Farouk El-Baz

Document Type

Article

Publication Date

1978

Abstract

The original lunar crust was physically and chemically mod- ified by meteorite bombardment and the subsequent transport and redistribution of its materials. The exposed relics of this crust form the lunar highlands, which are predominantly sculp- tured by over 30 large basins that range in diameter between 300 and 2000 km. These basins are equally distributed on the near and far lunar hemispheres. The largest, the South Pole/ Aitken basin on the far side, is among the oldest and is recog- nized by a subdued, discontinuous mountain chain surrounding a depression about 6 km deep. The youngest, the Orientale basin on the western limb, displays distinct mountain rings, a contin- uous lineated ejecta blanket surrounding the outer ring (930 km), and smooth plains and secondary craters farther out. Lu- nar basins display a continuous array of morpholoqies, from ex- tremely subdued to fairly crisp features. This negates the pos- sibility sculpting the crust by meteorite impacts in one catastrophic event. Lunar crustal materials most likely were bombarded continuously by large objects throughout the early lunar history, or between 4.6 and about 3.9 billion years ago. In addition to impact products, some highland units may be vol- canic in origin, including light plains on the lunar far side and spectrally distinct domes on the near side. Although stratigraphically very significant, mare materials cover less than 20% of the lunar surface and compose only 1% of the crust. Mare basalts were deposited within the lunar basins and surrounding troughs during three or four major episodes, between 3--85 and about 2.5 billion years canism indicates not only the chemistry of the amount and Zength of time of internal tiation. ago. Basaltic vol- the crust, but also planetary differentiation.

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