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Advertisement Advertisement Nature Astronomy * View all journals * Search * Log in * Explore content * About the journal * Publish with us * Subscribe * Sign up for alerts * RSS feed 1. nature 2. nature astronomy 3. articles 4. article * Article * Published: 15 July 2024 Radar evidence of an accessible cave conduit on the Moon below the Mare Tranquillitatis pit * Leonardo Carrer ORCID: orcid.org/0000-0003-4599-7900^1, * Riccardo Pozzobon ORCID: orcid.org/0000-0001-9183-645X^2,3,4,5, * Francesco Sauro ORCID: orcid.org/0000-0002-1878-0362^5, * Davide Castelletti^6, * Gerald Wesley Patterson^7 & * ... * Lorenzo Bruzzone ORCID: orcid.org/0000-0002-6036-459X^1 Show authors Nature Astronomy (2024)Cite this article * Metrics details Subjects * Geomorphology * Rings and moons Abstract Several potential subsurface openings have been observed on the surface of the Moon. These lunar pits are interesting in terms of science and for potential future habitation. However, it remains uncertain whether such pits provide access to cave conduits with extensive underground volumes. Here we analyse radar images of the Mare Tranquillitatis pit (MTP), an elliptical skylight with vertical or overhanging walls and a sloping pit floor that seems to extend further underground. The images were obtained by the Mini-RF instrument onboard the Lunar Reconnaissance Orbiter in 2010. We find that a portion of the radar reflections originating from the MTP can be attributed to a subsurface cave conduit tens of metres long, suggesting that the MTP leads to an accessible cave conduit beneath the Moon's surface. This discovery suggests that the MTP is a promising site for a lunar base, as it offers shelter from the harsh surface environment and could support long-term human exploration of the Moon. Access through your institution Buy or subscribe This is a preview of subscription content, access via your institution Access options Access through your institution Access through your institution Change institution Buy or subscribe Access Nature and 54 other Nature Portfolio journals Get Nature+, our best-value online-access subscription $29.99 / 30 days cancel any time Learn more Subscribe to this journal Receive 12 digital issues and online access to articles $119.00 per year only $9.92 per issue Learn more Buy this article * Purchase on Springer Link * Instant access to full article PDF Buy now Prices may be subject to local taxes which are calculated during checkout Additional access options: * Log in * Learn about institutional subscriptions * Read our FAQs * Contact customer support Fig. 1: Experimental results for the MTP (8.3355deg N, 33.222deg E) imaging with Mini-RF. [41550_2024_2302_Fig1_HTML] Fig. 2: Results for MTP radar simulations (8.3355deg N, 33.222deg E). [41550_2024_2302_Fig2_HTML] Fig. 3: Reconstructed MTP cave conduit based on an inversion of the the Mini-RF radar data. [41550_2024_2302_Fig3_HTML] Data availability The Mini-RF data are available through NASA's Planetary Data System Geoscience Node (https://pds-geosciences.wustl.edu/). Wagner and Robinson's^17 internal morphology point cloud of the MTP is available at https://zenodo.org/records/6622042. The LROC NAC images and DTMs used in this study are publicly available through the Planetary Data System LROC Node at https://wms.lroc.asu.edu/. The data supporting this study are openly available at Zenodo via https://doi.org/10.5281 /zenodo.11005458 (ref. ^28). Code availability All the relevant analyses on the experimental data were performed with MATLAB. RaySAR is open source and available at https:// github.com/StefanJAuer/RaySAR. References 1. Greeley, R. Lava tubes and channels in the lunar Marius Hills. Moon 3, 289-314 (1971). Article ADS Google Scholar 2. 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III Mare Tranquillitatis: basalt emplacement history and relation to lunar samples. J. Geophys. Res.: Planets 101, 23213-23228 (1996). Article ADS Google Scholar 19. Wagner, R. V. & Robinson, M. S. Occurrence and origin of lunar pits: observations from a new catalog. In Proc. 52nd Lunar and Planetary Science Conference (Lunar and Planetary Institute, 2021). 20. Wynne, J. J. et al. Planetary caves: a Solar System view of processes and products. J. Geophys. Res.: Planets 127, e2022JE007303 (2022). Article ADS Google Scholar 21. Cushing, G. E. Candidate cave entrances on Mars. J. Cave Karst Stud. 74, 33-47 (2012). Article Google Scholar 22. Sharma, R. & Srivastava, N. Detection and classification of potential caves on the flank of Elysium Mons, Mars. Res. Astron. Astrophys. 22, 065008 (2022). Article ADS Google Scholar 23. Henriksen, M. R. et al. Extracting accurate and precise topography from LROC narrow angle camera stereo observations. Icarus 283, 122-137 (2017). Article ADS Google Scholar 24. Auer, S., Hinz, S. & Bamler, R. Ray-tracing simulation techniques for understanding high-resolution SAR images. IEEE Trans. Geosci. Remote Sens. 48, 1445-1456 (2009). Article ADS Google Scholar 25. Head, J. W. III. Lunar volcanism in space and time. Rev. Geophys. 