Abstract

The major stages of lunar evolution as derived from information obtained from the Apollo and Luna missions can be defined as follows:

1. Beginning—4.55 eons. The moon formed contemporaneously with the Earth. Geophysical data and parentless Pb and volatiles in pyroclastic glasses of lmbrium age derived from deep source regions indicate that initially undifferentiated material accreted below about 480 km.

2. Magma Ocean—4. 5–4.4(?) eons. Accretionary melting, volatile depletion, and crystal settling and floating differentiated the outer 400–500 km of the moon into an anorthositic crust 50–70 km thick and an ultramafic upper mantle containing uTKREEP residual liquids.

3. Cratered Highlands—4.4(?)–4.2(?) eons. Impacts capable of forming craters at least 50 km in diameter saturated the Pre-Nectarian lunar crust, producing intense crustal brecciation and increased thermal insulation of the interior.

4. Old Large Basins and Crustal Strengthening—4.2(?)–3.9 eons. Pre-Nectarian large basins formed with rapid isostatic adjustment of the crust. Residual uTKREEP liquids apparently moved upward into the pervasively fractured crust. The removal of underlying liquid combined with interlocking intrusions strengthened the crust.

5. Young Large Basins—3.9–3.8 eons. Large impact basins of Nectarian and Imbrium age formed in a crust strong enough to support mascons and mass deficiencies. Materials related to this stage of large basin formation, including ejecta blankets, debris flows, and shock melted lava, covered most of the near side, if not most of the Moon. A lunar cataclysm of this age does not appear to be required to explain the rarity of older, datable breccia samples.

6. Basaltic Maria—3.8–3.0(?) eons. Mare basalt, produced by downwardly progressing partial melting of the mantle, erupted at the lunar surface and intruded the subsurface. The sequence of near-side surface eruptions proceeded roughly radially from northern Tranquillitatis, the region of minimum gravitational potential. Early volatile-driven eruptions of crustal debris contributed to light plains deposits. Late mafic pyroclastic materials appear to be a mixture of differentiated mantle and undifferentiated volatiles derived from below 400 km.

7. Mature Surface—3.0(?) eons to present. Formation of rayed craters occurred in all regions. Maturation of the lunar regolith continued, including implantation of solar wind gases. Bright swirls formed over large areas.

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