Are pit craters habitable? Geological analysis and description of their structural potential as lunar bases.

Introduction:

Subsidence or collapse craters, also called pit craters, can be found on the Moon and throughout the Solar System, including our planet. These depressions in the terrain are not formed by the impact of any projectile but rather by the collapse of the soil over a void, usually creating circular-shaped hollows. There are several formations causes, such as the collapse of the roof of underground lava tubes or dikes [1]; collapsed magmatic chambers under loose material; tectonic movements produced by faults; or caused by the presence of water or ice in the subsurface [e.g., 2]. 

Similar to how lava tube pits are formed on the Earth, on the Moon, these features could be the entrance to underground caves formed when lava ceilings are not resistant enough to support their weight and collapse [3, 4]. Some of them could even be interconnected if their origin is the same volcanic tube through which lava flowed in the past. Consequently, planetary pit craters and potential subsurface caves are promising astrobiological regions due to their properties to preserve their own microclimate [5], the shielding they offer against radiation (below or at 6 meters depth) [6] and the protection they offer against harsh surface conditions. There is also direct evidence found of surface exposed water ice in the lunar polar regions [7] and in simple craters [8], so they could have water ice reservoirs inside or near them. Caves have been described as possible first human settlements on the Moon and Mars, offering a permanent and safe refuge for astronauts and equipment storage [9]. On top of that, the protection that is providing this natural shelter offers an additional interest: an intact lava tube in pristine conditions gives us a lot of useful information to better understand the geological history of the Moon.

The aim of this study is to define a list of the best pit craters, suitable for establishing an exploration lunar base, so that future missions to the Moon can benefit from this information.

Methodology:

We analyze a catalogue of 278 pit craters from the atlas obtained thanks to NASA's Lunar Reconnaissance Orbiter mission to detail their possible habitability, giving priority to the potential lava tube candidates found [10, 11], and considering the following points of view: morphological characteristics; the proximity of each other’s and its possible internal connections; associations with various geological structures (such as lobate scarps, wrinkle ridges, maria or impact craters); their possible origin; geological materials and their proximity to areas abundant in resources ISRU (water ice, REE or other materials); their proximity to areas more suitable for landing on the Moon; proximity to current and future human and robotic missions.

We mapped and located the best candidates by overlaying current pit crater locations from the LROC atlas to different global and regional lunar datasets such as the global geologic map of the Moon (USGS, [12]), in order to obtain the information on the different regions on the moon and their properties (material type and age); the Kaguya (SELENE) mission maps, covering latitudes from -60° to 60°; and the north and south pole LOLA LRO lunar maps, that provide the information on the elevation profile. A wrinkle ridges layer is also added to find some clues about how the pit craters in an area might be connected and how large the subsurface caves would be.

To analyze each of the pit craters a python library is developed, extending the capabilities of the python QGIS modules and its main objective is to relate the pit craters with their origin, according to the region where they are located, their morphological properties and their relative location to nearby pit craters or resources as water ice or regolith. 

Summary and future work:

Python library will have as a primary output an ordered list of the best candidates for pit craters to establish a human lunar base. Further on, our observations and individual selection of the most potentially suitable pits can be analyzed in depth using the gravitational field of the Moon or by checking LRS radar data (SELENE mission).

Acknowledgement:

LMP acknowledges support from the Margarita Salas UCM postdoctoral grants funded by the Spanish Ministry of Universities with European Union funds - NextGenerationEU.

References:

[1]: Favre, G. (1993), Proceedings of the 3rd International Symposium on Vulcanospeleology, 37–41; [2] Boston, P. J. (2004), Encyclopedia of Caves and Karst Science, 355–358; [3]: Hong, I-S., et. al. (2014), J. Astron. Space Sci. 31(2), 131-140; [4]: Okubo, C. H., and Martel, S.J., (1998). J. of Volcanology and Geothermal Research 86, 1–18; [5]: Phillips-Lander, C., et al. (2020) MACIE Decadal Survey white paper; [6]: De Angelis G, et al. (2002). LPSCXXXIII; [7]: Li, S., et al. (2018). PNAS, 115 (36) 8907-8912; [8]: Rubanenko, L., et al. (2019) Nat. Geosci. 12, 597-601; [9] Titus, T.N., et al. (2021), Nat Astron 5, 524-525; [10]: Coombs C.R., et al. (1992) 2nd Lunar Bases and Space Activities of the 21st Century, 1, 219-229; [11] Wagner, R.V. and Robinson, M. S. (2021) 51th LPSC, #2530; [12]: Fortezzo, C.M., et al. (2020) 51st LPSC, #2760