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The Moon Is Hardly as Cold as It Looks

New observations point to a layer deep under the lunar surface kicking out perpetual warmth, thanks to Earth's gravity.
National Astronomical Observatory of Japan

The Moon has its very own internal furnace, not completely unlike Earth's. It's not nearly as hot or large or powerful as our own planet's internal (or mostly internal) miasma, but it's there, forming a soft, warm layer between the satellite's metallic core and its rock mantle. This assertion is old, but unproven: common models have the Moon radiating heat from its core, but it's a difficult thing to demonstrate.

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Now, a research team based at the Planetary Science Institute, an arm of the China University of Geoscience, is offering some proof. By measuring the Moon's tidal distortions—how it's stretched and deformed by Earth's relatively powerful gravity—the team reached the conclusion that a cold, hard interior could not account for the degree of deformation seen in observations made by the Japanese Selenological and Engineering Explorer (SELENE) probe, or those made with other similar instruments.

Early indications of the Moon's interior came courtesy of Apollo astronauts in the 1960s. The experiments performed  were impressive (and sketchy-sounding), involving an array of geophones, mortar shells, and a remarkable arsenal of explosives. The result of these "active" seismic experiments and their passive descendants (seismic instruments that stuck around the lunar surface until 1977 listening for "moonquakes") was a remarkable description of the sphere's interior, but one that still left enough blank space such that conjecture has maintained a leading role in describing the Moon's deep geology. As recently as 2011, a NASA team released new research—which described an Earth-like core for the Moon—based on Apollo's data, now nearly a half century old.

An exaggerated rendering of the Moon under tidal pressure/NAOJ

While the new research,  published recently in the journal Nature, in part just confirms older predictions or even assumptions, it offers at least one rather peculiar suggestion: the Moon's future is warm, or at least not completely cold.

First, understand that the progression of the Moon's guts is generally toward cooling, as the intense heat generated during a chaotic lunar infancy of nonstop bombardment by meteors and asteroids gradually dissipates. It's possible to see the effects of this cooling with just a pair of binoculars in the form of extreme cliffs and other surface geologic disruptions, the results of the Moon's interior shrinking as it cools; you might imagine it as a sort of negative pressure on the sphere's surface. The new suggestion is that the Moon's future is not entirely cold and dark at all—this soft, molten layer, located at the deepest edge of the lunar mantle, will likely stay warm and soft. The release of heat from the layer is expected to remain "exquisitely balanced" with the amount of new heat generated within it.

This layer will stay soft and warm so long as the Moon is gravitationally linked to Earth, at least. The heat, according to the new research, is thought to be the result of tidal deformations. Pressure, after all, is heat, or is at least heat's best friend. So, so long as Earth keeps trying to stretch the Moon out, some interior part of the Moon is going to stay warm and soft. The tidal forces/heat hypothesis isn't brand new, but the team behind the current study notes that they're the first to isolate it to a specific zone.

"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," Prof. Junichi Haruyama, one of the current study's investigators, said in an emailed statement. "Therefore, the Moon had been believed to be cool and hard, even in its deeper parts. 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."