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Lunar Pits: A Journey Beneath the Moon’s Surface

Brent Garry

by Brent Garry

Research Space Scientist, NASA Goddard Space Flight Center

Neil Armstrong was the first person to walk on the Moon, but who will be the first person to walk underneath the lunar surface?

Twelve humans have walked on the surface of the Moon, but the next generation of lunar explorers could venture beneath the lunar surface. Our entrance to the lunar underground could be through several pits recently discovered in the desolate landscape by the Japanese Space Agency’s (JAXA) Kaguya mission and further studied in detail by NASA’s Lunar Reconnaissance Orbiter (LRO) (Fig. 1A)[1, 2]. Dozens of pits have been found in both the lunar maria (dark regions on the Moon composed of basaltic lava flows) and melt flows associated with impact craters [3]. From a scientific perspective, we are curious as to how the pits form and what they can reveal about the geologic history of the Moon. From an exploration perspective, pits have long been considered potential shelters for future crews to protect them from the dangers of radiation[4][5][6].

Finding these pits was the first step, but where do they really lead? Since the extent of the void space beneath the rim of each pit can only be imaged a few meters back due to limitations in the viewing angle by LRO from orbit, what lies beneath is still unknown. There could be a wall of rock only a few meters from the edge of our visibility creating a cave that leads to nowhere. Or, in an ideal case, the pit could be a collapsed roof of an ancient lava tube that extends back several hundred meters or more. The geologic mystery surrounding these lunar pits and the potential for robotic and human exploration are the catalyst for our team to study How do volcanic pits form?, Where do they lead?, and Can they provide safe shelter for human crews?  We need to answer these questions as best as we can through analog studies before we explore the lunar pits with astronauts. The RIS4E team will spend the next four years investigating pits in Hawaii and New Mexico as part of this effort.

Figure 1. (A) A pit (120 m wide, 70 m deep) in Mare Ingenii on the far side of the Moon. LROC image: M123485893RE, 0.55 m/pixel [NASA/Arizona State University/Lunar Reconnaissance Orbiter Camera]. (B) Two pits (~40-50 m wide, up to 70 m deep) on Kilauea volcano along the Mauna Iki trail (19° 21' 18" 155° 18' 49") that lead to a lava tube system [Google Earth].

Figure 1. (A) A pit (120 m wide, 70 m deep) in Mare Ingenii on the far side of the Moon. LROC image: M123485893RE, 0.55 m/pixel [NASA/Arizona State University/Lunar Reconnaissance Orbiter Camera]. (B) Two pits (~40-50 m wide, up to 70 m deep) on Kilauea volcano along the Mauna Iki trail (19° 21′ 18″ 155° 18′ 49″) that lead to a lava tube system [Google Earth].

Several geologic processes can form pits in volcanic terrains. These include collapse above lava tubes (Fig. 2) and the inflation of a lava flow around a pre-existing topographic high (Fig. 3). The pits shown here in Hawaii lead to a lava tube that extends beneath the surface, while the pictured pits in New Mexico only extend a few meters back from overhanging rim (Fig. 3A,B). Part of the risk in sending a crew to explore a lunar pit is that it may not lead to an extensive cave system that could be used as a shelter. To minimize this risk, we can explore pit here on Earth to determine if there are characteristics that are commonly associated with pits that form above lava tube systems beneath the surface and apply those lessons and observations to the lunar pits.

Our main goal in studying the geologic characteristics and formation of pits at lava flows in Hawaii (Fig. 2) and New Mexico (Fig. 3) is to better understand the pits, so that we have a better idea on what we might expect to discover inside the lunar pits. One instrument our team is using to study the pits is a Light Detection and Ranging (LiDAR) system (Fig. 2A). The LiDAR emits an eye-safe LASER and measures the reflected laser light to create a cloud of individual points that represent the topography. The data can be used to create shaded relief surfaces, colorized elevation maps, and contour maps (Fig. 2C). This past September, we deployed the terrestrial scanning LiDAR to survey two pits in Hawaii. We completed 12 individual scans to capture the interior of these pits. The scans are then mosaicked to create a digital terrain model. The detail of the scans is high enough to capture the individual layers exposed in the walls, similar to the layers observed in the lunar pits. We can manipulate the data to gain new perspectives on the morphology of the pits (Fig. 4). This is just one instrument amongst a suite of field instruments the RIS4E team will use to study the pits and lava flows. Stay tuned as the RIS4E team studies terrestrial analogs of volcanic pits to help us prepare for robotic and human exploration of pits on the Moon!

 

Figure 2. (A) Riegl Vz-400 LiDAR scanner with a Nikon camera and Differential GPS on top set up next to the rim of a volcanic pit in Hawaii. (B) View looking out of a pit above a lava tube in Hawaii. Future astronauts could have a similar view from inside a lunar pit. (C) LiDAR data of the two pits in Hawaii (Fig. 1B) from a side perspective showing how deep the pits go beneath the surface (up to 70 m deep).

Figure 2. (A) Riegl Vz-400 LiDAR scanner with a Nikon camera and Differential GPS on top set up next to the rim of a volcanic pit in Hawaii. (B) View looking out of a pit above a lava tube in Hawaii. Future astronauts could have a similar view from inside a lunar pit. (C) LiDAR data of the two pits in Hawaii (Fig. 1B) from a side perspective showing how deep the pits go beneath the surface (up to 70 m deep).

 

Figure 3. Pits can also form in inflated lava flows. RIS4E team geologists (A) Brent Garry and (B) Jake Bleacher each stand next to pits at the McCarty's lava flow in New Mexico. Notice how there is an overhanging rim, but the void space does not extend more than a few meters beneath the rim. (C, D) Aerial perspectives of pits at the McCarty's lava flows (Courtesy of Jim Zimbelman).

Figure 3. Pits can also form in inflated lava flows. RIS4E team geologists (A) Brent Garry and (B) Jake Bleacher each stand next to pits at the McCarty’s lava flow in New Mexico. Notice how there is an overhanging rim, but the void space does not extend more than a few meters beneath the rim. (C, D) Aerial perspectives of pits at the McCarty’s lava flows (Courtesy of Jim Zimbelman).

Video: Click here to fly through the LiDAR scans of two pits in Hawaii.

References

[1] Haruyama, J., et al., (2009). Possible lunar lava tube skylight observed by SELENE cameras. Geophysical Research Letters, 36(21), L21206, doi:10.1029/2009GL040635.

[2] Robinson, M. S., et al., (2012). Confirmation of sublunarean voids and thin layering in mare deposits. Planetary and Space Science, 69(1), 18-27.

[3] Wagner, R. V., & Robinson, M. S. (2014). Distribution, formation mechanisms, and significance of lunar pits. Icarus, 237, 52-60.

[4] Horz, F. (1985). Lava tubes-Potential shelters for habitats. In Lunar bases and space activities of the 21st century, 1, 405-412.

[5] Coombs, C. R., & Hawke, B. R. A. Y. (1992, September). A search for intact lava tubes on the Moon: Possible lunar base habitats. In Lunar Bases and Space Activities of the 21st Century,1, 219-229.

[6] Angelis, D. G., Wilson, J. W., Clowdsley, M. S., Nealy, J. E., Humes, D. H., & Clem, J. M. (2002). Lunar lava tube radiation safety analysis. Journal of radiation research, 43(Suppl), S41-S45.

 

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