeared in the journal Planet, modeled how the Moon's deep structure could concentrate energy supplied by Earth's tidal pull, according to a release from Higher Education Press. ^[1]

The attraction of the idea is straightforward: the Moon's geological history may depend partly on the planet beside it. But the size of the effect turns sharply on the ancient orbit the researchers put into their model. ^[1]

Two deep pockets of heat

Tides do more than move ocean water. Gravity can also deform a solid body, and repeated deformation can turn mechanical energy into heat. In the new simulations, the important question is where inside the Moon that energy would accumulate. ^[1]

The researchers included a proposed layer near the core-mantle boundary that would deform much more readily than the material around it. Their calculations produced two symmetric patches of enhanced heating near the equatorial part of that boundary, which they named the Tidal Heating Anomaly Zone. ^[1]

Concentrating energy in a small volume matters because a local hot region can behave differently from a mantle warmed evenly. The team proposes that these deep pockets could affect temperatures and the upward movement of mantle material, potentially helping heat reach regions closer to the surface. ^[1]

In the strongest numerical example described in the release, the affected region occupied about 2.5% of the mantle, or one-fortieth of it. The model put roughly 150 gigawatts of tidal heating into that region. ^[1]

That is a measure of the rate at which heat was generated, not a temperature or an amount of lava. To explain volcanism, the energy would still have to influence the material from which magma formed. The proposed upward movement of hot mantle material supplies a possible route. ^[1]

The orbit controls the answer

For conditions around 3.2 billion years ago, the researchers assumed that the Moon took about 16 Earth days to complete an orbit. They then tested different values of orbital eccentricity, the measure of how stretched an orbit is rather than circular. ^[1]

At an eccentricity of 0.5, a strongly elongated orbit, the model generated about 150 gigawatts of heating. At 0.05, a much rounder orbit, it generated 1.5 gigawatts. Changing that orbital assumption cut the heating by a factor of 100. ^[1]

That comparison makes the ancient orbit central to the hypothesis. A mechanism capable of delivering substantial heat under one set of conditions can deliver far less under another. Establishing which conditions the Moon actually experienced, and how long they lasted, is therefore essential to judging its contribution. ^[1]

The high-output scenario also describes a time roughly 1.17 billion years before the Chang'e-5 basalts formed. It provides a possible part of the Moon's thermal history, rather than a calculation made at the date of those eruptions. ^[1][2]

The rocks set the test

The Chang'e-5 result rests on a different kind of evidence. Researchers used lead-lead isotope measurements to date basalt fragments to 2,030 million years old, with an uncertainty of four million years. They interpreted that age as evidence that lunar magmatic activity persisted until at least two billion years ago. ^[2]

Those rocks changed the chronology. The tidal study now tries to help explain the energy supply. Its contribution is a specific, testable proposal about where heat could have gathered inside the Moon, rather than a new date for its volcanic activity. ^[1][2]

The simulations do not demonstrate that tidal heating caused the Chang'e-5 eruptions, or that the 150-gigawatt rate persisted through lunar history. They also do not fully account for energy absorbed during melting or heat lost as magma rises, according to the study's release. The likelihood and duration of the high-eccentricity conditions remain open questions. ^[1]

The next step is to test whether the proposed deep heating survives more complete calculations, then check its consequences against lunar observations and returned samples. The researchers call for both. The rocks have established how late the Moon erupted. The task now is to trace the heat that made those eruptions possible. ^[1][2]