Trafficability and excavatability of icy lunar regolith simulants quantified using cone penetration

Abstract

This article outlines the experimental equipment and techniques used to quantify how ice content and microstructure influence the geotechnical behavior of icy lunar regolith simulants, using cone penetration tests. It addresses two primary research questions: (1) What is the relationship between ice microstructure and geotechnical behavior for icy lunar regolith simulants, and (2) How can this knowledge improve excavation and trafficability on the Moon. Unsintered, pressure sintered, ice-cemented, and vapor deposited icy regolith simulants were systematically studied at ice contents from 0.1 to 30 wt.% and bulk densities between 1.5 and 1.8 g cm\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^{-3}$$\end{document}. Sample and probe temperatures were maintained at \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$-180 \pm 5\,^{\circ }\text {C}$$\end{document} (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$93 \pm 5\text { K}$$\end{document}) and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$-190\,^{\circ }\text {C}$$\end{document} to \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$-110\,^{\circ }\text {C}$$\end{document} (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$83\text { K}$$\end{document} to \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$163\text { K}$$\end{document}), respectively. Trafficability was estimated using the cone index gradient (G, \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\text {kPa}\,\text {mm}^{-1}$$\end{document}). After correcting for lunar gravity (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$G_{\text {Lunar}}$$\end{document}) and experimental penetration depths to 30 mm, a vapor deposited sample with 6.88 wt.% ice was found to be 3.92 times stronger than a pressure sintered sample containing 8 wt.% ice, and 1.23 times stronger than the upper bound of in-situG values measured in the Descartes Highlands during Apollo 16. The energy expenditure per unit mass of excavated water (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\eta _{\text {exc}}$$\end{document}, J/g) decreased with increasing ice content for pressure sintered samples, falling from 0.35 J/g at 1 wt.% to 0.11 J/g at 30 wt.% (a 3.2-fold decrease). In contrast, \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\eta _{\text {exc}}$$\end{document} increased in ice-cemented samples from 2.74 J/g at 1 wt.% to 143.03 J/g at 12 wt.% (a 52-fold increase). In other words, pressure sintered samples became 3.2 times more energy efficient to excavate, while ice-cemented samples became 52 times less efficient. This work provides the first quantitative geotechnical measurements of vapor deposited icy simulants and demonstrates that ice-cemented simulants can exhibit strength several orders of magnitude higher than other morphologies, even under identical conditions. Consequently, the excavation of water-ice within PSRs may be more economically viable, while also offering improved trafficability compared to dry regolith. Consequently, icy lunar regolith may be easier to excavate and traverse than previously thought. These results also suggest that studies of ice-cemented simulants may overestimate excavation resistance and trafficability if lunar ice instead occurs predominantly in granular, sintered, or vapor deposited morphologies. Given the widespread use of ice-cemented simulants, rocks and concrete analogues, this also raises questions on their continued use as general-purpose proxies for all possible icy regolith morphologies.