Age of Lunar Polar Volatile Deposits Based on Modeling of Distribution and Heterogeneity

Permanently shadowed regions (PSRs) of the Moon host volatiles buried amongst the regolith. In the coldest regions of the Moon, temperatures are cold enough that water ice is stable against sublimation for on the order of a billion years, and immobile to thermal diffusion. In these regions, impact gardening is expected to be the primary process affecting the distribution of water ice. Although measurements have confirmed the presence of ice on the Moon, comparison of data from neutrons, IR, UV, and radar indicates there is clearly heterogeneity in the distribution of volatiles laterally, with depth, and from one PSR to another.

This 3-D Monte Carlo model statistically analyzes the effects of impact gardening on the heterogeneity of water in the lunar polar regions with the goal of linking spatial scales of heterogeneity to clues about the local history of volatiles. The model simulates the evolving topography and ice distribution in lunar regolith from impacts. Summing over a large set of runs, the model produces an expected average time evolution of ice retention, lateral distribution, and depth distribution. The results can be presented as various instrumentation would observe them to compare to existing data.

For an ice layer starting out 10 cm thick, the existing radar and neutron data are consistent with the ice being <1 Gyr old. For thicker initial ice layers, one would expect a stronger radar signature than is apparent now. For a thinner initial ice layer, modeling results suggest a much younger feature, which would be accompanied by a greater expression of exposed frost on the surface than is presently observed.

The model is then applied to predict the prevalence of accessible volatiles by drilling or other means of subsurface access as a function of the depth to which the drill can reach. This can be used for trade studies on the drill depth versus the number of drill sites for future mission planning or for in situ mining of water ice operations.

The model demonstrates the erosion of an ice layer by impacts as time progresses. The time soonest after emplacement is the most critical for ice retention. In some locations, the ice layer becomes protected from erosion by ejecta blankets emplaced over the layer. Impacts produce a heterogeneous distribution of ice on scales relevant for in situ sampling of ice deposits.