The formation of multi-ring basins dominated the early geological evolution of the Moon. The five basins described in the preceding chapters represent a spectrum of basin ages, sizes and morphologies. By comparing the similarities and differences among these basins, some general inferences may be made regarding the process of formation of multi-ring basins on the Moon. I here synthesize the information described in the previous chapters to develop a model for the formation and geological evolution of multi-ring basins on the Moon. This model is incomplete, but several puzzling aspects of basin geology can be explained satisfactorily through this approach. At various points in the following discussion, please refer to preceding sections in the text.

Composition and structure of the lunar crust

The crust of the Moon is heterogeneous on a local and a regional scale; the impact targets for lunar multi-ring basins were similarly heterogeneous. The crustal thickness at the basin target sites was widely varied, ranging from 50 km thick for parts of the Imbrium basin to over 120 km thick for the Orientale highlands (Bills and Ferrari, 1976). Moreover, lithospheric conditions during the era of basin-forming impacts changed with time in response to rapidly changing thermal conditions within the Moon 4 Ga ago (Hubbard and Minear, 1975; Solomon and Head, 1980). The older basins formed in a relatively thin, easily penetrated lithosphere that gave rise to extensive post-impact modification.