New Insights Into Why The Moon's Near And Far Sides Are So Different

The Moon's two faces: (left) near side and (right) far side. NASA

The Moon is tidally locked to the Earth, which means it's always showing the same face to our planet. The near side, which we can see quite clearly, is pocked by the maria, darker regions of volcanic origin, and has a thinner crust. This was expected to be the case for the far side too, but six decades of observations have revealed a completely different face.

A new paper, published in Nature Geoscience, suggests an intriguing explanation for some of the differences of the two faces of the Moon. Researchers suggest that the extent of the maria, the lunar “seas”, on the near side is due to a self-amplifying process caused by the chemical composition of the magma that formed them.

The lunar maria are basaltic planes caused by ancient volcanic eruptions that cover 31 percent of the surface of the near side. Compared to the rest of the Moon, they are rich in KREEP, an acronym that stands for Potassium (the chemical element K), rare earth elements, and Phosphorus (P). They also sport an abundance of Uranium (U) and Thorium (Th), both of which are radioactive.

The team believes that this special mix allowed for the formation of the maria. The radioactive elements release a lot of heat and this led to rocks being melted. The KREEP makes the melting point for those rocks lower, making this a winning combo for lava to flow across the lunar near side.  

“Because of the relative lack of erosion processes, the Moon’s surface records geological events from the Solar System’s early history,” co-author Matthieu Laneuvillr from the Earth-Life Science Institute, said in a statement. “In particular, regions on the Moon’s near side have concentrations of radioactive elements like U and Th unlike anywhere else on the Moon. Understanding the origin of these local U and Th enrichment can help explain the early stages of the Moon’s formation and, as a consequence, conditions on the early Earth.”

Distribution of thorium on the lunar surface from the Lunar Prospector mission. Thorium is highly correlated heat-producing with other radioactive elements, with most of it being present on the Earth-facing side. Laneuville, M. et al (2013) Journal of Geophysical Research: Planets.

The Moon is believed to have formed right after the Earth’s formation when a planetoid the size of Mars slammed into our infant planet, throwing a lot of material into orbit. Half of that fell back to Earth, while the rest ended up being the Moon.

The study suggests that the special composition of the maria has played a role in the general evolution of our satellite. The volcanic activity lasted longer there than in the more reflective highlands, creating the features that we can see today. The team suggests that this process might not be exclusive to the Moon and might be discovered on other bodies in the Solar System.

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