Exploring subsurface extent and physical properties of lunar lava tubes using surface microgravity survey

Lunar lava tubes are of significant interest for potential human exploration of the Moon, therefore it is important to study their subsurface extent and physical properties in detail. The underground existence of the lava tubes in the mare regions due to past volcanic activity has been predicted in the past (e.g. 1,2) and orbiting missions for gravity surveys (GRAIL) and radar investigations (Lunar Radar Sounder onboard SELENE) have identified candidate locations of these subsurface structures (e.g. 3-5). But only global constraints could be inferred, because the spatial resolution achievable from orbit is not sufficient compared to the estimated extents of the lava caves. To better understand the real extent and depth of the lava tubes, measurements on the lunar surface have to be performed. In this study we propose surface microgravity survey techniques to measure gravitational anomalies. A gravimeter experiment on a lunar rover would map out the spatial variation of the surface gravity and constrain subsurface voids shown as mass deficits accurately when traversing a lava tube. Microgravity survey yield very high spatially resolved gravity mapping. Microgravity surveys is a geophysical mapping technique commonly used on Erath to aid the location of buried features such as faults, sinkholes, tunnels and voids associated with mines, quarries as well as lava tubes 6. The Traverse Gravimeter Experiment (TGE) on the Apollo 17 Lunar mission, allowed to determine presence and properties of a higher density lava flow 7,8. Microgravimeter measurements can be made across linear profiles and/or equally spaced grids at survey stations in varying increments dependent on the depth of investigation.Here, we will present the lunar microgravimetric survey concept to explore subsurface extent and physical properties of lunar lava caves. For different scenarios consisting of different depth size etc. of subsurface lava tubes, the gravitational anomalies will be calculated and the traverse gravity survey will be simulated 9. The measurements concepts and instrument requirements will be presented. A possible application of this study is the robotic explorer concept LunarLeaper 10, recently selected from the ESA 2023 Small Missions for Exploration call for a Pre-Phase A study.These measurements are ideally supported by ground penetrating radar (GPR) measurements, a technique commonly used on Earth to map shallow subsurface structures using electromagnetic reflection waves. GPR has been successfully applied to various problems in the geological, archaeological, and engineering fields as well as on mapping of lava tubes 11-14. This combined, non-intrusive method, can help to characterize the physical properties of the lava tubes, as for example their stability and the composition of the surrounding material. These are important considerations for future human exploration of the Moon.References: 1 Murase and McBirney, 1970, Science 167, 1491. 2 Greeley, 1971, The moon, 3, doi:10.1007/BF00561842. 3 Chappaz et al., 2017, Geophysical Research Letters, 44, doi :10.1002/2016GL071588. 4 Kaku et al., 2017, Geophysical Research Letters, 44, doi.org/10.1002/2017GL074998. 5 Zhu et al., 2024, Icarus, doi:10.1016/j.icarus.2023.115814. 6 Deroussi et al., 2009, Journal of Volcanology and Geothermal Research, 184, doi:10.1016/j.jvolgeores.2008.10.002. 7 Talwani and Kahle, 1976, NASA STIRecon Tech. Rep. A, vol. 77. 8 Urbancic et al., 2017, Journal of Geophysical Research: Planets, 122, doi:10.1002/2017JE005296. 9 Noeker and Karatekin, 2022, EPSC2022-361, Europlanet Science Congress. 10 Mittelholz et al., 2024, EGU General Assembly, doi:10.5194/egusphere-egu24-21578 11 Esmaeili et al., 2020, Journal of Geophysical Research: Planets, 125, doi:10.1029/2019JE006138. 12 Miyamoto et al., 2005, Geophysical Research Letters, 32, doi:10.1029/2005GL024159. 13 Rowell et al., 2010 CREWES Research Report, 22. 14 Gmez-Ortiz, 2014, Journal of Applied Geophysics, 109, doi:10.1016/j.jappgeo.2014.07.009.