Radar Evidence of an Accessible Cave Conduit on the Moon below the Mare Tranquillitatis Pit

Radar Evidence of an Accessible Cave Conduit on the Moon below the Mare Tranquillitatis Pit. R. Pozzobon1,2,3 ([email protected]), L. Carrer4 ([email protected]), F. Sauro1,5, D. Castelletti6, G.W. Patterson7 & L. Bruzzone4 1) Department of Geosciences, University of Padova, Padua, Italy 2) Department of Physics and Astronomy, University of Padova, Padua, Italy 3) Centro di Ateneo di Studi ed Attività Spaziali 'G. Colombo', University of Padova, Padua, Italy 4) University of Trento, Trento, Italy 5) La Venta Geographic Exploration APS, Treviso, Italy 6) Capella Space Corporation, San Francisco, CA, USA 7) Johns Hopkins University Applied Physics Laboratory, Laurel, MD, USA Introduction: The presence of subsurface conduits and cave systems on the Moon has been a subject of speculation and debate for over 50 years [1,2]. While the existence of such features has been theorized based on observations of collapsed lava tubes and other geological evidence, direct confirmation of accessible underground voids has remained elusive. Several potential openings, known as lunar pits, have been identified on the lunar surface [3,4], but whether these pits lead to extensive underground cave systems and/or drained lava tubes is uncertain. Exploring and confirming the presence of such caves could provide vital insights into the geological evolution of the Moon, shedding light on the emplacement of lunar maria and the processes that shaped the lunar surface [5]. Additionally, these subsurface environments could offer stable temperatures and protection from cosmic radiation [6], making them ideal candidates for establishing future human habitats on the Moon and supporting long-term exploration efforts. Methodology: In this study, we analyzed radar data from the miniature radio-frequency (Mini-RF) instrument [7,8] onboard the Lunar Reconnaissance Orbiter (LRO) to investigate the Mare Tranquillitatis pit (MTP), one of the deepest known lunar pits [9]. The MTP is an elliptical skylight with vertical or overhanging walls and a sloping pit floor that appears to extend further underground, making it a promising candidate for accessing potential subsurface cave systems. Previous attempts to identify cave conduits near lunar pits using nadir-looking orbital groundpenetrating radar [10,11], gravimeters [12], and radiometers [13] have provided mixed results. Our approach leverages the unique capabilities of sidelooking orbital synthetic aperture radar (SAR) imaging systems to detect and characterize potential cave conduits in a way that has proven successful in terrestrial environments [14]. Results: Our analysis of the Mini-RF synthetic aperture radar (SAR) image of the MTP revealed an anomalous increase in radar echo power originating beyond the west side of the pit. Through detailed 3D radar simulations [14] and comparisons with the observed data, we determined that these anomalous reflections are consistent with the presence of a subsurface cave conduit. The simulations allowed us to estimate the geometric parameters of the hypothesized cave conduit and test various scenarios to find the best-fitting models. Figure 1: a, b) The Mini-RF SAR image (a) and its mchi polarimetric decomposition (b) reveal that the radar echoes from the MTP overhang exhibit singlebounce scattering, while the hypothesized cave conduit echoes show double-bounce scattering. This scattering behavior is consistent with the expected radar backscattering model for subsurface conduits. c, d) The Digital Terrain Model (DTM) from stereo observations (c) is limited to the visible pit walls in LRO images. A 3D radar simulation (d) based on this DTM cannot fully explain the anomalous radar echoes, suggesting the presence of an additional subsurface feature like a cave conduit extending beyond the visible pit floor. The red dashed circle in (d) delineates the pit edge, and the white arrow indicates the radar look direction. Two best-fitting models were identified: Model A represents a conduit with a floor slope of about 3°±2.5°, a maximum depth of 135 m from the surface, and an extension of around 25 m. Model B depicts a conduit with a roof slope of 55°±5°, a floor slope of 45°±5°, a maximum depth of 175 m, and an extension of 77 m. The estimated width of the conduit for both models is similar at 45 ± 7.5 m, although it is important to note that the Mini-RF data has limitations in measuring conduit widths