= Solution
The <lunar crustal dichotomy> combines several observables: the <far side of the Moon> has generally thicker, higher, feldspathic <lunar highlands>, whereas most visible <lunar maria> and the <Procellarum KREEP Terrane> occupy the <near side of the Moon>. These need not all have been produced by a single event. The old <anorthosite> crust formed mainly before the younger basaltic mare infill. Gravity-based crustal models combined with sample and seismic constraints demonstrate that crustal thickness and porosity must be considered alongside surface color.<The Crust of the Moon as Seen by GRAIL by Mark A. Wieczorek and colleagues>[Science 339, p. 671, crustal density, porosity and thickness constraints]{ref}
\b[An early thermal and chemical asymmetry is one physically motivated option.] In a <lunar magma ocean>, crystallizing, relatively buoyant <plagioclase> can accumulate to form feldspathic crust. On a young <Moon> that rapidly attained <synchronous rotation>, radiation from a still-hot nearby <Earth> would preferentially warm the nearside. The <primordial Earthshine hypothesis> proposes that this imposed a thermal gradient in the atmosphere and accreting material: crust-forming refractory constituents condensed preferentially toward the cooler farside, favoring thicker farside crust. This is more specific than simply saying that the farside cooled faster; it requires a persistent hemispheric chemical contrast.<Earthshine on a Young Moon: Explaining the Lunar Farside Highlands by Arpita Roy and others>[Sections 2–4, early tidal locking, condensation gradient and survival of compositional asymmetry]{ref}
Subsequent concentration of incompatible elements in <KREEP>-rich late liquids can help preserve a thermal asymmetry: potassium, uranium and thorium provide <radiogenic heating>, supporting later melting in the nearside source region. Lower surface elevation and a thinner crust also facilitate some magma transport and pooling, although crustal thickness alone cannot specify eruption rates. \b[The origin of the crustal contrast and the later production and emplacement of mare basalt are separate parts of the explanation.]
The thermal option is consistent with a thick feldspathic farside crust, a hemispheric chemical contrast and the existence of an early hot Earth. It is not uniquely demonstrated by those observations. Vigorous magma-ocean mixing could erase the proposed gradient; the required irradiation must persist during the relevant condensation and crust-building interval, not merely occur at some early time. A failure to find the predicted bulk compositional contrast in demonstrably ancient deep crust, or a chronology placing the asymmetry after complete crystallization, would challenge this particular mechanism. The existence of farside mare deposits and large thin-crust basins also warns against treating thickness as an on/off volcanic switch.
\b[Accretion of a smaller companion is a contrasting option.] A low-speed collision of a smaller co-orbiting moon with the lunar farside could add a broad layer of relatively cold silicate material rather than excavate an ordinary deep crater. Numerical impact models demonstrate that such a depositional event can build thick highlands and displace underlying residual liquid toward the opposite hemisphere.<Forming the lunar farside highlands by accretion of a companion moon by Martin Jutzi and others>[Nature 476, pp. 69–72, accretionary collision model]{ref} Broad crustal excess and asymmetric evolved material are compatible with that mechanism, but are not unique evidence for it. The necessary companion, collision timing and impact geometry must be dynamically feasible. A predicted buried depositional interface, age discontinuity or lateral fabric should survive somewhere; their systematic absence in well-resolved, unmodified ancient crust would count against the model. Closely related bodies could have similar isotopes, so failure to find a large isotopic difference alone would not disprove accretion.
Both scenarios allow later basin impacts, <partial melting> of the <lunar mantle> and mare emplacement to modify the original contrast. \b[A useful explanation must reproduce crustal mass, chemistry, chronology and volcanic distribution together, and state which observations would distinguish its initial cause.]
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