More than five decades of study have established the major

features of lunar geologic style and history. The most numerous and

significant landforms belong to a size-morphology series of simple

craters, complex craters, and ringed basins that were formed by

impacts.

Each crater and basin is the source of primary ejecta and

secondary craters that, collectively, cover the entire terra. The largest

impacts thinned, weakened, and redistributed feldspathic terracrustal

material averaging about 75 km in thickness. Relatively small

volumes of basalt, generated by partial remelting of mantle material,

were erupted through the thin subbasin and subcrater crust to form

the maria that cover 16 percent of the lunar surface. Tectonism has

modified the various stratigraphic deposits relatively little; most

structures are confined to basins and large craters.

This general geologic style, basically simple though complex in

detail, has persisted longer than 4 aeons (1 aeon = lo9 yr). Impacts

began to leave a visible record about 4.2 aeons ago, after the crust and

mantle had differentiated and the crust had solidified. At least 30

basins and 100 times that many craters larger than 30 km in diameter

were formed before a massive impact created the Nectaris basin about

3.92 aeons ago. Impacts continued during the ensuing Nectarian

Period at a lesser rate, whereas volcanism left more traces than

during pre-Nectarian time.

The latest basin-forming impacts created the giant and still-conspicuous Imbrium and Orientale basins during the Early Imbrian Epoch, between 3.85 and 3.80 aeons ago. The rate of crater-forming impacts continued to decline during the Imbrian Period. Beginning in the Late Imbrian Epoch, mare-basalt flows

remained exposed because they were no longer obscured by many

large impacts. The Eratosthenian Period (3.2-1.1 aeons ago) and the

Copernican Period (1.1 aeons ago to present) were times of lesser

volcanism and a still lower, probably constant impact rate. Copernican

impacts created craters whose surfaces have remained brighter

and topographically crisper than those of the more ancient lunar

features.

Fonte: Arizona State University - Space Exploration Resource

Adaptação: Avani Soares