Scale: 1 pixel = approximately 4.7 km. 10-degree grid; Lambert Azimuthal Equal Area Projection, Center 00NS 090E
|Copernican: Beginning with the formation of the
bright crater Copernicus, about 1.1 Ga. Formation of bright ray
Eratosthenian: Begins with the formation of the crater Eratosthenes. Fresh but non-rayed craters and late mare eruptions. 1.1 to 3.1 Ga.
Imbrian: Begins with the formation of the Imbrium Basin. Includes most mare eruptions. 3.1 to 3.8 Ga.
Nectarian: Begins with the formation of the Nectaris Basin, and includes most of the major impact basin events. 3.8 to 3.9 Ga.
pre-Nectarian: Before 3.9 Ga
This map clearly shows the asymmetry of maria distribution on the Moon. The densely spaced and large near side maria are on the left and the mare-poor far side on the right. Note that maria like Mare Smythii and Mare Australe taht are on the extreme edge of the Moon as seen from Earth are much less extensive and patchier than maria in the center of the near side. Some far side craters like Hertzsprung are plenty large enough to have major maria but are completely empty of lava plains.
The asymmetry appears to be due to the near side having thinner crust than the far side. Dense lava flows filling the near side basins actually make the near side more massive, and tidal friction locked the moon with the dense side facing earth. Why one side was more heavily impacted than the other and what connection that has with the difference in crustal thickness are unknown.
Created 11 April 2014, Last Update
12 March 2015
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