Formation & Geology of the Moon: Observation, Measurement & Evidence

The Moon has a small metallic core, a silicate mantle and crust, ancient bright highlands dominated by anorthositic rock, and darker basaltic maria. Its surface is covered by regolith produced by billions of years of impacts. With no substantial atmosphere, liquid water erosion, or active plate tectonics, old surfaces remain exposed for extraordinarily long times.

Apollo samples reveal ancient ages, silicate-rich composition, and isotopic similarities between Earth and Moon material. Crater counts distinguish older heavily cratered highlands from younger maria, while radiometric dating of returned rocks calibrates the crater record. Gravity and seismic measurements constrain the Moon's internal structure.

A giant collision can eject mantle-rich material into Earth orbit while leaving much of the colliding bodies' metal in their cores. As lunar material cooled, lower-density crustal minerals rose while denser material sank. Later impacts excavated basins, and heat-driven volcanism flooded some basins with basalt before the Moon's interior cooled enough for major volcanism to fade.

Apollo samples provide radiometric ages and composition, seismic instruments constrain the interior, orbital spectroscopy maps minerals, and crater counts compare relative surface ages. Isotopic similarities between Earth and Moon strongly constrain origin models, while differences in volatile content and detailed chemistry show that the impact and mixing history was complex.