Patterns of soil maturation and mixing at the Apollo 16 landing site: surface soils
Ray, James R.
Master of Science
An analysis of the Apollo 16 surface soil samples is presented in an effort to distinguish the separate effects due to mixing of unlike fines from those attributable purely to regolithic maturation processes. In establishing a background needed to accomplish this task, aspects of large-scale stratigraphy and geologic sequence in the central highlands are reviewed. Likewise, a comprehensive discussion of the interrelated phenomena associated with a soil's residence in the upper millimeter of the regolith follows. Data for major element chemistry and trapped inert gases are assembled from the literature and used to infer the provenance of archetypical soils and kinship relationships of remaining surface samples. The hypothesis of maturation domination of inert gas variations is considered and its consequences are delineated. It is found that this assumption, while adequate in explaining the gas properties of most soils, fails to account for about one-fifth of the samples, particularly soil 61221. An alternative approach is then presented which views the distinctive soils as remnants of ancient solar wind irradiation. In this context, unambiguous maturation effects are discernible only for the evolution of absolute gas concentrations and the elemental ratio 2^Ne/36^Ar. The unique soil 61221 is found to have excess argon relative to its very low state of maturity by a factor of about ten after correction for outgassing. This anomaly is ascribed to an irradiation between 3.5 and 4. Gy ago by a solar wind with particle flux greater than present by the same factor. A similar gas-rich component occurs in less pure form in other surface samples.