A Novel Hypothesis on the Compositional and Morphological Asymmetry Between the Moon's Near Side and Far Side: Gravitational Differentiation and Tidal Heating

The dichotomy between the lunar near side and far side remains one of the most enduring questions in planetary science. While the near side is dominated by dark volcanic maria and a thin crust, the far side is characterized by rugged highlands and a significantly thicker crust. Current theories, including the Giant Impact Hypothesis and the South Pole-Aitken (SPA) impact event, offer partial explanations but leave gaps regarding the specific concentration of heat-producing elements on the near side and the sustained volcanic activity that created the maria. This study proposes a novel hypothesis: during the Moon's early formation in a semi-molten state, Earth's strong tidal forces induced gravitational differentiation, causing dense radioactive elements (thorium and uranium) to migrate preferentially toward the near side. This concentration of heat-producing elements, combined with tidal heating from Earth's gravitational pull, generated sustained internal heating that powered prolonged volcanic activity on the near side. This dual heating mechanism explains both the compositional asymmetry (KREEP terrane enrichment) and the morphological differences (extensive maria formation). The hypothesis is supported by: (1) density data showing radioactive elements are 3-6 times denser than silicate mantle; (2) observational evidence of tidal heating in the lunar mantle (Harada et al., 2014); (3) thermal asymmetry measurements indicating 100-200K temperature difference between hemispheres (Nature, 2025); and (4) crustal thickness variations (30 km near side vs. 50 km far side). This work contributes to planetary science by proposing a testable mechanism that integrates gravitational physics with geochemical observations, offering specific predictions for future lunar missions and comparative studies of other tidally locked bodies.