Spatial Distribution of Water Ice Thickness Implied by Regolith Mixing in Permanently Shaded Regions in the Lunar South Pole

Abstract:

Water ice is present in permanently shaded regions (PSRs) in the lunar south pole. Its distribution may be controlled by mixing of materials, heat transfer, and 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 [1]. 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 [2, 3], 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. The 90% area fraction of the regolith mixing varies spatially by grid-size, ranging from ~1 m to ~64 m. Our interpretation is consistent with Cannon and Britt [4], who showed that water ice may be spatially distributed at meters to hectare scales. In our analyses, high values of mixing depths in some regions are due to large craters. Such process can mix surface water ice with regolith throughout the impact history of craters that host water ice and be preserved at greater depths. Because water ice is being mixed by impact gardening, impact heating processes may sublimate water ice and other volatiles present in the lunar regolith. However, they may be reaccumulated on new cratered topography, both developing lunar regolith and possibly thin water ice layers.

Ref.

[1] Hirabayashi et al (2018) JGRP 123 527-543. [2] Tye et al (2015) Icarus 225, 70-77. [3] Deutsch et al (2020) Icarus 336 113455. [4] Cannon and Britt (2020) Icarus 374 113778.