Global geologic context for lunar geochemistry

A gross subdivision of the Moon s surface is based on albedo. The bright regions are the highlands, which are clearly ancient because of their high densities of impact craters. The dark regions are younger maria, basaltic lavas that flooded the impact basins on the nearside of the Moon. Mare basalts are exposed over 17% of the lunar surface and probably comprise only 1% of the crustal volume. 

Schematic view of lunar stratigraphic and time units. The major rock and time periods are separated by Formations representing the ejecta of huge impact basins. Estimates of the absolute ages of these boundaries are shown on the right. 

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