A summary of relative ages of lunar nearside and farside plains

This report contains a summary of the moon-wide distribution of relative ages of lunar plains units determined with Apollo and Lunar Orbiter photographs. Relative ages are expressed as the largest diameter of craters (D_L, meters) in a mappable unit that are eroded to a slope of 1° by the net accumulated flux of small impacts. New data show that the technique used here generally requires a sample area less than 2,000 km² for satisfactory results. Additionally, the measurement of relative age (D_L) using this technique is unaffected by resolutions of about 0.15 D_L or smaller.

Results obtained indicate there are two types of plains: 1) light plains and 2) mare plains. The light plains units have two separate and distinct ages (D_L ≃ 550 m and D ≃ 1100 m). The range in ages for each of these units is so small that each could represent two separate synchronous lunar events. The maria were deposited in three major stages after the emplacement of the light plains units. The oldest mare units ( > 3.7 b.y. old) are in Mare Tranquillitatis, Mare Vaporum, southeastern Mare Imbrium, the dark ring around Mare Serenitatis and a thin band from the crater Reinhold to Sinus Medii. Young mare volcanism ( < 2.5 b.y. old) occurred in western Mare Serenitatis, southwestern Mare Imbrium and Oceanus Procellarum. All other mare areas investigated are of intermediate age.