Document Type

Article

Publication Date

2-1982

Abstract

Mare material is asymmetrically distributed on the Moon. The Farth- facing hemisphere, where the crust is believed co be 26 lom thinner than on the farside, contains subetantially 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), Ieaev (1.0 km), and Tsiolkovsky (1.75 km). We also studied crater frequency distribution in five farside mare unite (Aitken, Isaev, Lacus Solitudinis, Langemak, and Tsiolkovskiy) and one light plains unit (in Mendeleev). Nearly 10,000 farside craters were counted. Analyeis of the crater frequency on the light plains unit gives an age of 4.3 billion years. Crater.frequency distributions on the mare units indicate ages of 3.7 and 3.8 billion years, suggesting that the units are distributed over a narrow time period of approximately 100 million years. Returned lunar samples from nearside maria give dates as young as 3.1 billion years. The results of this study suggest that mare basalt emplace ment on the far side ceased before it did on the near side.

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