Cave Discovered on the Moon Opens New Possibilities for Human Colonization

A lunar pit in the Mare Tranquillitatis region. Credit: NASA/GSFC/Arizona State University

Nearly 55 years after the launch of Apollo 11, the first mission to land humans on the moon, scientists have discovered evidence of a vast cave system near the astronauts’ landing site.

Using radar images taken by NASA’s Lunar Reconnaissance Orbiter spacecraft in 2010, researchers were able to determine that huge pits seen in images of the Moon could actually be “skylights” leading to large caves and tunnels beneath the lunar surface.

They could prove extremely valuable to future astronauts hoping to settle on the Moon, serving as a practical shelter for a lunar base.

The cave is accessed through a pit in the Mare Tranquillitatis (Sea of Tranquility), a much-studied area. It is a large basin made mostly of basalt. Neil Armstrong and Buzz Aldrin landed in this area on July 20, 1969.

While it is unlikely that Mare Tranquillitatis will be the first place humans attempt to settle on the Moon, the existence of one cave makes the existence of others very likely, so scientists now expect there to be others in locations more suitable for human settlement.

Mare Tranquillitatis is not scientists’ first choice for a human base because it lacks one of the other important ingredients needed for survival. There is no ice at the lunar equator and therefore no easy access to water for astronauts to drink, produce oxygen, and break down for rocket fuel. This makes the equator an ideal place to land and visit, but a poor choice for setting up camp.

However, it is likely that ice exists at the lunar poles, thanks to the shadow that protects them from the sun’s harsh rays. The poles are therefore our first choice for starting to settle the Moon, as it reduces the amount of water we would need to carry with us.

The opening studied here is simply known as the Mare Tranquillitatis pit, and it is one of about 200 known openings on the lunar surface. It was first photographed in 2010 and is suspected to be a pit leading to a cave or tunnel system, but we had no way to confirm this until now.

In an article published in Astronomy of natureLeonardo Carrer of the University of Trento, Italy, and his colleagues report evidence that this pit does indeed lead to a cave below, and perhaps to a larger system of tunnels and conduits. The Mare Tranquillitatis pit is about 100 meters wide, with steep walls that extend between 130 and 170 meters, making it the deepest known lunar pit.

By reanalyzing the radar data and using computer simulations to reconstruct the pits, the scientists were able to determine that part of the radar signal reflected back to the satellite came from an underground conduit at least several dozen meters long. This suggests that the Mare Tranquillitatis pit leads to an accessible cave beneath the moon’s surface.

Exciting prospects

This discovery is extremely exciting, especially because it represents a promising potential location for future lunar shelters and bases. In addition to providing natural shelter from harmful cosmic rays, a cave system also provides a stable temperature.

The temperature of the lunar surface fluctuates wildly over the weeks due to the lack of an atmosphere to trap heat. During the lunar day, temperatures can reach 121 °C (250 °F) in the sun, then drop to -133 °C (-208 °F) after dark. The shade of an underground cave system should regulate the temperature to be much more constant, making it much easier to build a shelter inside.

Similarly, small asteroids often crash into the Moon due to the lack of atmospheric protection. It is therefore important to be in a shelter strong enough to survive an impact. A cave is the ideal solution to this problem.

While having a cave for shelter could reduce the amount of materials we need to take to the Moon to begin settling there and have a long-term human presence, there are still some hurdles to overcome.

For example, the walls of the pit are steep and over 100 metres high, meaning future explorers will need to find a safe way to descend into the caves and climb back up when they explore the surface.

This could take the form of ladders or more complex systems, such as jetpacks. Fortunately, the lunar gravity is weaker, so this is less of a problem than on Earth. The structural integrity of the caves will also need to be assessed before we move in.

The study also found that the underground system could be between 30 and 80 meters long and about 45 meters wide. It would therefore be large enough to accommodate several “moon homes.” The ground also appears to be flat enough to allow for construction on it without major work or preparation.

The most likely cause of lunar caves is that they are ancient lava tubes. These are tunnels that formed when the moon was still volcanically active, millions of years ago. Flowing lava can develop a continuous, hard crust, forming a roof over the still-flowing lava stream. A trough then exists when the lava stops flowing.

Even better than the Mare Tranquillitatis cave, the discovery of other similar structures near either lunar pole would be a great opportunity for astronauts to enjoy the best of both worlds: shelter from the harsh conditions of the Moon and access to the water ice that exists in the shadowed craters of the lunar poles. This would be a great opportunity to reduce the cost and difficulty of a prolonged installation on the Moon.

The fact that we can spot them from space also allows us to plan missions to use these natural shelters. This could mean that future astronauts will live in caves formed by volcanoes on the Moon.

More information:

Leonardo Carrer et al, Radar evidence of an accessible cave conduit on the Moon beneath the Mare Tranquillitatis pit, Astronomy of nature (2024). DOI: 10.1038/s41550-024-02302-y

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