Lunar megaregolith mixing by impacts: Evaluation of the non-mare component of mare soils

Abstract

The source of the non-mare component in the lunar mare soil samples is still an open question. A spatially-resolved numerical model tracing the diffusion of non-mare material was developed by means of the Monte Carlo method to study this issue in more detail. The vertically- and laterally-transported non-mare components are recorded separately to assess their concentration on the surface. We find a general higher efficiency of lateral transport than of vertical transport, but the opposite may occur within small zones of interest. The overall (background) distribution of non-mare component is estimated by averaging out regional variations of material composition caused by local impact mixing. We find that almost all the mare surface is mixed with non-mare material. If most of the mare regions have filled basaltic material about 500 m in thickness since the formation of basins, the average non-mare fraction in the top 5 m is about 0.1. The abundance of the non-mare component decreases with increasing distance from the mare-highland boundary, but the slope of the distance-dependence is shallower within ~100 km of the boundary than further away. By comparing the background composition derived from our model with the geochemical analysis and geological interpretation of the lunar mare soil samples, we infer the most plausible geologic processes that have significantly altered the material composition at the sampling sites: for the Apollo 15 and 17 mare soil samples, the large fraction of non-mare material is likely to have resulted from the downslope slumping or lateral transport of the nearby massifs. The Apollo 12 sampling site that is located interior domains of Oceanus Procellarum has a component of Copernicus ejecta. A mixing of both Copernicus ejecta and excavated local underlying material by high-velocity ejecta has altered the composition at the surface. The non-mare material contained in the Apollo 11 and Luna 24 mare soil samples could have been built up gradually by both long-time lateral and vertical mixing. The mare deposit at the Luna 16 landing site is likely to be relatively thin resulting in the abundant vertically transported non-mare component. Since the history of the lunar volcanic eruptions - deposit thicknesses, flux curves and onset of activity - has not been well constrained, we make simple estimates with the first-order accuracy. In addition, it was found that the vertically transported non-mare abundance in the top surface is influenced by both the cessation time of the major mare fillings and the total amount of mare deposit; the laterally-transported non-mare abundance in the top surface is mainly dependent on the cessation time of the major mare fillings. The peak time of eruption would not change the abundance of both the laterally and vertically transported non-mare component.