14, 265-300 (1976). Article ADS Google Scholar 26. Titus, T. N. et al. A roadmap for planetary caves science and exploration. Nat. Astron. 5, 524-525 (2021). Article ADS Google Scholar 27. Blank, J. G. et al. Planetary Caves as Astrobiology Targets. White Paper (National Academy of Sciences, 2018). 28. Carrer, L. et al. Dataset of 'Radar evidence of an accessible cave conduit on the Moon below the Mare Tranquillitatis pit'. Zenodo https://doi.org/10.5281/zenodo.11005458 (2024). Download references Acknowledgements We would like to acknowledge all members of the Topical Team on Planetary Caves of the European Space Agency for the useful discussion on the interpretation of our findings. 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, 'Attivita scientifiche per il radar sounder di EnVision fase B1'). Author information Authors and Affiliations 1. University of Trento, Trento, Italy Leonardo Carrer & Lorenzo Bruzzone 2. Department of Geosciences, University of Padova, Padua, Italy Riccardo Pozzobon 3. Department of Physics and Astronomy, University of Padova, Padua, Italy Riccardo Pozzobon 4. Centro di Ateneo di Studi ed Attivita Spaziali 'G. Colombo', University of Padova, Padua, Italy Riccardo Pozzobon 5. La Venta Geographic Exploration APS, Treviso, Italy Riccardo Pozzobon & Francesco Sauro 6. Capella Space Corporation, San Francisco, CA, USA Davide Castelletti 7. Johns Hopkins University Applied Physics Laboratory, Laurel, MD, USA Gerald Wesley Patterson Authors 1. Leonardo Carrer View author publications You can also search for this author in PubMed Google Scholar 2. Riccardo Pozzobon View author publications You can also search for this author in PubMed Google Scholar 3. Francesco Sauro View author publications You can also search for this author in PubMed Google Scholar 4. Davide Castelletti View author publications You can also search for this author in PubMed Google Scholar 5. Gerald Wesley Patterson View author publications You can also search for this author in PubMed Google Scholar 6. Lorenzo Bruzzone View author publications You can also search for this author in PubMed Google Scholar Contributions L.C. formulated the concept. L.C., D.C. and L.B. developed the radar theoretical model for explaining the observations. L.C., D.C., R.P. and F.S. designed the experiments. L.C., D.C. and L.B. analysed the radar data. R.P. produced the 3D models of the pit and cave-like conduit. R.P and F.S. provided the geological interpretation of the experimental results. L.B. supervised the research and the related funding project. All authors co-wrote the paper and discussed the results and the related implications. Corresponding authors Correspondence to Leonardo Carrer or Lorenzo Bruzzone. Ethics declarations Competing interests The authors declare no competing interests. Peer review Peer review information Nature Astronomy thanks Chunyu Ding, Tyler Horvath and Matthew Perry for their contribution to the peer review of this work. Additional information Publisher's note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. Extended data Extended Data Fig. 1 Wagner and Robinson^17 3D model of the Mare Tranquillitatis Pit with superimposed geometric quantities. (a) Geometric model with pit characteristics and radar incident rays. The incident radiation rays are depicted for \({\theta }_{L}\) equal to the one of the Mini-RF acquisition. (b) Geometric model detail depicting the parameters involved in the inversion of the cave conduit characteristics. Refer to methods for the description of the variables displayed in the figures. Extended Data Fig. 2 Comparison between the experimental X-band SAR image and the radar simulation and ground truth of a series of terrestrial analogue pits in Lanzarote, Spain (Lat = 29.165deg deg, Lon = -13.454deg deg). (a) Capella Space X-band (9.65 GHz) Very High Resolution Synthetic Aperture Radar image^14. Radar look direction is indicated with a white arrow. (b) 3D radar simulation^24 without subsurface Lidar 3D digital model. (c) 3D radar simulation^24 with subsurface Lidar 3D digital model. The red lines identify the radar response originating from the conduit interior. (d) 3D Lidar scans and drone photogrammetry of the surface (transparency) and the subsurface^14. Color coding from red to green indicate a progressive increase of the points depth. (e) Superimposition of a detail of the Synthetic Aperture Radar image (Jameo Redondo and Cumplido) with the 3D Lidar scans and drone photogrammetry of the surface and the subsurface^14. Extended Data Fig. 3 Additional examples of tested models and 3D Radar Simulations Results. 3D Radar Simulation assuming (a) roof and floor slope of 10deg, (b) roof and floor slope of 20deg, (c) roof and floor slope of 50deg, (d) roof and floor slope of 60deg, (e) roof and floor slope of 80deg, (f) roof and floor slope of 50deg and 40deg, (g) roof and floor slope of 60deg and 40deg, (h) roof and floor slope of 50deg and 60deg. The red shape marks the outline of the anomaly in the experimental data (Fig. 1a). Extended Data Fig. 4 Examples of the evaluated models latitudinal power profiles. 