アポロとルナ計画で採取した月物質の年代と月の進化

Radiometric ages have been determined over the samples of Apollo 11, 12, 14, 15, 16 and 17 and Luna 16 and 20 rocks and soils. From the agedating data of the lunar materials it is attempted to initiate a framework for lunar chronology. The basalts from mare regions have not undergone shock metamorphism and brecciation caused by the impact events. The ages of mare basalts from Apollo 11, 12, 14, 15, and 17 and Luna 16 date the period of flooding by basaltic lava on mare basins. The highland samples from Apollo 14, 15, 16, and 17 and Luna 20 abound with breccias-rock fragments and soils that have adhered because of the heat and pressure of impacts. Ejecta thrown up by the mare basin impacts formed the highlands. The highland rock samples (anorthositic gabbro composition but breccia texture) give a date for the cataclysm of mare basin excavation and highland formation. The ages for the mare volcanism are: Oceanus Procellarum 3.16-3.36 b.y.; Mare Imbrium 3.28-3.44 b.y.; Mare Fecunditatis 3.4-3.5 b.y.; Mare Tranquillitatis 3.56-3.83 b.y.; Mare Serenitatis 3.71 -3.79 b.y.; pre-Mare Imbrium 3.95-4.0 b.y. For the highland formation: Cayley Plains 〜3.84 b.y.: Fra Mauro Hills 〜3.88 b.y.; Apennine Mountains 〜3.88 b.y.; Apollonius Mountains 〜3.90 b.y. Descartes Mountains 〜3.93 b.y.; Taurus-Littrow Highlands 〜3.98 b.y. For the impact mare basin formation: Orientale Basin 〜3.84 b.y.; Imbrium Basin 〜3.88 b.y.; Crisium Basin 〜3.90 b.y.; Nectaris Basin 〜3.93 b.y.; Serenitatis Basin 〜3.98 b.y. The moon originated about 4.6 b.y. ago. The outermost layers of the moon must have been very hot at that time, by virtus of conservation of accretional energy, as inferred from the wholerock and soil Rb-Sr model ages of 4.6 b.y. Cooling within the outer layers commences after the period of intensive rapid heating. The flooding of the mare basins with basaltic lavas spans a time interval between 3.16 and 4.0 b.y. ago. This shows a second heating of the moon due to long-lived radioactive energy. The highlands are built up by the ejecta from the large (basin) impacts, in especial, from Orientale, Imbrium, Crisium, Nectaris, and Serenitatis events. Ages of highland samples cluster in the interval 3.84 to 4.05 b.y. and indicate that most of the major lunar basins formed in this period by a cataclysmic bombardment of the surface by many large bodies. The failure to discover lunar volcanic rocks which were formed about 4.0 to 4.6 b.y. ago, and younger than those about 3.16 b.y. ago is an important constraint on the lunar thermal history. The moon has been slowly dying since 3.16 b.y. ago, in contrast to the earth, which is, and may be, as active as ever. This results from the fact that the moon is a small body in comparison with the earth. The lunar chronology in this study is consistent with the theoretical thermal calculation (IRIYAMA and SHIMAZU, 1967; REYNOLDS et al., 1972).