The hemispheric contrast involving thicker, generally higher feldspathic crust on the far side of the Moon, compared with generally thinner crust, more extensive lunar maria and the concentration of KREEP-rich material on the near side of the Moon. Crustal mass, topography, chemistry and later volcanic deposits are different observables. A proposed mechanism must explain their relationship rather than identify the contrast solely with visible dark maria.
A proposed explanation of the lunar farside highlands in which irradiation from a nearby hot early Earth imposes a thermal gradient on a synchronously rotating, accreting Moon. Preferential condensation and deposition of refractory crust-forming material toward the cooler farside seeds a lasting chemical and crustal-thickness asymmetry. It requires early synchronous rotation, appropriate transport times and incomplete homogenization by subsequent mixing. It is a testable formation hypothesis, distinct from ordinary present-day illumination of the lunar night side.[ref]
Earthshine on a Young Moon: Explaining the Lunar Farside Highlands by Arpita Roy and others. Sections 2–4, physical requirements and chemical-gradient mechanism
A nearside lunar geochemical region enriched in KREEP-associated material and heat-producing elements. Its distribution offers a constraint on the Moon's chemical and thermal evolution. Enrichment can help sustain melting but does not, by itself, identify the process that first made the hemispheric crustal contrast.
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