3D Radar Simulation assuming (a) only the surface elevation model, (b) Wagner and Robinson's^17 3D Pit Model (surface and overhang), (c) model A (roof and floor slope of 3deg), (d) model B (roof and floor slope of 55deg and 45deg), (e) conduit roof and floor slope of 5deg, (f) conduit roof and floor slope of 50deg, (g) conduit roof and floor slope of 60deg, (h) conduit roof and floor slope of 70deg, (i) conduit roof and floor slope of 50deg and 40deg, (l) conduit roof and floor slope of 60deg and 40deg, (m) conduit roof and floor slope of 20deg and (n) conduit roof and floor slope of 80deg. The normalized power profiles are evaluated at a fixed latitude of about 8.335deg. The two power peaks of about 0 dB and -10 dB are the overhang and conduit response, respectively. There is a discrepancy of about 10 dB between the experimental and simulated data in the level of the power response from the lunar surface. This implies that the simulator, as expected, is correctly estimating the scattering contribution from the pit, but underestimating the diffuse scattering contribution from the lunar surface by about 10 dB. However, this does not affect the general validity of the results. The large negative peak of the simulations corresponds to the interior of the pit. This is not shown in the experimental data as due to the Mini-RF dynamic range. Extended Data Fig. 5 Results on selection of the best-fitting model through correlation analysis between experimental and simulated radar data. (a) Values of the correlation coefficient (see Methods) between experimental and simulated data versus the roof and floor slopes. The black arrow represents the uncertainty with respect to the best fit model denoted as B. (b) Maximum value of the correlation coefficient versus the roof's slope. As a result of the radar ambiguity in determining the cave parameters, the two models denoted as A and B are possible. The range of plausible slopes for which the correlation coefficient yields a high value is in line with what predicted by the radar geometric model for estimating the cave conduit slope from the radar image (see Methods). The correlation coefficient value for the simulated data based on the sole Wagner and Robinson overhang model^ 17 is equal to 0.66. Extended Data Fig. 6 Comparison between LROC NAC image and the meshed model of the MTP. (a) LROC NAC image M155016845R at 0.41 m/pixel resolution. Notably, two large boulders of 8-10 m of size are located in the south-western side of the MTP's floor. These were not modelled in the procedural rock population generation as they were considered outliers in the global population and also they do not affect the outputs of the simulated Mini-RF response. (b) Shaded meshed model of the MTP with the central pit bottom populated by the procedurally generated rocks with geometry nodes with random spatial distribution and a size distribution between 1 m and 4 m. This particular range of size has been selected based on the boulder's size that can be observed from LROC NAC images of the MTP. (c, d, e, f, g) Transparency view of the modelled conduit in plan-view and in perspective view. The LROC NAC DEM and the photogrammetric model by Wagner and Robinson^17 are in orange whereas the procedurally generated pit and cave used for the simulations of the subsurface response to Mini-RF are in cyan. The presence or absence of a cave is simulated, and diameter ranges are displayed here starting from 30, 50, 100 and 200 m. The checkboxes show whether the output of the Mini-RF simulation matches with the observed data or not. Extended Data Fig. 7 3D Radar simulations results for different values of the conduit width. 3D Radar Simulation assuming a conduit width of (a) 15 m, (b) 30 m, (c) 55 m, (d) 100 m and (e) 200 m. (f) Value of the radar measured conduit versus the simulated model cave width. The red shape marks the outline of the anomaly in the experimental data (Fig. 1a). Rights and permissions Springer Nature or its licensor (e.g. a society or other partner) holds exclusive rights to this article under a publishing agreement with the author(s) or other rightsholder(s); author self-archiving of the accepted manuscript version of this article is solely governed by the terms of such publishing agreement and applicable law. Reprints and permissions About this article Check for updates. Verify currency and authenticity via CrossMark Cite this article Carrer, L., Pozzobon, R., Sauro, F. et al. Radar evidence of an accessible cave conduit on the Moon below the Mare Tranquillitatis pit. Nat Astron (2024). https://doi.org/10.1038/s41550-024-02302-y Download citation * Received: 20 October 2023 * Accepted: 23 May 2024 * Published: 15 July 2024 * DOI: https://doi.org/10.1038/s41550-024-02302-y Share this article Anyone you share the following link with will be able to read this content: Get shareable link Sorry, a shareable link is not currently available for this article. 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