Earth’s gravity acted on a young Moon covered in molten rock. Tidal heating pushed crust toward the far side and left the near side rich in volcanic materials, according to research published in the journal National Science Review.
Researchers studying rocks returned by China’s Chang’e-6 mission propose that tidal forces from a much closer early Earth drove this split by reshaping the Moon’s interior magma ocean. The side of the Moon facing Earth looks entirely different from the side facing away, a lopsided contrast that has puzzled scientists since the first far-side photographs in 1959.
Chinese Academy of Sciences geologist Zongyu Yue told PBS NewsHour that the Chang’e-6 samples could address one of the most fundamental scientific questions in lunar science research.
Scientists have proposed several competing hypotheses over the decades to explain the lunar asymmetry, including giant impacts and uneven crust growth. None of these ideas has gained universal acceptance,
according to a ScienceDaily research summary.
Far Side Samples Reveal Distinct Crust
Chang’e-6 landed in the South Pole-Aitken basin and returned to Earth in June 2024 with about 1,935 grams of soil and rock. The study was led by Hejiu Hui of Nanjing University and published in the journal National Science Review. It compared far-side samples with near-side rocks collected by Apollo astronauts and China’s Chang’e-5 lander.
The analysis revealed that the crust tells a distinct story. Far-side samples showed a magnesium-rich crust. The Chang'e-6 samples were not rich in KREEP, a mix of potassium, rare earth elements, and phosphorus, indicating that little KREEP-rich material reached that side of the Moon.
Tidal Flexing Heats Near Side
After the Moon became tidally locked, permanently showing the same face to Earth, tidal flexing heated the near side more intensely than the far side. The researchers outlined a process acting on the early lunar body to explain these differences.
That uneven thermal input triggered lateral thermal convection within the lunar magma ocean. Hot magma flowed toward the far side, carrying magnesium-rich material and building up a thicker crust there. The last dregs of melt, rich in KREEP and titanium-bearing minerals, collected primarily beneath the near side. This accumulation fueled the heavy volcanism that created the dark maria plains visible from Earth today.
Gravity Flexes Early Moon Interior
None of these ideas has gained universal acceptance,
according to a ScienceDaily research summary of competing hypotheses proposed over the decades.
Tidal heating is a well-documented phenomenon elsewhere in the solar system; Jupiter’s gravity continuously flexes the interior of its moon Io, making it the most volcanically active body known. The new model applies a related mechanism to the early Moon when Earth’s gravitational pull was significantly stronger.