Document Type

Article

Publication Date

6-30-1982

Abstract

Marc material is asymmetrically distributed on the Moon. The Earth-facing hemisphere, where the crust is bclieved to be 26 km thinner than on the farside. contains substantially more basaltic mare material. Using Lunar Topographic Orthophoto Maps. We calculated the thickness of the mare material in three farside craters, Aitken (0.59 km), Isaev (1.0 km), and Tsiolkovskiy (1.75 km). we also studicd crater frequency dist ribution in five farside mare units (Aitken, Isacv, Lacus Solitudinis, Langemak, and Tsiolkovskiy) and one light plains unit (in Mendeleev). Nearly 10000 farsidc craters were counted, Analysis of the crater frequency on the light plains unit gives an age of 4.3 billion yr. Crater frequency distributions on the mare units indicate ages of 3.7 and 3.8 billion yr, suggesting that the units are distributed over narrow time period of approximately 100 million yr. Returned lunar umples from nearside matia give dates as young as 3,1 billion yr. The results of this study suggest that mare basalt cmplacement on the far side ccased before it did on the near side.

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