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Background. Lava tubes are caves that form when the surface of a lava flow cools and solidifies, while molten rock underneath continues to flow and eventually drains away: a tube-shaped underground cave is then created. Lava tubes are known to exist on the Moon and Mars where they may form in either volcanic or large impact crater settings. Motivation. Lava tubes on the Moon and Mars are of interest for space settlement because caves have been proposed as natural shelters that future human explorers could occupy. Caves would in principle protect dwellers from surface radiation, wide temperature swings, micrometeorite impacts, and rocket exhaust blast. Some lava tubes on the Moon and Mars might also be cold enough to trap water ice, which, if water ice is actually present, might offer access to an important resource, and in the case of Mars, also to potential underground habitats for microbial life. Goal and Objectives. The main goal of our study was to begin to learn more about the physical and geological reality of lava tubes on Earth, specifically with respect to their habitability. Our objectives focus on two issues of central relevance to assessing the potential habitability of lava tubes on Earth, and by extension on the Moon and Mars: 1) Geotechnical characteristics: How accessible and safe are lava tubes? What makes some lava tubes experience collapse? 2) Relation to ice: Why, when and how does ice occur and evolve in lava tubes? Approach and Findings. 1) To begin surveying the geotechnical characteristics of lava tubes, we carried out a review of the published technical literature on the topic. Our finding is that while there is an abundance of archaeological and historical reports and maps documenting the access, exploration, investigation, use, and occupation of lava tubes, including many anecdotal mentions of collapse features inside lava tubes, there are essentially no published quantitative studies of the geotechnical properties of lava tubes using modern standard geotechnical metrics. 2) To begin examining the relationship between lava tubes and ice, we carried out a field investigation of the Lofthellir Lava Tube Ice Cave in Iceland in which massive ice is known to exist. Our field observations suggest that the ice inside Lofthellir is predominantly meteoric in origin (water from atmospheric precipitation or condensation) rather than from volcanic venting. In other words, the water inside the cave came mostly, if not exclusively, from “above”, not “below”. We also report two important new findings: a) Underground micro-glaciers are recognized for the first time; they form in the lave tube as a result of accumulation, slow movement, and loss of water and massive ice under the effect of gravity; b) Gelifraction, the fracturing of rocks by freezing of water trapped in joints, is an important cause of breakup and collapse inside this ice-rich lava tube. Conclusions. 1) Our literature survey indicates that we still have little quantitative knowledge about the geotechnical properties of lava tubes on Earth, and therefore limited understanding of the factors that influence their evolution on our planet, let alone on the Moon or Mars; 2) Our field study of an ice-rich lava tube in Iceland suggests that, if ice were able to accumulate inside lava tubes on the Moon or Mars - regardless of its origin -, a) gravity might produce underground micro-glaciers, while b) gelifraction might increase the risks of rock fall and collapse (compared to ice-free caves). Next Steps. The following is recommended: A) Carry out detailed geotechnical assays of a series of lava tubes on Earth that captures their diversity in composition, size, shape, age, and weathering history, to understand better the key factors that might control the evolution of lava tubes on the Moon and Mars; B) Further investigate Lofthellir and other ice-rich lava tubes on Earth to understand better if and how ice might occur and evolve inside lava tubes on the Moon and Mars; C) Identify, design, and test robotic systems and astronaut hardware that will enable safe and productive exploration of lava tubes on the Moon and Mars. Additional Accomplishments: Our project also enabled, and benefitted from, other research and outreach activities that collectively help advance the human exploration and settlement of space: i) By combining resources from other research projects funded by NASA and other partners, we carried out at Lofthellir the first successful flight of, and 3D-mapping by, a lidar-equipped drone inside a lava tube and icerich cave; ii) our fieldwork at Lofthellir was filmed and will be featured in an upcoming episode of What on Earth? (Season 6), the award-winning TV documentary series and most-watched program on the US Science Channel; iii) Results of our research will be presented at the upcoming 50th Lunar and Planetary Science Conference in Houston, TX (18-22 March 2019) (Lee et al. 2019)
Lava tubes, Moon, Mars (Planet)
Lee, Pascal, "Habitability of lava tubes on the Moon and Mars: lessons from Earth" (2019). KIP Articles. 5844.