CONTRIBUTIONS OF IMPACT MIXING TO THE SPATIAL DISTRIBUTION OF WATER ICE IN PERMANENTLY SHADED LUNAR SOUTH POLAR CRATERS

Water ice is present in permanently shaded regions (PSRs) in the lunar south pole. Its distribution may be controlled by mixing of materials and heat transfer, and then is transported and cold-trapped in PSRs; however, these explanations may be incomplete because meteorite impacts may gradually mix material in the lunar surface. We analyze the spatial distribution of mixing due to impact cratering events and give interpretations of water ice deposit thickness. By obtaining the crater distributions on the permanently shadowed floors of complex craters around the lunar south pole, we constrain the intensity of regolith mixing and thus the distribution of water ice.

We investigated regolith mixing depths around the lunar south pole by applying crater distributions and a statistical model. Craters were counted in permanently shaded floors of Haworth, Shoemaker and Faustini. Crater size frequency distributions (CSFDs) were obtained at multiple grid sizes. We first mapped the spatial distribution by calculating the number of craters in a moving neighborhood of 2.5, 5.0 and 7.5 km. While we confirmed that the cumulative number of counted craters at a global scale was consistent with previous works, our results showed spatial heterogeneity of the crater distribution.

By looking at the rate of crater emplacements, we computed the spatial distribution of the mixing at which a regolith layer was disturbed once. At the crater floor scale, the mixing depth varies between each target crater and ranges from ~0.8 m to ~3 m at the 99.99% mixing level. The results show vertical and lateral heterogeneity of material mixing ranging from ~7 cm to ~13 m (upper limit) and ~4 cm to ~7 m (lower limit). The highest material mixing depth values are in areas that mostly contain larger craters compared to smaller ones. Grids that mostly contain small crater populations show shallower material mixing depths. The mixing process strongly depends on the crater distribution, suggesting depths of ~4 cm to ~13 m and becoming deeper in the presence of larger craters. Our model implies that impact-driven mixing processes lower the water ice density on a surface. Such low-density anomalies infer recent impact mixing processes. If this is the case, the time evolution of water ice in these regions would rather be short, leading to active variations in its distribution.