A Potentially Habitable Moon Cave

For decades, scientists have been searching for lunar caves deep and protected enough to serve as safe habitats for long-term manned missions. Recently, an Italian research team reported preliminary evidence of a horizontal cave located at the base of a shaft in the "Sea of Tranquility", which may fulfill these criteria. Discovered through radar scans, the cave's existence still needs confirmation through physical inspection to determine its accessibility and suitability for habitation.  If it proves to be such a cave, this discovery would not only validate the search method used but also likely pave the way for locating additional caves.

 Indications of a Horizontal Lunar Cave with Surface Access | Illustration by A. Romeo, incorporating elements from NASA imagery.

Peeking Beneath the Surface 

The six human missions to the Moon were very brief, with each lasting no more than three days during the Apollo program's final missions. In the coming years, NASA plans to land humans on the Moon again as part of the Artemis program, this time preparing to establish infrastructure for extended stays. The lunar surface, however, presents numerous hazards, including extreme temperatures, micro-meteorite impacts, and intense radiation. A practical solution is to utilize subsurface habitats, which offer considerable protection, and to minimize exposure to the harsh airless lunar environment,  venturing out only when conditions are relatively safer

Experts estimate that the optimal way to achieve this is by utilizing existing lunar caves. This approach would eliminate the need to transport heavy drilling and excavation equipment to the Moon, thereby shortening the timeline for establishing an underground base and significantly cutting mission costs. Presently, the cost of landing one kilogram of equipment on the Moon is estimated at about one million dollars.

The lunar surface is dotted with numerous openings that, according to satellite images, appear to constitute vertical shafts extending tens of meters deep or more. Researchers believe there are also many "lava tubes" on the Moon—horizontal caves that form remnants of volcanic activity, which could be very suitable for housing a manned base. Some of these tubes are visible in satellite images as slightly raised formations, resembling plant roots near the surface. In other cases, surface features suggest collapsed ceilings of such horizontal caves. However, these observations provide only indirect evidence of the existence of such caves.

The openings of the vertical shafts have also only been studied remotely, by utilizing satellites orbiting the Moon. In-depth analysis of optical images and measurements from other instruments allows researchers to estimate the geometry and depth of these openings, assess the continuity of the underground cavities, and even study temperature fluctuations within the shafts. However, researchers have yet to find evidence of the most sought-after site: a horizontal cave accessible from the surface via one of these shafts. Discovering such a cave would provide a tangible location for exploration, initially by robotic vehicles and eventually by human missions.

Studied only remotely so far: The entrance to the shaft known as MTP in the 'Sea of Tranquility,' at the bottom of which the entrance to the horizontal cave might be located  | Satellite image: NASA

Horizon at the Bottom of the Shaft

A team of researchers led by scientists from the University of Trento in Italy reported such a discovery in the journal Nature Astronomy, utilizing advanced radar measurement analysis techniques. They examined data from NASA's Lunar Reconnaissance Orbiter (LRO), which has been orbiting the Moon since 2009. The team focused on measurements from the mini-RF radar instrument, analyzing data collected during the satellite's early years of operation, up to 2011. They focused on radar signals transmitted at oblique angles rather than directly overhead. Their objective was to detect whether the signals penetrated into a potential horizontal cave extending from the shaft's bottom—rather than being reflected directly upwards—and thus to confirm the existence of such caves through radar scans.

The analysis of the measurements revealed signs of a cave at the bottom of a well-known shaft located in the "Sea of Tranquility"—the area where Apollo 11, the first manned spacecraft on the Moon, landed in July 1969. This shaft, known as MTP, has been extensively studied through satellite imagery. It features a pit with a funnel-shaped upper section and steep walls that descend to a depth of approximately 135 meters below the lunar surface at its deepest point. The diameter of the shaft ranges between 80 and 100 meters

Based on the analysis of the measurements and comparison with computer simulations, the researchers believe that there is a cave at the bottom of the MTP with a width of about 45 meters. According to them, the findings match two possible models: one of an almost horizontal cave with a slope of only about three degrees and a length of 25 meters; the other of a steeper cave with a slope of about 45 degrees and a length of 77 meters. The horizontal cave was “probably an empty lava tube,” said the head of the research team, Lorenzo Bruzzone, to The Guardian, adding that such features could serve as human habitats for future explorers as they were “a natural shelter against the harsh lunar environment”.

The cave begins below. A 3D simulation of the MTP vertical shaft based on satellite imagery and measurements | Source: Research article "Lunar Pit Morphology: Implications for Exploration" by R. V. Wagner, M. S. Robinson.

Scientific Potential

Satellite images of the Moon’s surface have revealed over a hundred shaft openings, but MTP is currently the only one where a potential horizontal cave has been discovered at its base. Researchers are hopeful that more such caves will be found, which may require a new satellite equipped with suitable instrumentation. The radar aboard NASA's Lunar Reconnaissance Orbiter (LRO) was not specifically designed for this purpose and is generally limited to studying shafts that are 80 meters in diameter or larger.

"There are likely shafts that constitute lava tube openings on both the Moon and Mars. What’s novel here is the apparent confirmation of the existence of a horizontal cave at the bottom of a shaft," explained Prof. Oded Aharonson from the Department of Earth and Planetary Sciences at the Weizmann Institute of Science, who was not involved in the current research. "The researchers developed a model of the underground topography using the radar's exceptional data, and their findings appear plausible."

"If a lunar colony does become a reality, it will need to protect both people and equipment, and lava tubes are ideal for this purpose, not only on the Moon but also on Mars," added Aharonson, who conducts research on such caves "They can provide protection from intense solar storms similar to those experienced in recent months. While Earth's magnetic field shields us from these storms, the Moon and Mars lack a global magnetic field, making radiation levels during such events potentially hazardous."

In addition to providing protection, such caves may also have important scientific potential, says Aharonson. "The underground areas in the caves are likely more insulated from temperature changes than the surface. If there were microbial life on Mars, one could speculate that they survived in warmer places, like inside caves, making it an excellent place to search for them."