The Promising Cave on the Moon

For decades, scientists have been searching for caves on the Moon that are deep and protected enough to serve as safe living spaces for long-term manned missions. Now, researchers from Italy report for the first time signs of a horizontal cave at the bottom of a shaft in the "Sea of Tranquility" region, which may meet these requirements. The cave was discovered through radar scans, and of course, it needs physical inspection to verify the findings and to assess if it is indeed accessible and suitable for habitation as the researchers expect. If it turns out to be such a cave, it will also validate the search method and likely pave the way for locating additional caves. 

Initial signs of a horizontal cave with an exit to the lunar surface | Rendering: A. Romeo, with components from NASA images.

Peek into the Ground

The six human visits to the moon were very brief, lasting three days at most during the last missions of the Apollo program. In the coming years, NASA plans to land humans on the moon again as part of the Artemis program, this time preparing infrastructure for extended stays. However, the lunar surface is quite dangerous: astronauts are exposed to extreme temperatures, micro-meteorite impacts, and intense radiation. The obvious solution is to stay below the surface most of the time, which provides substantial protection, and minimize exposure to the airless open environment only when the outside conditions are less dangerous.

Experts estimate that the optimal way to achieve this is to utilize existing caves on the moon. This would avoid the need to transport heavy drilling and excavation equipment to the moon, shorten the time from landing to establishing an underground base, and significantly reduce mission costs. Currently, the cost of landing one kilogram of equipment on the moon is estimated at about one million dollars.

The lunar surface is punctuated with many holes that, at least in satellite images, appear to be vertical shafts tens of meters deep or more. Researchers believe there are also many "lava tubes" on the moon, horizontal caves that are remnants of volcanic activity on the young moon, which could be very suitable for housing a manned base. Some of them appear slightly raised above the surface in satellite images, like plant roots close to the ground. In other cases, we can infer from a particular surface formation that it is the outline of such a horizontal cave whose ceiling has collapsed inward. But these are only indirect evidence of the existence of such caves.

The openings of the vertical shafts have also been studied only remotely, from satellites orbiting the moon. In-depth analysis of optical images and measurements from other instruments allows researchers to estimate the geometry and depth of the openings, try to analyze the continuity of the underground cavities, and even study temperature fluctuations within the openings. But so far, researchers have not been able to find evidence of the most sought-after site: a horizontal cave with surface access through such a shaft. Finding such a cave would allow scientists to point to a physical site that could be explored on the moon, initially with robotic vehicles, and later by sending humans there.

So far studied only remotely. 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, using new radar measurement analysis methods. The researchers used measurements taken by the American satellite LRO, which has been orbiting the moon since 2009. One of its instruments is a radar called mini-RF, and the researchers analyzed measurements made by this radar in the early years of the satellite's operation, until 2011. They searched for measurements where the signal was transmitted not when the satellite was directly above the shafts but at an oblique angle. They hoped that the signal reaching the bottom of the shaft this way would not be directly reflected upwards but would penetrate into the horizontal cave extending from its bottom—if such a cave existed—and that the radar scan could confirm its existence.

The analysis of the measurements revealed signs that there is indeed such a cave at the bottom of a well-known shaft located in the "Sea of Tranquility"—the area where the first manned spacecraft on the moon, Apollo 11, landed in July 1969. The shaft, known as MTP, has already been extensively studied in satellite images. It is a pit with a funnel-shaped upper part and fairly steep walls below, reaching a depth of about 135 meters below the surface at its deepest point. The shaft itself is between 80 and 100 meters in diameter.

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. The findings, according to them, 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 a lava tube that remained empty," said the head of the research team, Lorenzo Bruzzone, to The Guardian, adding that such caves could provide living space for astronauts because they are "a natural shelter from the harsh environment of the lunar surface."

 The cave begins below. A 3D simulation of the MTP vertical shaft based on satellite images 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 so far revealed over a hundred such shaft openings, but MTP is the only one where a horizontal cave has apparently been discovered at the bottom. The researchers hope to discover more such caves, which might require a new satellite with suitable instrumentation. The radar on LRO was not designed for such missions and is only suitable for studying shafts with a diameter of 80 meters or more.

"There are shafts that are probably openings of lava tubes both on the moon and on Mars. The novelty here is that there is apparently confirmation of the existence of a horizontal cave at the bottom of the 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. "They developed a model of the underground topography using the radar's exceptional data, and their findings seem plausible."

"A lunar colony, if it indeed comes to be, will need to protect people and equipment, and lava tubes are the ideal place for this, not only on the moon but also on Mars," added Aharonson, who is himself involved in research on such caves. "They can provide protection from strong solar storms like those that hit us in recent months. On Earth, we are relatively protected from them thanks to our magnetic field, but on the moon and Mars, there is no global magnetic field, and the radiation levels during such events can be dangerous."

In addition to protection, such caves may also have important scientific potential, says Aharonson. "The underground areas in the caves are probably more protected 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 look for them."