Abstract:
Comparative frequency distributions of aluminum, magnesium, titanium, iron, and thorium concentrations from orbital geochemical data show a chemical dichotomy between the lunar nearside and farside terra surfaces. Nearside terra rock types (indicated by modes in the histograms) are more mafic and less aluminous than those on the farside. To explore this chemical asymmetry, terra regions exhibiting either anomalously high or low Mg/Al concentration ratios were examined. Low Mg/Al material excavated from depth by basin and large crater impacts indicates that the highly 'anorthositic' farside crustal composition is also present in the subsurface of the near side at widely scattered locations. Study of high Mg/Al areas of the nearside terra suggests resurfacing of the preexisting terra, possibly by impact excavation of local mafic 'pockets' in the crust or early extrusion of volcanic materials.
