Conjectures about the evolution of the moon

Abstract

The principal questions about the derivation of the lunar surface have not yet been settled: is it the surface left over from the process of accumulation of the Moon, or is it a surface generated by magmatic processes on the Moon and subsequently altered by further infall from outside? The evidence derived from many sources now favors the former. Seismic data suggest an absence of bedrock down to a depth of several kilometers, and instead a compacted powder only. The ‘mascon’ evidence can be understood as a consequence of major impacts in a deep porous layer. The great abundance of cosmic ray tracks in most soil samples demands a much greater cosmic ray dosage than present rates would cause in the age of the Moon, unless the dust represented infallen material previously irradiated. The nuclear age, since freezing, of the dust is greater than that of the rocks found. The chemical composition of the dust is not the same as of the rocks. Strict layering of the dust has been seen, implying some process other than meteoritic impacts for its generation and deposition. These and other effects found can be understood in the framework of a cold accumulation description, in which the surface layers represent the last addition of meteoritic infall of a basaltic material similar to, but not identical with the present basaltic achondrites. The possible relation of this material to oceanic basalt on Earth is mentioned.