larger than 55 m due to the specific acquisition geometry and conduit geometry [14]. All the simulations with conduits wider than 55 m retured positive results, making this measure a minimum envelope of size. The radar data also provided insights into the scattering properties of the conduit through polarimetric analysis. The m-chi polarimetric decomposition revealed that the reflections from the hypothesized conduit exhibit singleand doublebounce scattering, which is compatible with the expected scattering mechanism from subsurface conduits observed in terrestrial analogues [14]. Furthermore, our simulations tested alternative hypotheses, such as the presence of a closed magmatic chamber or a void due to tectonic extension [9], but these scenarios did not match the observed radar data. The simulations strongly support the interpretation of the anomalous reflections originating from an accessible cave conduit extending from the MTP. Notably, the best-fitting models align with the hypothesis of a lava tube roof collapse origin for the MTP, where the material collapsed from the roof during the pit's formation would have formed a cone of detritus corresponding to the observable pit floor, with the conduit extending further underground [9]. Conclusions: Our study provides the first direct radar evidence for the presence of an accessible subsurface cave conduit below the Mare Tranquillitatis pit on the lunar surface. This discovery suggests that the MTP could offer access to an extensive lunar cave system, making it a promising target for future robotic and human exploration missions. Exploring such caves could yield unprecedented insights into the emplacement of lunar maria and the evolution of lunar volcanism by providing access to samples of superposed lava flows with different ages [15]. Additionally, these subsurface environments could offer stable temperatures and protection from cosmic radiation [6], making them ideal candidates for establishing future human habitats on the Moon and supporting long-term exploration efforts. While the current resolution of the Mini-RF data limits our ability to fully characterize the conduit's dimensions and extent, our work demonstrates the viability of using orbital synthetic aperture radar (SAR) imaging for detecting and assessing the accessibility of lunar conduits extending from surface pits [14]. The presented methodology could be substantially expanded if radar orbital sensors with higher resolution, capable of resolving the interior of all known lunar pits [3], are deployed in lunar orbit. A comprehensive survey of all identified lunar pits using this approach could reveal the most promising access points for subsurface exploration and habitation, providing unprecedented information on the potential for installing human lunar bases in environments protected from the harsh surface conditions [6]. The discovery of an accessible lunar cave conduit below the MTP represents a significant step forward in our understanding of the Moon's subsurface environment and highlights the potential for future exploration missions. As we continue to explore the Moon and prepare for future human missions, the identification and characterization of such subsurface features will be crucial in enabling safe and sustainable operations, while also advancing our scientific knowledge of the lunar geological history [16]. Acknowledgements: We thank the Topical Team of Planetary Caves of the European Space Agency. Capella Space X-band SAR imagery was provided by Capella Space under the Open Data Community programme. This work was supported by the Italian Space Agency (Contract No. 2022-23-HH.0, ‘Attività scientifiche per il radar sounder di EnVision fase B1’). References: [1] Greeley, R. Moon 3, 289–314 (1971). [2] Halliday & William, R. Bull. Natl Speleol. Soc. 28, 167–170 (1966). [3] Wagner, R.V. & Robinson, M.S. Icarus 237, 52-60 (2014). [4] Haruyama, J. et al. Geophys. Res. Lett. 36, L21206 (2009). [5] Head, J.W. III. Rev. Geophys. 14, 265-300 (1976). [6] Horz, F. in Lunar Bases and Space Activities of the 21st Century (Mendell, W.W., ed.) 405-412 (1985). [7] Nozette, S. et al. Space Sci. Rev. 150, 285-302 (2010). [8] Raney, R.K. et al. Proc. IEEE 99, 808-823 (2010). [9] Wagner, R.V. & Robinson, M.S. J. Geophys. Res.: Planets 127, e2022JE007328 (2022). [10] Kaku, T. et al. Geophys. Res. Lett. 44, 10155-10165 (2017). [11] Donini, E. et al. IEEE Trans. Geosci. Remote Sens. 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