Development of the mare regolith: Some model considerations

Abstract

Mare regolith is fragmental debris of variable thickness that lies upon fractured bedrock. Its origin by impact comminution of primarily local basaltic rocks is widely accepted, but the consequences of such an origin are not appreciated fully. This investigation uses results obtained in an earlier Monte Carlo study by Oberbecket al. (1973) to shed light on those consequences by evaluating regolith growth and mixing as a function of time. Results reported are for average cases and must be used with caution. Each small area of the lunar surface has experienced a unique history and results based on averages may have no application to specific cases. Consideration of average processes is useful, nevertheless, when this limitation is kept in mind. The study demonstrates that regolith growth is self regulated and has the same trend and nearly the same terminal growth rates whatever the history of bombardment: rapid initial accumulation followed by diminishing rates of growth. Mixing and all other processes investigated are growth regulated. Mixing increases as growth slows, but never to the extent that the regolith is homogenized. Because the average regolith is never homogenized, products of growth regulated processes are preserved in the stratigraphy. Differences in material properties are to be expected in vertical sections of the regolith, therefore, but this model is not sufficiently refined to permit prediction of all possible trends. It does indicate, however, that deeper levels contain thinner depositional units, lesser quantities of meteoritic and exotic components, and more debris derived from shallow levels in the mare basalts than material in near surface layers. Additionally, neutron fluence production is regulated by the growth process, but because rates of growth do not differ much over the last aeon, whatever the total age or early bombardment history, values of surface fluence may be similar in many areas whatever their age.